6.1: Introduction to Ancient Greece
Introduction to Ancient Greece
Ancient Greek culture spans over a thousand years, from the earliest civilizations to the cultures that became the Ancient Greeks.
Illustrate a timeline of ancient Greece from the Bronze Age to the Hellenistic period
Key Points
- Ancient Greek culture is noted for its government, art, architecture, philosophy, and sports, all of which became foundations for modern western society. It was admired and adopted by others, including Alexander the Great and the Romans, who helped spread Greek culture around the world. Before Greek culture took root in Greece, early civilizations thrived on the Greek mainland and the Aegean Islands. The fall of these cultures and the aftermath, known as the Dark Age, is believed to be the time when the Homeric epics were first recited.
- Greek culture began to develop during the Geometric, Orientalizing, and Archaic periods, which lasted from 900 to 480 BCE. During this time the population of city-states began to grow, Panhellenic traditions were established, and art and architecture began to reflect Greek values .
- The Early, High, and Late Classical periods in Greece occurred from 480 to 323 BCE. During these periods, Greece flourished and the polis of Athens saw its Golden Age under the leadership of Pericles . However, city-state rivalries lead to wars, and Greece was never truly stable until conquered.
- The Hellenistic period in Greece is the last period before Greek culture becomes a subset of Roman hegemony . This period occurs from the death of Alexander the Great in 323 BCE, to the Greek defeat at the Battle of Actium in 30 BCE. It marks the spread of Greek culture across the Mediterranean.
Key Terms
- polis : A city, or a city-state. Its plural is poleis.
Ancient Greek Culture
Ancient Greek culture covers over a thousand years of history, from the earliest civilizations in the area to the cultures that became the Ancient Greeks. Following a Greek Dark Age, Greece once more flourished and developed into the ancient culture that we recognize today .
Greek culture is based on a series of shared values that connected independent city-states throughout the region, and expanded as far north as Mount Olympus. Greek society was insular, and loyalties were focused around one’s polis (city-state). Greeks considered themselves civilized and considered outsiders to be barbaric.
While Greek daily life and loyalty was centered on one’s polis, the Greeks did create leagues, which vied for control of the peninsula, and were able to unite together against a common threat (such as the Persians).
Greek culture is focused on their government, art, architecture, philosophy, and sport. Athens was intensely proud of its creation of democracy, and citizens from all poleis (city-states) took part in civic duties. Cities commissioned artists and architects to honor their gods and beautify their cities.
Greek philosophers, mathematicians, and thinkers are still honored in society today. As a religious people, the Greeks worshipped a number of gods through sacrifices, rituals, and festivals.
Bronze Age and Proto-Greek Civilizations
Cycladic Civilization
During the Bronze Age, several distinct cultures developed around the Aegean. The Cycladic civilization, around the Cyclades Islands, thrived from 3,000 to 2,000 BCE. Little is known about the Cycladic civilization because they left no written records. Their material culture is mainly excavated from grave sites, which reveal that the people produced unique, geometric marble figures.
Minoan Civilization
The Minoan civilization stretches from 3700 BCE until 1200 BCE, and thrived during their Neopalatial period (from 1700 to 1400 BCE), with the large-scale building of communal palaces. Numerous archives have been discovered at Minoan sites; however their language, Linear A , has yet to be deciphered. The culture was centered on trade and production, and the Minoans were great seafarers on the Mediterranean Sea.
Mycenaean Civilization
A proto-Greek culture known as the Mycenaeans developed and flourished on the mainland, eventually conquering the Aegean Islands and Crete, where the Minoan civilization was centered. The Mycenaeans developed a fractious, war-like culture that was centered on the authority of a single ruler. Their culture eventually collapsed, but many of their citadel sites were occupied through the Greek Dark Age and rebuilt into Greek city-states.
The Dark Age
From around 1200 BCE, the palace centers and outlying settlements of the Mycenaeans’ culture began to be abandoned or destroyed. By 1050 BCE, the recognizable features of Mycenaean culture had disappeared.
Many explanations attribute the fall of the Mycenaean civilization and the collapse of the Bronze Age to climatic or environmental catastrophe, combined with an invasion by the Dorians or by the Sea Peoples, or to the widespread availability of edged weapons of iron, but no single explanation fits the available archaeological evidence.
This two- to three-century span of history is also known as the Homeric Age. It is believed that the Homeric epics The Iliad and The Odyssey were first recited around this time.
The Geometric and Orientalizing Periods
The Geometric period (c. 900–700 BCE), which derives its name from the proliferation of geometric designs and rendering of figures in art, witnessed the emergence of a new culture on the Greek mainland. The culture’s change in language, its adaptation of the Phoenician alphabet, and its new funerary practices and material culture suggest the ethnic population changed from the mainland’s previous inhabitants, the Mycenaeans.
During this time, the new culture was centered on the people and independent poleis, which divided the land into regional populations. This period witnessed a growth in population and the revival of trade.
The Orientalizing period (c. 700–600 BCE) is named for the cultural exchanges the Greeks had with Eastern, or Oriental civilizations. During this time, international trade began to flourish. Art from this period reflects contact with locations such as Egypt, Syria, Assyria, Phoenicia, and Israel.
Archaic Greece
Greece’s Archaic period lasted from 600 to 480 BCE, in which the Greek culture expanded. The population in Greece began to rise and the Greeks began to colonize along the coasts of the Mediterranean and the Black Sea. The poleis at this time were typically ruled by a single ruler who commanded the city by force.
For the city of Athens, this led to the creation of democracy. Several city-states emerged as major powers, including Athens, Sparta, Corinth, and Thebes. These poleis were often warring with each other, and formed coalitions to gain power and allies. The Persian invasion of Greece in 480 BCE marked the end of the Archaic period.
Classical Greece
The era of Classical Greece began in 480 BCE with the sacking of Athens by the Persians. The Persian invasion of Greece, first lead by Darius I and then by his son Xerxes, united Greece against a common enemy.
With the defeat of the Persian threat, Athens became the most powerful polis until the start of the Peloponnesian War in 431 BCE. These wars continued on and off until 400 BCE. While marred by war, the Classical period saw the height of Greek culture and the creation of some of Greece’s most famous art and architecture.
However, peace and stability in Greece was not achieved until it was conquered and united by Macedonia under the leadership of Philip II and Alexander the Great in the mid-third century BCE.
Hellenistic Greece
The Hellenistic period began with the death of Alexander the Great in 323 BCE, and ended with the Roman victory at the Battle of Actium in 30 BCE. Greece poleis spent this time under the hegemony of foreign rulers, first the Macedons and then the Romans, starting in 146 BCE.
New centers of Hellenic culture flourished through Greece and on foreign soil, including the cities of Pergamon, Antioch, and Alexandria—the capitals of the Attalids, Seleucids, and Ptolemies.
The Ancient Greek Gods and Their Temples
Greek religion played a central and daily role in the life of ancient Greeks, and group worship was centered on the temple and cult sites.
Describe the ways in which Greek life and art was influenced by the gods
Key Points
- The history of the Greek pantheon begins with the primordial deities Gaia and Uranus and their children, the Titans. The pantheon of Greek gods consisted of twelve Olympian gods plus a variety of additional principal and minor gods and goddesses. The gods had human characteristics and personalities, and their lives were detailed by the mythologies told about them.
- The gods played a central role in Greek daily life. They were consulted, blamed, and honored for a variety of reasons, including natural occurrences (from earthquakes to rain), as well as for the public and private affairs of the polis and its people.
- The mythologies and cult worship of heroes also played an important role in Greek religion and ritual . Heroes—especially Perseus, Hercules , Theseus, and those involved in the Trojan War—were often depicted in art, and the location of their feats became cult sites.
- The temple was considered the home of the god and was often an expensive and lavishly decorated building. The temple included a naos , the main room that held the cult statue. Offerings and dedications were left for the gods, and sacrifices took place outdoors.
Key Terms
- primordial : Existing at or before the beginning of time.
- demigod : A half-god or hero; the offspring of a deity and a mortal.
- libation : The act of pouring a liquid or liquor, usually wine, either on the ground or on a victim in sacrifice, in honor of some deity.
- naos : The central room in the god’s temple, where a cult statue of the god is erected.
- polytheistic : A religious system whose members worship many deities.
- votive : A small religious offering deposited at a temple without the purpose of display or retrieval.
Greek religious traditions encompassed a large pantheon of gods, complex mythologies, rituals, and cult practices. Greece was a polytheistic society, and looked to its gods and mythology to explain natural mysteries as well as current events. Religious festivals and ceremonies were held throughout the year, and animal sacrifice and votive offerings were popular ways to appease and worship the gods. Religious life, rituals, and practices were one of the unifying aspects of Greece across regions and poleis (cities, or city-states , such as Athens and Sparta).
Greek Gods
Greek gods were immortal beings who possessed human-like qualities and were represented as completely human in visual art. They were moral and immoral, petty and just, and often vain. The gods were invoked to intervene and assist in matters large, small, private and public.
City-states claimed individual gods and goddess as their patrons . Temples and sanctuaries to the gods were built in every city. Many cities became cult sites due to their connection with a god or goddess and specific myths. For instance, the city of Delphi was known for its oracle and sanctuary of Apollo, because Apollo was believed to have killed a dragon that inhabited Delphi.
The history of the Greek pantheon begins with the primordial deities Gaia (Mother Earth) and Uranus (Father Sky), who were the parents of the first of twelve giants known as Titans. Among these Titans were six males and six females.
- The males were named Oceanus, Hyperion, Coeus, Crius, Iapetus, and Kronos.
- The females were named Themis, Mnemosyne, Tethys, Theia, Phoebe, and Rhea.
Kronos eventually overthrew Uranus and ruled during a mythological Golden Age. Over time, he and Rhea had twelve children who would become the Olympian gods. However, Kronos heard a prophecy that his son would overthrow him, as he did to Uranus. In an effort to avert fate, he ordered Rhea to allow him to devour each of the children upon their birth.
The Olympian Gods
Best known among the pantheon are the twelve Olympian gods and goddesses who resided on Mt. Olympus in northern Greece. Zeus, the youngest son of Rhea and Kronos, was hidden from his father, instead of being swallowed.  These children were Zeus’s siblings, and together they overthrew Kronos, making Zeus the father of gods and men.
Violence and power struggles were common in Greek mythology, and the Greeks used their mythologies to explain their lives around them, from the change in seasons to why the Persians were able to sack Athens.
The traditional pantheon of Greek gods includes
- Zeus, the king of gods and the ruler of the sky,
- Zeus’ two brothers, Poseidon (who ruled over the sea) and Hades (who ruled the underworld).
- Zeus’s sister and wife, Hera, the goddess of marriage, who is frequently jealous and vindictive of Zeus’s other lovers.
- Their sisters Hestia, the goddess of the hearth, and Demeter, the goddess of grain and culture .
- Zeus’s children:
- Athena (goddess of warfare and wisdom).
- Hermes (a messenger god and god of commerce).
- the twins Apollo (god of the sun, music, and prophecy) and Artemis (goddess of the hunt and of wild animals).
- Dionysos (god of wine and theatre).
- Aphrodite (goddess of beauty and love), who was married to Hephaestus (deformed god of the forge).
- Ares (god of war and lover of Aphrodite) are also part of the traditional pantheon.
- Hephaestus was in some mythologies the son of Zeus while in others the fatherless son of Hera.
Heroes
Heroes, who were often demigods , were also important characters in Greek mythology. The two most important heroes are Perseus and Hercules.
Perseus
Perseus is known for defeating the Gorgon, Medusa. He slew her with help from the gods: Athena gave him armor and a reflective shield, and Hermes provided Perseus with winged sandals so he could fly.
Hercules
Hercules was a strong but unkind man, a drunkard who conducted huge misdeeds and social faux pas. Hercules was sent on twelve labors to atone for his sins as punishment for his misdeeds. These deeds, and several other stories, were often depicted in art, on ceramic pots, or on temple metopes . The most famous of his deeds include slaying both the Nemean Lion and the Hydra, capturing Cerberus (the dog of the underworld), and obtaining the apples of the Hesperides.
Theseus
A third hero, Theseus, was an Athenian hero known for slaying King Minos’s Minotaur . Other major heros in Greek mythology include the warriors and participants of the Trojan War, such as Achilles, Ajax, Odysseus, Agamemnon, Paris, Hector, and Helen.
Hero cults were another popular form of Greek worship that involved the honoring of the dead, specifically the dead heroes of the Trojan War. The sites of hero worship were usually old Bronze Age sites or tombs that the ancient Greeks recognized as important or sacred, which they then connected to their own legends and stories.
Sacred Spaces
Greek worship was centered on the temple. The temple was considered the home of the god, and a cult statue of the god would be erected in the central room, or the naos. Temples generally followed the same basic rectangular plan, although a round temple, known as a tholos , were used at some sites in starting in the Classical period.
Temples were oriented east to face the rising sun. Patrons would leave offerings for the gods, such as small votives, large statues, libations or costly goods. Due to the wealth dedicated to the gods, the temples often became treasuries that held and preserved the wealth of the city. Greek temples would be extensively decorated, and their construction was a long and costly endeavor.
Rituals and animal sacrifices in honor of the god or goddess would take place outside, in front of the temple. Rituals often included a large number of people, and sacrifice was a messy business that was best done outdoors. The development and decoration of temples is a primary focus in the study of Greek art and culture.
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1.7: The Foundations of Communication References
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Contributions and Affiliations
- Survey of Communication Study. Authored by : Scott T Paynton and Linda K Hahn. Provided by : Humboldt State University. Located at : en.wikibooks.org/wiki/Survey_of_Communication_Study. License : CC BY-SA: Attribution-ShareAlike


9.2: Heart
The vital importance of the heart is obvious. If one assumes an average rate of contraction of 75 contractions per minute, a human heart would contract approximately 108,000 times in one day, more than 39 million times in one year, and nearly 3 billion times during a 75-year lifespan. Each of the major pumping chambers of the heart ejects approximately 70 mL blood per contraction in a resting adult. This would be equal to 5.25 liters of fluid per minute and approximately 14,000 liters per day. Over one year, that would equal 10,000,000 liters or 2.6 million gallons of blood sent through roughly 60,000 miles of vessels. In order to understand how that happens, it is necessary to understand the anatomy and physiology of the heart.
Location of the Heart
The human heart is located within the thoracic cavity, medially between the lungs in the space known as the mediastinum. Figure 9.2 shows the position of the heart within the thoracic cavity. Within the mediastinum, the heart is separated from the other mediastinal structures by a tough membrane known as the pericardium, or pericardial sac, and sits in its own space called the pericardial cavity. The dorsal surface of the heart lies near the bodies of the vertebrae, and its anterior surface sits deep to the sternum and costal cartilages. The great veins, the superior and inferior venae cavae, and the great arteries, the aorta and pulmonary trunk, are attached to the superior surface of the heart, called the base. The base of the heart is located at the level of the third costal cartilage, as seen in Figure 9.2. The inferior tip of the heart, the apex, lies just to the left of the sternum between the junction of the fourth and fifth ribs near their articulation with the costal cartilages. The right side of the heart is deflected anteriorly, and the left side is deflected posteriorly. It is important to remember the position and orientation of the heart when placing a stethoscope on the chest of a patient and listening for heart sounds, and also when looking at images taken from a midsagittal perspective. The slight deviation of the apex to the left is reflected in a depression in the medial surface of the inferior lobe of the left lung, called the cardiac notch.
Figure 9.2: Location of the Heart
Everyday Connection
CPR
The position of the heart in the torso between the vertebrae and sternum (see Figure 9.2 above for the position of the heart within the thorax) allows for individuals to apply an emergency technique known as cardiopulmonary resuscitation (CPR) if the heart of a patient should stop. By applying pressure with the flat portion of one hand on the sternum in the area between the lines in Figure 9.3, it is possible to manually compress the blood within the heart enough to push some of the blood within it into the pulmonary and systemic circuits. This is particularly critical for the brain, as irreversible damage and death of neurons occur within minutes of loss of blood flow. Current standards call for compression of the chest at least 5 cm deep and at a rate of 100 compressions per minute, a rate equal to the beat in “Staying Alive,” recorded in 1977 by the Bee Gees. If you are unfamiliar with this song, a version is available on www.youtube.com. At this stage, the emphasis is on performing high-quality chest compressions, rather than providing artificial respiration. CPR is generally performed until the patient regains spontaneous contraction or is declared dead by an experienced healthcare professional.
When performed by untrained or overzealous individuals, CPR can result in broken ribs or a broken sternum, and can inflict additional severe damage on the patient. It is also possible, if the hands are placed too low on the sternum, to manually drive the xiphoid process into the liver, a consequence that may prove fatal for the patient. Proper training is essential. This proven life-sustaining technique is so valuable that virtually all medical personnel as well as concerned members of the public should be certified and routinely recertified in its application. CPR courses are offered at a variety of locations, including colleges, hospitals, the American Red Cross, and some commercial companies. They normally include practice of the compression technique on a mannequin.
Figure 9.3: Proper CPR Position
Visit the American Heart Association website to help locate a course near your home in the United States. There are also many other national and regional heart associations that offer the same service, depending upon the location.
Shape and Size of the Heart
The shape of the heart is similar to a pinecone, rather broad at the superior surface and tapering to the apex. A typical heart is approximately the size of your fist: 12 cm (5 in) in length, 8 cm (3.5 in) wide, and 6 cm (2.5 in) in thickness. Given the size difference between most members of the sexes, the weight of a female heart is approximately 250–300 grams (9 to 11 ounces), and the weight of a male heart is approximately 300–350 grams (11 to 12 ounces). The heart of a well-trained athlete, especially one specializing in aerobic sports, can be considerably larger than this. Cardiac muscle responds to exercise in a manner similar to that of skeletal muscle. That is, exercise results in the addition of protein myofilaments that increase the size of the individual cells without increasing their numbers, a concept called hypertrophy. Hearts of athletes can pump blood more effectively at lower rates than those of nonathletes. Enlarged hearts are not always a result of exercise; they can result from pathologies, such as hypertrophic cardiomyopathy. The cause of an abnormally enlarged heart muscle is unknown, but the condition is often undiagnosed and can cause sudden death in apparently otherwise healthy young people.
Chambers and Circulation through the Heart
The human heart consists of four chambers: The left side and the right side each have one atrium and one ventricle. Each of the upper chambers, the right atrium (plural = atria) and the left atrium, acts as a receiving chamber and contracts to push blood into the lower chambers, the right ventricle and the left ventricle. The ventricles serve as the primary pumping chambers of the heart, propelling blood to the lungs or to the rest of the body.
There are two distinct but linked circuits in the human circulation called the pulmonary and systemic circuits. Although both circuits transport blood and everything it carries, we can initially view the circuits from the point of view of gases. The pulmonary circuit transports blood to and from the lungs, where it picks up oxygen and delivers carbon dioxide for exhalation. The systemic circuit transports oxygenated blood to virtually all of the tissues of the body and returns relatively deoxygenated blood and carbon dioxide to the heart to be sent back to the pulmonary circulation.
The right ventricle pumps deoxygenated blood into the pulmonary trunk, which leads toward the lungs and bifurcates into the left and right pulmonary arteries. These vessels in turn branch many times before reaching the pulmonary capillaries, where gas exchange occurs: Carbon dioxide exits the blood and oxygen enters. The pulmonary trunk arteries and their branches are the only arteries in the post-natal body that carry relatively deoxygenated blood. Highly oxygenated blood returning from the pulmonary capillaries in the lungs passes through a series of vessels that join together to form the pulmonary veins—the only post-natal veins in the body that carry highly oxygenated blood. The pulmonary veins conduct blood into the left atrium, which pumps the blood into the left ventricle, which in turn pumps oxygenated blood into the aorta and on to the many branches of the systemic circuit. Eventually, these vessels will lead to the systemic capillaries, where exchange with the tissue fluid and cells of the body occurs. In this case, oxygen and nutrients exit the systemic capillaries to be used by the cells in their metabolic processes, and carbon dioxide and waste products will enter the blood.
The blood exiting the systemic capillaries is lower in oxygen concentration than when it entered. The capillaries will ultimately unite to form venules, joining to form ever-larger veins, eventually flowing into the two major systemic veins, the superior vena cava and the inferior vena cava, which return blood to the right atrium. The blood in the superior and inferior venae cavae flows into the right atrium, which pumps blood into the right ventricle. This process of blood circulation continues as long as the individual remains alive. Understanding the flow of blood through the pulmonary and systemic circuits is critical to all health professions.
Figure 9.4: Dual System of the Human Blood Circulation
Membranes, Surface Features, and Layers
Our exploration of more in-depth heart structures begins by examining the membrane that surrounds the heart, the prominent surface features of the heart, and the layers that form the wall of the heart. Each of these components plays its own unique role in terms of function.
Membranes
The membrane that directly surrounds the heart and defines the pericardial cavity is called the pericardium or pericardial sac. It also surrounds the “roots” of the major vessels, or the areas of closest proximity to the heart. The pericardium, which literally translates as “around the heart,” consists of two distinct sublayers: the sturdy outer fibrous pericardium and the inner serous pericardium. The fibrous pericardium is made of tough, dense connective tissue that protects the heart and maintains its position in the thorax. The more delicate serous pericardium consists of two layers: the parietal pericardium, which is fused to the fibrous pericardium, and an inner visceral pericardium, or epicardium, which is fused to the heart and is part of the heart wall. The pericardial cavity, filled with lubricating serous fluid, lies between the epicardium and the pericardium.
In most organs within the body, visceral serous membranes such as the epicardium are microscopic. However, in the case of the heart, it is not a microscopic layer but rather a macroscopic layer, consisting of a simple squamous epithelium called a mesothelium, reinforced with loose, irregular, or areolar connective tissue that attaches to the pericardium. This mesothelium secretes the lubricating serous fluid that fills the pericardial cavity and reduces friction as the heart contracts.
Figure 9.5: Pericardial Membranes and Layers of the Heart Wall
Surface Features of the Heart
Inside the pericardium, the surface features of the heart are visible, including the four chambers. There is a superficial leaf-like extension of the atria near the superior surface of the heart, one on each side, called an auricle—a name that means “ear like”—because its shape resembles the external ear of a human (Figure 9.6). Auricles are relatively thin-walled structures that can fill with blood and empty into the atria or upper chambers of the heart. You may also hear them referred to as atrial appendages. Also prominent is a series of fat-filled grooves, each of which is known as a sulcus (plural = sulci), along the superior surfaces of the heart. Major coronary blood vessels are located in these sulci. The deep coronary sulcus is located between the atria and ventricles. Located between the left and right ventricles are two additional sulci that are not as deep as the coronary sulcus. The anterior interventricular sulcus is visible on the anterior surface of the heart, whereas the posterior interventricular sulcus is visible on the posterior surface of the heart. Figure 9.6 illustrates anterior and posterior views of the surface of the heart.
Figure 9.6: External Anatomy of the Heart
Layers
The wall of the heart is composed of three layers of unequal thickness. From superficial to deep, these are the epicardium, the myocardium, and the endocardium. The outermost layer of the wall of the heart is also the innermost layer of the pericardium, the epicardium, or the visceral pericardium discussed earlier.
The middle and thickest layer is the myocardium, made largely of cardiac muscle cells. It is built upon a framework of collagenous fibers, plus the blood vessels that supply the myocardium and the nerve fibers that help regulate the heart. It is the contraction of the myocardium that pumps blood through the heart and into the major arteries. The muscle pattern is elegant and complex, as the muscle cells swirl and spiral around the chambers of the heart. They form a figure 8 pattern around the atria and around the bases of the great vessels. Deeper ventricular muscles also form a figure 8 around the two ventricles and proceed toward the apex. More superficial layers of ventricular muscle wrap around both ventricles. This complex swirling pattern allows the heart to pump blood more effectively than a simple linear pattern would. The i mage belowillustrates the arrangement of muscle cells.
Figure 9.7: Heart Musculature
Although the ventricles on the right and left sides pump the same amount of blood per contraction, the muscle of the left ventricle is much thicker and better developed than that of the right ventricle. In order to overcome the high resistance required to pump blood into the long systemic circuit, the left ventricle must generate a great amount of pressure. The right ventricle does not need to generate as much pressure, since the pulmonary circuit is shorter and provides less resistance. The image below illustrates the differences in muscular thickness needed for each of the ventricles.
Figure 9.8: Differences in Ventricular Muscle Thickness
The innermost layer of the heart wall, the endocardium, is joined to the myocardium with a thin layer of connective tissue. The endocardium lines the chambers where the blood circulates and covers the heart valves. It is made of simple squamous epithelium called endothelium, which is continuous with the endothelial lining of the blood vessels.
Once regarded as a simple lining layer, recent evidence indicates that the endothelium of the endocardium and the coronary capillaries may play active roles in regulating the contraction of the muscle within the myocardium. The endothelium may also regulate the growth patterns of the cardiac muscle cells throughout life, and the endothelins it secretes create an environment in the surrounding tissue fluids that regulates ionic concentrations and states of contractility. Endothelins are potent vasoconstrictors and, in a normal individual, establish a homeostatic balance with other vasoconstrictors and vasodilators.
Internal Structure of the Heart
Recall that the heart’s contraction cycle follows a dual pattern of circulation—the pulmonary and systemic circuits—because of the pairs of chambers that pump blood into the circulation. In order to develop a more precise understanding of cardiac function, it is first necessary to explore the internal anatomical structures in more detail.
Septa of the Heart
The word septum is derived from the Latin for “something that encloses;” in this case, a septum (plural = septa) refers to a wall or partition that divides the heart into chambers. The septa are physical extensions of the myocardium lined with endocardium. Located between the two atria is the interatrial septum. Normally in an adult heart, the interatrial septum bears an oval-shaped depression known as the fossa ovalis, a remnant of an opening in the fetal heart known as the foramen ovale. The foramen ovale allowed blood in the fetal heart to pass directly from the right atrium to the left atrium, allowing some blood to bypass the pulmonary circuit. Within seconds after birth, a flap of tissue known as the septum primum that previously acted as a valve closes the foramen ovale and establishes the typical cardiac circulation pattern.
Between the two ventricles is a second septum known as the interventricular septum. Unlike the interatrial septum, the interventricular septum is normally intact after its formation during fetal development. It is substantially thicker than the interatrial septum, since the ventricles generate far greater pressure when they contract.
The septum between the atria and ventricles is known as the atrioventricular septum. It is marked by the presence of four openings that allow blood to move from the atria into the ventricles and from the ventricles into the pulmonary trunk and aorta. Located in each of these openings between the atria and ventricles is a valve, a specialized structure that ensures one-way flow of blood. The valves between the atria and ventricles are known generically as atrioventricular valves. The valves at the openings that lead to the pulmonary trunk and aorta are known generically as semilunar valves. The interventricular septum is visible in the image below. In this figure, the atrioventricular septum has been removed to better show the bicupid and tricuspid valves; the interatrial septum is not visible, since its location is covered by the aorta and pulmonary trunk. Since these openings and valves structurally weaken the atrioventricular septum, the remaining tissue is heavily reinforced with dense connective tissue called the cardiac skeleton, or skeleton of the heart. It includes four rings that surround the openings between the atria and ventricles, and the openings to the pulmonary trunk and aorta, and serve as the point of attachment for the heart valves. The cardiac skeleton also provides an important boundary in the heart electrical conduction system.
Figure 9.9: Interior Structure of the Heart
Right Atrium
The right atrium serves as the receiving chamber for blood returning to the heart from the systemic circulation. The two major systemic veins, the superior and inferior venae cavae, and the large coronary vein called the coronary sinus that drains the heart myocardium empty into the right atrium. The superior vena cava drains blood from regions superior to the diaphragm: the head, neck, upper limbs, and the thoracic region. It empties into the superior and posterior portions of the right atrium. The inferior vena cava drains blood from areas inferior to the diaphragm: the lower limbs and abdominopelvic region of the body. It, too, empties into the posterior portion of the atria, but inferior to the opening of the superior vena cava. Immediately superior and slightly medial to the opening of the inferior vena cava on the posterior surface of the atrium is the opening of the coronary sinus. This thin-walled vessel drains most of the coronary veins that return systemic blood from the heart. The majority of the internal heart structures discussed in this and subsequent sections are illustrated in the image above.
While the bulk of the internal surface of the right atrium is smooth, the depression of the fossa ovalis is medial, and the anterior surface demonstrates prominent ridges of muscle called the pectinate muscles. The right auricle also has pectinate muscles. The left atrium does not have pectinate muscles except in the auricle.
The atria receive venous blood on a nearly continuous basis, preventing venous flow from stopping while the ventricles are contracting. While most ventricular filling occurs while the atria are relaxed, they do demonstrate a contractile phase and actively pump blood into the ventricles just prior to ventricular contraction. The opening between the atrium and ventricle is guarded by the tricuspid valve.
Right Ventricle
The right ventricle receives blood from the right atrium through the tricuspid valve. Each flap of the valve is attached to strong strands of connective tissue, the chordae tendineae, literally “tendinous cords,” or sometimes more poetically referred to as “heart strings.” There are several chordae tendineae associated with each of the flaps. They are composed of approximately 80 percent collagenous fibers with the remainder consisting of elastic fibers and endothelium. They connect each of the flaps to a papillary muscle that extends from the inferior ventricular surface. There are three papillary muscles in the right ventricle, called the anterior, posterior, and septal muscles, which correspond to the three sections of the valves.
When the myocardium of the ventricle contracts, pressure within the ventricular chamber rises. Blood, like any fluid, flows from higher pressure to lower pressure areas, in this case, toward the pulmonary trunk and the atrium. To prevent any potential backflow, the papillary muscles also contract, generating tension on the chordae tendineae. This prevents the flaps of the valves from being forced into the atria and regurgitation of the blood back into the atria during ventricular contraction.The image below shows papillary muscles and chordae tendineae attached to the tricuspid valve.
Figure 9.10: Chordae Tendineae and Papillary Muscles
The walls of the ventricle are lined with trabeculae carneae, ridges of cardiac muscle covered by endocardium. In addition to these muscular ridges, a band of cardiac muscle, also covered by endocardium, known as the moderator band reinforces the thin walls of the right ventricle and plays a crucial role in cardiac conduction. It arises from the inferior portion of the interventricular septum and crosses the interior space of the right ventricle to connect with the inferior papillary muscle.
When the right ventricle contracts, it ejects blood into the pulmonary trunk, which branches into the left and right pulmonary arteries that carry it to each lung. The superior surface of the right ventricle begins to taper as it approaches the pulmonary trunk. At the base of the pulmonary trunk is the pulmonary semilunar valve that prevents backflow from the pulmonary trunk.
Left Atrium
After exchange of gases in the pulmonary capillaries, blood returns to the left atrium high in oxygen via one of the four pulmonary veins. While the left atrium does not contain pectinate muscles, it does have an auricle that includes these pectinate ridges. Blood flows nearly continuously from the pulmonary veins back into the atrium, which acts as the receiving chamber, and from here through an opening into the left ventricle. Most blood flows passively into the heart while both the atria and ventricles are relaxed, but toward the end of the ventricular relaxation period, the left atrium will contract, pumping blood into the ventricle. This atrial contraction accounts for approximately 20 percent of ventricular filling. The opening between the left atrium and ventricle is guarded by the mitral valve.
Left Ventricle
Recall that, although both sides of the heart will pump the same amount of blood, the muscular layer is much thicker in the left ventricle compared to the right. Like the right ventricle, the left also has trabeculae carneae, but there is no moderator band. The mitral valve is connected to papillary muscles via chordae tendineae. There are two papillary muscles on the left—the anterior and posterior—as opposed to three on the right.
The left ventricle is the major pumping chamber for the systemic circuit; it ejects blood into the aorta through the aortic semilunar valve.
Heart Valve Structure and Function
A transverse section through the heart slightly above the level of the atrioventricular septum reveals all four heart valves along the same plane. The valves ensure unidirectional blood flow through the heart. Between the right atrium and the right ventricle is the right atrioventricular valve, or tricuspid valve. It typically consists of three flaps, or leaflets, made of endocardium reinforced with additional connective tissue. The flaps are connected by chordae tendineae to the papillary muscles, which control the opening and closing of the valves.
Figure 9.11: Heart Valves
Emerging from the right ventricle at the base of the pulmonary trunk is the pulmonary semilunar valve, or the pulmonary valve; it is also known as the pulmonic valve or the right semilunar valve. The pulmonary valve is comprised of three small flaps of endothelium reinforced with connective tissue. When the ventricle relaxes, the pressure differential causes blood to flow back into the ventricle from the pulmonary trunk. This flow of blood fills the pocket-like flaps of the pulmonary valve, causing the valve to close and producing an audible sound. Unlike the atrioventricular valves, there are no papillary muscles or chordae tendineae associated with the pulmonary valve.
Located at the opening between the left atrium and left ventricle is the mitral valve, also called the bicuspid valve or the left atrioventricular valve. Structurally, this valve consists of two cusps, known as the anterior medial cusp and the posterior medial cusp, compared to the three cusps of the tricuspid valve. In a clinical setting, the valve is referred to as the mitral valve, rather than the bicuspid valve. The two cusps of the mitral valve are attached by chordae tendineae to two papillary muscles that project from the wall of the ventricle.
At the base of the aorta is the aortic semilunar valve, or the aortic valve, which prevents backflow from the aorta. It normally is composed of three flaps. When the ventricle relaxes and blood attempts to flow back into the ventricle from the aorta, blood will fill the cusps of the valve, causing it to close and producing an audible sound.
In the image above, the two atrioventricular valves are open and the two semilunar valves are closed. This occurs when both atria and ventricles are relaxed and when the atria contract to pump blood into the ventricles. The image below shows a frontal view. Although only the left side of the heart is illustrated, the process is virtually identical on the right.
Figure 9.12: Blood Flow from the Left Atrium to the Left Ventricle
Image a above shows the atrioventricular valves closed while the two semilunar valves are open. This occurs when the ventricles contract to eject blood into the pulmonary trunk and aorta. Closure of the two atrioventricular valves prevents blood from being forced back into the atria. This stage can be seen from a frontal view in image b above.
Figure 9.13: Blood Flow from the Left Ventricle into the Great Vessels
When the ventricles begin to contract, pressure within the ventricles rises and blood flows toward the area of lowest pressure, which is initially in the atria. This backflow causes the cusps of the tricuspid and mitral (bicuspid) valves to close. These valves are tied down to the papillary muscles by chordae tendineae. During the relaxation phase of the cardiac cycle, the papillary muscles are also relaxed and the tension on the chordae tendineae is slight (image b above). However, as the myocardium of the ventricle contracts, so do the papillary muscles. This creates tension on the chordae tendineae (image b above), helping to hold the cusps of the atrioventricular valves in place and preventing them from being blown back into the atria.
The aortic and pulmonary semilunar valves lack the chordae tendineae and papillary muscles associated with the atrioventricular valves. Instead, they consist of pocket-like folds of endocardium reinforced with additional connective tissue. When the ventricles relax and the change in pressure forces the blood toward the ventricles, the blood presses against these cusps and seals the openings.


4.3: Overview- Types of Writing
Now that you have had the opportunity to develop your skills related to reading and comprehension and information literacy, it is time to shift to writing. In your post-secondary nursing program, you will be expected to engage in a variety of types of writing .
Early on in your academic trajectory, you may be asked to simply describe or summarize a text and/or construct an annotated bibliography. You might be asked to do some simple evaluation/critique/analysis of a text. Throughout your program, you will consistently be required to engage in reflective writing. As you progress in your program, you will be asked to engage in more challenging forms of writing including advanced synthesis and critique.
You will often be required to use a combination of types of writing within one assignment. For example, reflective writing will require you to reflect and sometimes analyze. See Table 4.1 for a brief description of common types of writing. Each type of writing is described in more detail in the next sections.
Table 4.1: Common types of writing
| Type of writing | Explanation |
|---|---|
|
Descriptive |
You write a summary about a text you’ve read. Descriptive writing includes a description of the main points and usually does not include your personal opinion or critique. |
|
Reflective |
You think deeply and write about an experience or an event or something you have read. In nursing, you are expected to engage in reflective writing related to your clinical practice. |
|
Analytical |
This genre of writing moves beyond description and involves examining the issue or text closely and looking at its parts to understand the whole. |
|
Persuasive |
Your job is to persuade, influence, convince, and inspire your audience to believe in your point of view on a topic that involves multiple viewpoints and opinions. |
|
Critical |
This genre of writing involves a detailed assessment or evaluation of a text. To some degree, this is the highest level of writing because it is both analytical and persuasive. |
|
Editorial |
This is sometimes referred to as an opinion or perspective piece in which you incorporate an educated opinion from a unique viewpoint on a particular issue. |
|
Literature reviews |
There are many types of writing associated with literature reviews. Overall, you will synthesize a body of literature on a particular topic. |
Student Tip
Know what is expected of you
The most important starting point is that you understand what type of writing is expected for a course or an assignment. Start by closely reviewing and highlighting keywords in the assignment guidelines and if provided, the marking rubric. What verbs do the guidelines use? Reflect? Analyze? Critique? Or something else? These starting points will point you in the right direction.
Listen to the Audio Podcast 4.1 about various types of writing by Kerry McNamara, M.Ed., MFA, Composition Instructor, Tidewater Community College.
Audio Podcast 4.1 : Various types of writing by Kerry McNamara [6:18]
The following discussion provides more details about each type of writing for your assignments.


1.29: Contract Verbs
Module 24
© 2021 Philip S. Peek, CC BY 4.0 https://doi.org/ 10.11647/OBP.0264. 29
The Verb
In Greek and in English verbs have the same definition and functions. Verbs are words that represent actions ( throw ) and states of being ( be or exist ). They differ in the same fundamental way that Greek nouns differ from their English counterparts: they use endings to create meaning in a way that English does not. The Greek verb ( ῥῆμα ) in its finite form has an ending that indicates what person and number the subject is. The Greek infinitive has an ending that indicates that it is unmarked for person and number .
Contract Verbs
As you have seen in Modules 10 , 17 , and 19 , contractions occur when the stem of the first or second principal part ends in an alpha or epsilon. Contractions also occur when the stem ends in an omicron. These vowels contract with the initial vowel or diphthong of the ending in specific ways.
Accenting Contract Verbs
When creating the form of a contract verb, you must accent before you contract: ποιοῦμεν < ποιέομεν , not ποίουμεν .
Contract Verbs in Alpha
If the stem of principal part I or II ends in alpha, the alpha contracts with the endings in accordance with the following chart:
|
|
Present Indicative Active of ὁράω
To the present active, middle, and passive tense stem ὁρα - , add the correct endings.
|
Verb Form |
English Equivalent |
Person and Number |
|
ὁρῶ ( ὁράω ) |
I see |
1st person singular |
|
ὁρᾷς ( ὁράεις ) |
you see |
2 nd person singular |
|
ὁρᾷ ( ὁράει ) |
he, she, it sees |
3 rd person singular |
|
ὁρῶμεν ( ὁράομεν ) |
we see |
1 st person plural |
|
ὁρᾶτε ( ὁ ράετε ) |
you see |
2 nd person plural |
|
ὁρῶσι ( ν ) ( ὁράουσι ( ν )) |
they see |
3 rd person plural |
Present Infinitive Active of ὁράω
To the tense stem ὁρα - , add the active infinitive ending.
|
ὁρᾶν ( ὁράειν < ὁράεεν ) |
- Note the absence of the iota subscript.
Imperfect Indicative Active of ὁράω
To the augmented tense stem ἑωρα - , add the correct endings.
|
Verb Form |
English Equivalent |
Person and # |
|
ἑώρων ( ἑώραον ) |
I was seeing, used to see, saw |
1 st person singular |
|
ἑώρᾱς ( ἑώραες ) |
you were seeing, used to see, saw |
2 nd person singular |
|
ἑώρᾱ ( ἑώραε ) |
he, she, it was seeing, used to see, saw |
3 rd person singular |
|
ἑωρῶμεν ( ἑωράομεν ) |
we were seeing, used to see, saw |
1 st person plural |
|
ἑωρᾶτε ( ἑωράετε ) |
you were seeing, used to see, saw |
2 nd person plural |
|
ἑώρων ( ἑώραον ) |
they were seeing, used to see, saw |
3 rd person plural |
- nu-movable is NEVER added to the third person singular of the imperfect indicative active of contracted verbs. Thus ἑώρᾱ but ἐβάδιζε ( ν ).
- The accent on a contracted ultima is a circumflex. Accents elsewhere are determined by the rules for possibilities of accent.
Contract Verbs in Epsilon
If the stem of principal part I or II ends in epsilon, the epsilon contracts with the endings in accordance with the following chart:
|
|
Present Indicative Active of δοκέω
To the present active, middle, and passive tense stem δοκε - , add the correct endings.
|
Verb Form |
English Equivalent |
Person and Number |
|
δοκῶ ( δοκέω ) |
I think |
1 st person singular |
|
δοκεῖς ( δοκέεις ) |
you think |
2 nd person singular |
|
δοκεῖ ( δοκέει ) |
he, she, it thinks |
3 rd person singular |
|
δοκοῦμεν ( δοκέομεν ) |
we think |
1 st person plural |
|
δοκεῖτε ( δοκέετε ) |
you think |
2 nd person plural |
|
δοκοῦσι ( ν ) ( δοκέουσι ( ν )) |
they think |
3 rd person plural |
Present Infinitive Active of δοκέω
To the present active, middle, and passive tense stem δοκε - , add the active infinitive ending.
|
δοκεῖν ( δοκέειν ) |
Imperfect Indicative Active of δοκέω
To the augmented tense stem ἐδοκ - , add the correct endings.
|
Verb Form |
English Equivalent |
Person and # |
|
ἐδόκουν ( ἐδόκεον ) |
I was thinking, used to think, thought |
1 st person singular |
|
ἐδόκεις ( ἐδόκεες ) |
you were thinking, used to think, thought |
2 nd person singular |
|
ἐδόκει ( ἐδ όκεε ) |
he, she, it was thinking, used to think, thought |
3 rd person singular |
|
ἐδοκοῦμεν ( ἐδοκέομεν ) |
we were thinking, used to think, thought |
1 st person plural |
|
ἐδοκε ῖτε ( ἐδοκέετε ) |
you were thinking, used to think, thought |
2 nd person plural |
|
ἐδόκουν ( ἐδόκεον ) |
they were thinking, used to think, thought |
3 rd person plural |
- nu-movable is NEVER added to the third person singular of the imperfect indicative active of contracted verbs. Thus ἐδόκει but ἐβάδιζε ( ν ).
- The accent on a contracted ultima is a circumflex. Accents elsewhere are determined by the rules for possibilities of accent.
Contract Verbs in Omicron
If the stem of principal part I or II ends in omicron, the omicron contracts with the endings in accordance with the following chart:
|
|
Present Indicative Active of ἀξιόω
To the present active, middle, and passive tense stem ἀξιο - , add the correct endings.
|
Verb Form |
English Equivalent |
Person and Number |
|
ἀξιῶ ( ἀξιόω ) |
I esteem |
1 st person singular |
|
ἀ ξιοῖς ( ἀξιόεις ) |
you esteem |
2 nd person singular |
|
ἀξιοῖ ( ἀξιόει ) |
he, she, it esteems |
3 rd person singular |
|
ἀξιοῦμεν ( ἀξιόομεν ) |
we esteem |
1 st person plural |
|
ἀξιοῦτε ( ἀξιόετε ) |
you esteem |
2 nd person plural |
|
ἀξιοῦσι ( ν ) ( ἀξιόουσι ( ν )) |
they esteem |
3 rd person plural |
Present Infinitive Active of ἀξιόω
To the present active, middle, and passive tense stem ἀξιο - , add the active infinitive ending.
|
ἀξιοῦν ( ἀξιόειν < ἀξιόεεν ) |
- Note the absence of the iota.
Imperfect Indicative Active of ἀξιόω
To the augmented tense stem ἠ ξιο - , add the correct endings.
|
Verb Form |
English Equivalent |
Person and # |
|
ἠξίουν ( ἠξίοον ) |
I was esteeming, used to esteem, esteemed |
1 st person singular |
|
ἠξ ίους ( ἠξίοες ) |
you were esteeming, used to esteem, esteemed |
2 nd person singular |
|
ἠξίου ( ἠξίοε ) |
he, she, it was esteeming, used to esteem, esteemed |
3 rd person singular |
|
ἠξιοῦμεν ( ἠξιόομεν ) |
we were esteeming, used to esteem, esteemed |
1 st person plural |
|
ἠξιοῦ τε ( ἠξιόετε ) |
you were esteeming, used to esteem, esteemed |
2 nd person plural |
|
ἠξίουν ( ἠξίοον ) |
they were esteeming, used to esteem, esteemed |
3 rd person plural |
- nu-movable is NEVER added to the third person singular of the imperfect indicative active of contracted verbs. Thus ἠξίου but ἐβάδιζε ( ν ).
- The accent on a contracted ultima is a circumflex. Accents elsewhere are determined by the rules for possibilities of accent.
Practice Translating Contract Verbs. Translate the below sentences, adapted from a variety of ancient Greek writers. To come to an accurate understanding of the sentences, use your knowledge of endings and their functions. If you forget an ending or function, remember to use the Case and Function Chart in Appendix I and the Adjective, Adverb, Noun, and Pronoun Chart in Appendix VIII to assist you. After you finish translating each sentence, check your understanding with the answer in the Answer Key . After you have read through all of the sentences once, read them again at least two more times. In each subsequent reading, your understanding improves.
- ταύτην μὲν οὖν χρὴ νομίζειν οὐ τὴν αἰτίαν τῶν πάντων κακῶν εἶναι (Lysias).
- μετὰ δὲ ταύτην ἡ Δράκοντος πολιτεία ἦν , ἐν ᾗ καὶ τῶν νόμων ἀριθμὸν ἔγραψε πρῶτον . ἐν ἐκείνοις τοῖς νόμοις μία ζημία , θάνατος . οἱ θεσμοὶ ἐκαλοῦντο χαλεποί .
- ὁ μὲν τὰ ἔπη ἐν ἐλαφρῷ ποιεῖ καὶ ἄπαρνός ἐστι μὴ νοσέειν· οἱ δὲ αὐτῷ πιστεύουσιν ὥστε αὐτὸν κτείν ουσι καὶ ἐσθίουσι (Herodotos).
- αἱ γυναῖκες δοκοῦσιν ἐμοὶ ταὐτὰ * τοῖς ἀνδράσι ποιεῖν . * ταὐτά = τὰ αὐτά
- τὴν αὐτὴν ζημίαν καὶ ἐπὶ ταῖς παλλακαῖς τε καὶ γυναιξὶν ἠξίου γίγνεσθαι (Lysias).
- πίστεις οὐ γὰρ ἔχω εἰπεῖν ἀλλ ’ ἢ ταύτας ὅτι οὐδὲν εἰς τοῦτον πρότερον ποιήσ ω ἢ πρὸς αὐτοὺς λέγω . αἱροῦμαι λέγειν πρῶτον μᾶλλον ἢ ποιεῖν .
- οὐ γὰρ ἀξιῶ οὔτ ’ ἐγὼ φεύγειν τὴν ἐμαυτοῦ πατρίδα οὔτε ἀξιῶ τοῦτον τοσαύτην δίκην παρ ’ ἐμοῦ λαβεῖν .
- μετὰ δὲ ταῦτα , ἐπειδὴ ἔκτεινεν οὓς αὐτὸς περὶ πλείστου ποιεῖται , δρόμῳ ἔφυγε .
- ἐπεὶ ταῦτα πρὸς ἐμαυτὸν ἐνόουν , ἔγνων ὡς γραῦς ἐμὲ διδράσκειν ἕτοιμον οὐ βλέψει (Lucian).
- οἶδα ὅτι αὐτὸς φέρω εὐγενῶς τὰ ἐν ποσὶ καὶ προθύμως βαδίζω καὶ μὴ νοῶ τὰ κακὰ ἃ οἴσω . ἐδόκει ἐμοὶ φεύγειν τάχιστα (Lucian).
Adverbs, Conjunctions, and Verbs
|
* αἱρέω , αἱ ρήσω , εἷλον ( ἑλεῖν ) take, seize, grab, capture; (mid.) choose |
* καλέω , καλέω , ἐκάλεσα call |
|
* ἀξιόω , ἀξιώσω , ἠξίωσα deem worthy, think fit + ‘ x ’ in acc. + inf. ; expect + ‘ x ’ in acc. + inf; to deem ‘ x ’ in acc. worthy of ‘ y ’ in gen. |
κτείνω , κτενέω , ἔκτεινα or ἔκτανον kill, slay, slaughter |
|
βαδίζω , βαδιέω or βαδίσω , ἐβάδισα go, walk |
* λαμβάνω , λήψομαι , ἔλαβον take, receive; capture |
|
βλέπω , βλέψω , ἔβλεψα see, have the power of sight |
* λέγω , λέξω or ἐρέω , ἔλεξα or εἶπον say, tell, speak |
|
* γίγνομαι , γενήσομαι , ἐγενόμην be, be born, happen, become |
νοέω , νοήσω , ἐνόησα or ἔνωσα perceive by the eyes, observe, notice; intend |
|
* γιγνώσκω , γνώσομαι , ἔγνων know, recognize; decide + inf. |
* νομίζω , νομιέω , ἐνόμισα believe, think, have the custom of, hold as custom |
|
* γράφω , γράψω , ἔγραψα write |
νοσέω , νοσήσω , ἐνόσησα be sick, ail |
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διδράσκω , δράσομαι , ἔδρην run away, escape |
* ὁράω ( ἑώραον ), ὄψομαι , εἶδον ( ἰδεῖν ) see |
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* δοκέω , δόξω , ἐ ́ δοξα seem, think; seem best, think best + inf. |
πιστεύω , πιστεύσω , ἐπίστευσα trust, believe, confide in, rely on + dat. |
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* ἔγνων ( γιγνώσκω ) I thought |
* ποιέω , ποιήσω , ἐποίησα do, make, cause; (mid.) consider |
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* εἰπεῖν > λέγω |
προθύμως eagerly, earnestly |
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ἐ πειδή when, since |
* φέρω , οἴσω , ἤνεγκα or ἤνεγκον bring, bear, carry; endure |
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ἐσθίω , ἔδομαι , ἔφαγον or ἤδεσα eat |
* φεύγω , φεύξομαι , ἔφυγον flee; be banished, be in exile; be a defendant |
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εὐγενῶς well, nobly |
* χρή ( inf. χρῆναι ; imp. ἐχρῆν or χρῆν ; fut. χρήσει ), χρῆσται it is necessary + inf. |
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* ἔχω , ἕξω or σχήσω , ἔσχον have, hold; be able + inf.; καλῶς ἔχειν to be well |
Adjectives, Nouns, and Pronouns
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αἴτιος , αἰτίᾱ , αἴτιον responsible for, the cause of, guilty of + gen. |
ἑτοῖμος , ἑτοίμη , ἑτοῖμον at hand, ready, prepared, able + inf. |
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* ἀληθής , ἀληθέ ς true |
ζημία , ζημίας ἡ loss, damage, penalty |
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* ἀνήρ , ἀνδρός ὁ man, husband |
θάνατος , θανάτου ὁ death |
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* ἄξιος , ἀξ ίᾱ , ἄξιον worthy, deserving + gen. |
θεσμός , θεσμοῦ ὁ law |
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ἄπαρνος , ἄπαρνον denying + inf. or gen. |
* κακός , κακή , κακόν bad, evil, cowardly |
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* ἀριθμός , ἀριθμοῦ ὁ number |
* νόμος , νόμου ὁ law, custom |
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γραῦς , γραός ἡ old woman |
παλλακή , παλλακῆς ἡ concubine |
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* γυνή , γυναικός ἡ woman, wife |
πίστις , πίστεως (- ιος ) ἡ faith, honesty, trust, promise |
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* δίκη , δίκης ἡ custom, usage; judgment; order, right; penalty, sentence; lawsuit |
πολιτεία , πολιτείας ἡ government, commonwealth, constitution |
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Δράκων , Δράκοντος ὁ Drako , an Athenian lawgiver, living about the 7th century B.C.E. |
πούς , ποδός ὁ foot ; κατὰ πόδας on the heels |
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δράκων , δράκοντος ὁ a snake |
* πρότερος , προτέρᾱ , πρότερον prior, before, sooner |
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δρόμος , δρόμου ὁ course, race, running; flight, escape; race-course |
* πρῶτος , πρώτη , πρῶτον first, for the present, just now |
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* ἐμαυτοῦ , ἐμαυτῆς , ἐ μαυτοῦ myself |
* τοσοῦτος , τοσαύτη , τοσοῦτο so much, so many |
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ἔπος , ἔπεος ( ἔπους ) τό word, speech; song; saying; verse |
- The asterisk indicates the top 250 most frequently occurring vocabulary, which you are to memorize.
Practice Translating . Translate the sentences below, which have been adapted from Euripides’ Helen ( Ἑλένη ). Remember the meanings and functions of the cases presented in Module 7 . Use your memory to identify endings and their functions. If you forget an ending or a function, consult the Adjective, Adverb, Noun, and Pronoun Chart in Appendix VIII and the Case and Function Chart in Appendix I. Check your understanding with the translations in the Answer Key , making sure that you understand why each word translates as it does. Now go back and read each sentence two or three times, noticing with each rereading how much better your understanding of the sentence becomes. Make this a habit and you will improve quickly.
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Γραῦ ς : τίς πρὸς πύλαις ; οὐκ ἀπαλλ άξῃ δόμων ; πρὸς αὐλείοις στή σεις πύλαις ὄχλον παρέχων δεσπόταις ; εἰ σὺ εἶ Ἕλλην , καταθανῇ· αὐτοῖς γ ὰρ οὐκ ἐπιστροφαί . |
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Μενέλαος : ὦ γραῖα , σοὶ τὰ αὐτὰ ταῦτ ’ ἔπη καὶ ἄλλως λέγειν ἔξεστι· πείσομαι γὰρ ἀ λλὰ δεῖ ἀνιέναι λόγον . |
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5 |
Γραῦς : χρὴ δ ’ ἀπέρχεσθαι . ἐμοὶ γὰρ τοῦ το πρόσκειται , ξένε· ἔξεστι μηδένα πελάζειν τοῖς Ἑλλήνων δόμοις . |
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Μενέλαος : τί προσειλέεις χεῖρα καί με ὠθῇ βίᾳ ; |
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Γραῦς : πείθῃ οὐδὲν τούτων ἃ λέγω . σὺ δ ’ αἴτιος . |
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Μενέλαος : κελεύω σοι ἀγγέλλειν δεσπόταις τοῖς σοῖς . . . |
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10 |
Γραῦς : πικρῶς ἄρ ’ οἶμαι γ ’ εἰ ἀγγέλλω τοὺς σοὺς λόγους . |
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Μενέλαος : . . . ναυαγὸς ἥκω ξένος , ἀσύλητον γένος . |
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Γραῦς : ὥρα οἶκον πρὸς ἄλλον νῦ ν ἀντὶ τοῦδ ’ ἔρχεσθαι . |
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Μενέλαος : οὔκ , ἀ λλ ’ ἔσω παρελεύσομαι καὶ σύ μοι πείσ ῃ . |
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Γραῦς : ὀχληρῶς ἔχεις καὶ τάχ ’ ὠθήσομαι βίᾳ . |
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15 |
Μενέλαος : τὰ κλεινὰ ποῦ ἐστί μοι στρατεύ ματα ; |
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Γραῦς : χῶρός τις ἵνα που σεμνὸς εἶ ἀλλ ’ οὐκ ἐνθ άδε . |
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Μενέλαος : ὦ δαῖμον , ὡς ἀνάξι ’ ἀτιμούμεθα . |
Adverbs and Verbs
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ἀγγέλλω , ἀγγελέω , ἤγγειλα announce, report, tell |
καταθνῄσκω , καταθανέομαι , κατέθανον die, perish |
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ἄλλως otherwise, differently |
* κελεύω , κελεύσω , ἐκέλευσα bid, order, command |
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ἀν ίημι , ἀνήσω , ἀνῆκα send up or forth ; let go; abate; loose; allow + inf.; dissolve |
* λέγω , λέξω or ἐρέω , ἔλεξα or εἶπον say, tell, speak |
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ἀπαλλάττω , ἀπαλλάξω , ἀπήλλαξα set free, release, deliver from; escape; depart |
οἶμαι I think |
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ἀπέρχομαι , ἀπελεύ σομαι , ἀπῆλθον depart, leave |
ὀχληρῶς bothersome, irksome |
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ἀτιμόω , ἀτιμώσω , ἠ τίμωσα dishonor, punish |
παρέρχομαι , παρελεύσομαι , παρῆλθον come forward, pass by, go by |
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* δεῖ it is necessary + ‘x’ in gen. or dat. or acc. + inf., δεῖ ἐλθεῖν it is necessary to come |
* πείθω , πείσω , ἔπεισα persuade + inf.; (mid. or pass.) listen to, obey + dat. or gen. |
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* εἰμί , ἔσομαι be, be possible |
πελάζω , πελάσω , ἐπέλᾰσα approach, come near + dat. |
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ἐνθάδε here |
πικρῶς bitterly |
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ἔξεστι it is possible |
ποῦ where, in what place |
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* ἔρχομαι , ἐλεύ σομαι , ἦλθον come, go |
προσειλέω , προσειλήσω press, force |
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ἔσω within, inside |
πρόσκειμαι be placed at; lie by, lie upon |
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* ἔχω , ἕξω or σχήσω , ἔσχον have, hold; be able + inf.; καλῶς ἔχειν to be well |
τάχα quickly, presently; perhaps |
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* ἥκω , ἥξω have come, be present |
* χρ ή ( inf. χρῆναι ; imp. ἐχρῆν or χρῆν ; fut. χρήσει ), χρ ῆσται it is necessary + inf. |
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* ἵστημι , στήσω , ἔστηκα stand, make stand , place |
ὠθέω , ὠθήσω , ὤθησα push, shove, thrust; (mid.) press forward |
Adjectives, Nouns, Pronouns
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αἴτιος , αἰτίᾱ , αἴτιον responsible, guilty |
κλεινός , κλεινή , κλεινόν famous |
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* ἄλλος , ἄλλη , ἄλλο another, other |
* λόγος , λόγου ὁ word, speech, story; reason, account |
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ἀνάξιος , ἀναξίᾱ , ἀνάξιον (- ος , - ον ) unworthy + gen. |
ναυαγός , ναυαγόν shipwrecked |
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ἀσύλητος , ἀσύλητον inviolate |
ξένος ( ξεῖνος ), ξένου ( ξείνου ) ὁ stranger; guest-friend |
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αὔλειος , αὐλεία , αὔλειον (- ος , - ον ) of the courtyard |
οἶκος , οἴκου ὁ house, palace |
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βία , βίας ἡ strength, force |
ὄχλος , ὄχλου ὁ throng, mob; trouble |
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* γένος , γένεος (- ους ) τό race, kind, sort; birth, origin |
* παρέχων , παρέχοντος ὁ furnishing; causing; allowing |
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γραῖα , γραίᾱς ἡ old lady |
πύλη , πύλης ἡ gates |
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δαίμων , δαίμονος ὁ god, deity |
σεμνός , σεμνή , σεμνόν revered, holy |
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δεσπότης , δεσπότου ὁ master |
* σός , σή , σόν your |
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δόμος , δόμου ὁ house, houses |
στράτευμα , στρατεύματος τό campaign; army |
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* Ἕλλην , Ἕλληνος ἡ ὁ Greek |
* χείρ , χειρός ἡ ( dat pl. χερσίν ) hand; force, army |
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ἔπος , ἔπεος ( ἔπους ) τό word, speech; song; saying; verse |
χῶρος , χώρου ὁ ground, place |
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ἐπιστροφή , ἐπιστροφῆς ἡ turning; business |
ὥρα , ὥρας ἡ season, period, time |
- The asterisk indicates the top 250 most frequently occurring vocabulary, which you are to memorize.
Practice Parsing Greek Sentences. Parse each word of the sentence found below. For nouns and pronouns, give case and function. For verbs, give person, number, tense, mood, and voice. For adverbs and conjunctions, identify them. For prepositional phrases, give the preposition and the preposition’s object. For adjectives, specify the noun they agree with in gender, number, and case.
ὀχληρῶς ἔχεις καὶ τ άχ ’ ὠθήσομαι βίᾳ .
Check your answers with those in the Answer Key .
Prophecy and Seers
Many Greeks believed in the existence of a multitude of divine beings and believed that the deities took such an interest in mortal affairs that they intervened, caused events to happen, and predicted the future. Seers or prophets served as intermediaries between the divine and mortal spheres. The Greeks consulted seers for a variety of different reasons including decisions like should they marry, should they go to war, and who their parents were. Called a bastard while dining at a banquet, Oidipous consulted the oracle at Delphi to find out who his parents were. Uncertain about whether he should wage war against the Persians, Kroisos (Croesus), king of the Lydians, consulted many oracles to determine which ones were genuine. Kroisos’ test affirmed the validity of the oracle of Apollo at Delphi. Kroisos offered the oracle abundant gifts and asked if he should wage war against the Persians. To both Oidipous and Kroisos the oracle gave ambiguous answers.  To Kroisos it replied that if he went to war, he would destroy a great empire. Oidipous tried to keep the oracle’s prediction from coming true by avoiding his parents. In fleeing from the prophecy and whom he thought were his parents, Oidipous fulfilled it, killing his biological dad Laios and marrying his birth mother Jocasta. Kroisos waged war against the Persians and destroyed a great empire, his own. To the common question of marrying, the Delphic oracle gave straightforward yes or no answers. Consulting the oracle about possibly doing something that was considered wrong—like handing over to an enemy someone to whom you have granted asylum—could result in divine vengeance and your death (Herodotos, Book I.159). When the Greeks, intent on sailing to Troy to recover Helen, were at Aulis stranded by contrary winds, they consulted a seer who replied that favorable winds would be granted if Agamemnon sacrificed his daughter Iphigeneia to the goddess Artemis. It may be that Artemis’ demand for a sacrifice was her divine punishment of Agamemnon for wanting to wage the Trojan war, causing much needless death and suffering. Dreams and portents also serve as intermediaries, giving mortals an indication of what may or may not happen. Today prediction, prophecy, and portents remain an integral part of our culture and play a significant role in literature, in science, and in people’s belief systems. Einstein’s theory of relativity predicts that the past, present, and future all exist concurrently. Time has been proven to be relative, slowing down the faster we approach the speed of light. If the future is already here, does this mean that prediction is merely the statement of what already is?
Module 24 Top 250 Vocabulary to be Memorized. Like learning the alphabet and endings, memorizing vocabulary is essential to acquiring language. The better you memorize the top 250 most frequently occurring vocabulary words, the greater mastery of the language you will have.
Adjective and Nouns
ἡμέρᾱ , ἡμέρᾱς ἡ day
ἱππεύς , ἱππῆος ( ἱππέως ) ὁ knight, cavalryman; horseman, rider
ἵππος , ἵππου ἡ ὁ horse; (fem.) cavalry
νέος , νέᾱ , νέον new, fresh, young; strange, unexpected
ποταμός , ποταμοῦ ὁ river
τέλος , τέλεος ( τέλους ) τό end, boundary; power; office; (acc.) finally
τόπος , τόπου ὁ place, spot
Verbs
πάσχω , πείσομαι , ἔπαθον , πέπονθα , ------, ------ suffer, have done to one
φεύγω , φεύξομαι , ἔφυγον , πέφευγα , ------, ------ flee; be banished; be in exile; be a defendant


13.2: DNA Replication
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- Ying Liu
- City College of San Francisco
Learning Objectives
- Explain the meaning of semiconservative DNA replication
- Explain why DNA replication is bidirectional and includes both a leading and lagging strand
- Explain why Okazaki fragments are formed
- Describe the process of DNA replication and the functions of the enzymes involved
- Identify the differences between DNA replication in bacteria and eukaryotes
- Explain the process of rolling circle replication
The elucidation of the structure of the double helix by James Watson and Francis Crick in 1953 provided a hint as to how DNA is copied during the process of replication. Separating the strands of the double helix would provide two templates for the synthesis of new complementary strands, but exactly how new DNA molecules were constructed was still unclear. In one model, semiconservative replication, the two strands of the double helix separate during DNA replication, and each strand serves as a template from which the new complementary strand is copied; after replication, each double-stranded DNA includes one parental or “old” strand and one “new” strand. There were two competing models also suggested: conservative and dispersive, which are shown in Figure \(\PageIndex{1}\).
Matthew Meselson (1930–) and Franklin Stahl (1929–) devised an experiment in 1958 to test which of these models correctly represents DNA replication (Figure \(\PageIndex{2}\)). They grew E. coli for several generations in a medium containing a “heavy” isotope of nitrogen ( 15 N) that was incorporated into nitrogenous bases and, eventually, into the DNA. This labeled the parental DNA. The E. coli culture was then shifted into a medium containing 14 N and allowed to grow for one generation. The cells were harvested and the DNA was isolated. The DNA was separated by ultracentrifugation, during which the DNA formed bands according to its density. DNA grown in 15 N would be expected to form a band at a higher density position than that grown in 14 N. Meselson and Stahl noted that after one generation of growth in 14 N, the single band observed was intermediate in position in between DNA of cells grown exclusively in 15 N or 14 N. This suggested either a semiconservative or dispersive mode of replication. Some cells were allowed to grow for one more generation in 14 N and spun again. The DNA harvested from cells grown for two generations in 14 N formed two bands: one DNA band was at the intermediate position between 15 N and 14 N, and the other corresponded to the band of 14 N DNA. These results could only be explained if DNA replicates in a semiconservative manner. Therefore, the other two models were ruled out. As a result of this experiment, we now know that during DNA replication, each of the two strands that make up the double helix serves as a template from which new strands are copied. The new strand will be complementary to the parental or “old” strand. The resulting DNA molecules have the same sequence and are divided equally into the two daughter cells.
Exercise \(\PageIndex{1}\)
What would have been the conclusion of Meselson and Stahl’s experiment if, after the first generation, they had found two bands of DNA?
DNA Replication in Bacteria
DNA replication has been well studied in bacteria primarily because of the small size of the genome and the mutants that are available. E. coli has 4.6 million base pairs (Mbp) in a single circular chromosome and all of it is replicated in approximately 42 minutes, starting from a single origin of replication and proceeding around the circle bidirectionally (i.e., in both directions). This means that approximately 1000 nucleotides are added per second. The process is quite rapid and occurs with few errors.
DNA replication uses a large number of proteins and enzymes (Table \(\PageIndex{1}\)). One of the key players is the enzyme DNA polymerase, also known as DNA pol. In bacteria, three main types of DNA polymerases are known: DNA pol I, DNA pol II, and DNA pol III. It is now known that DNA pol III is the enzyme required for DNA synthesis; DNA pol I and DNA pol II are primarily required for repair. DNA pol III adds deoxyribonucleotides each complementary to a nucleotide on the template strand, one by one to the 3’-OH group of the growing DNA chain. The addition of these nucleotides requires energy. This energy is present in the bonds of three phosphate groups attached to each nucleotide (a triphosphate nucleotide), similar to how energy is stored in the phosphate bonds of adenosine triphosphate (ATP) (Figure \(\PageIndex{3}\)). When the bond between the phosphates is broken and diphosphate is released, the energy released allows for the formation of a covalent phosphodiester bond by dehydration synthesis between the incoming nucleotide and the free 3’-OH group on the growing DNA strand.
Initiation
The initiation of replication occurs at specific nucleotide sequence called the origin of replication, where various proteins bind to begin the replication process. E. coli has a single origin of replication (as do most prokaryotes), called oriC , on its one chromosome. The origin of replication is approximately 245 base pairs long and is rich in adenine-thymine (AT) sequences.
Some of the proteins that bind to the origin of replication are important in making single-stranded regions of DNA accessible for replication. Chromosomal DNA is typically wrapped around histones (in eukaryotes and archaea) or histone-like proteins (in bacteria), and is supercoiled, or extensively wrapped and twisted on itself. This packaging makes the information in the DNA molecule inaccessible. However, enzymes called topoisomerases change the shape and supercoiling of the chromosome. For bacterial DNA replication to begin, the supercoiled chromosome is relaxed by topoisomerase II, also called DNA gyrase. An enzyme called helicase then separates the DNA strands by breaking the hydrogen bonds between the nitrogenous base pairs. Recall that AT sequences have fewer hydrogen bonds and, hence, have weaker interactions than guanine-cytosine (GC) sequences. These enzymes require ATP hydrolysis. As the DNA opens up, Y-shaped structures called replication forks are formed. Two replication forks are formed at the origin of replication, allowing for bidirectional replication and formation of a structure that looks like a bubble when viewed with a transmission electron microscope; as a result, this structure is called a replication bubble. The DNA near each replication fork is coated with single-stranded binding proteins to prevent the single-stranded DNA from rewinding into a double helix.
Once single-stranded DNA is accessible at the origin of replication, DNA replication can begin. However, DNA pol III is able to add nucleotides only in the 5’ to 3’ direction (a new DNA strand can be only extended in this direction). This is because DNA polymerase requires a free 3’-OH group to which it can add nucleotides by forming a covalent phosphodiester bond between the 3’-OH end and the 5’ phosphate of the next nucleotide. This also means that it cannot add nucleotides if a free 3’-OH group is not available, which is the case for a single strand of DNA. The problem is solved with the help of an RNA sequence that provides the free 3’-OH end. Because this sequence allows the start of DNA synthesis, it is appropriately called the primer. The primer is five to 10 nucleotides long and complementary to the parental or template DNA. It is synthesized by RNA primase, which is an RNA polymerase. Unlike DNA polymerases, RNA polymerases do not need a free 3’-OH group to synthesize an RNA molecule. Now that the primer provides the free 3’-OH group, DNA polymerase III can now extend this RNA primer, adding DNA nucleotides one by one that are complementary to the template strand (Figure \(\PageIndex{1}\)).
Elongation
During elongation in DNA replication, the addition of nucleotides occurs at its maximal rate of about 1000 nucleotides per second. DNA polymerase III can only extend in the 5’ to 3’ direction, which poses a problem at the replication fork. The DNA double helix is antiparallel; that is, one strand is oriented in the 5’ to 3’ direction and the other is oriented in the 3’ to 5’ direction (see Structure and Function of DNA ). During replication, one strand, which is complementary to the 3’ to 5’ parental DNA strand, is synthesized continuously toward the replication fork because polymerase can add nucleotides in this direction. This continuously synthesized strand is known as the leading strand. The other strand, complementary to the 5’ to 3’ parental DNA, grows away from the replication fork, so the polymerase must move back toward the replication fork to begin adding bases to a new primer, again in the direction away from the replication fork. It does so until it bumps into the previously synthesized strand and then it moves back again (Figure \(\PageIndex{4}\)). These steps produce small DNA sequence fragments known as Okazaki fragments, each separated by RNA primer. Okazaki fragments are named after the Japanese research team and married couple Reiji and Tsuneko Okazaki, who first discovered them in 1966. The strand with the Okazaki fragments is known as the lagging strand, and its synthesis is said to be discontinuous.
The leading strand can be extended from one primer alone, whereas the lagging strand needs a new primer for each of the short Okazaki fragments. The overall direction of the lagging strand will be 3’ to 5’, and that of the leading strand 5’ to 3’. A protein called the sliding clamp holds the DNA polymerase in place as it continues to add nucleotides. The sliding clamp is a ring-shaped protein that binds to the DNA and holds the polymerase in place. Beyond its role in initiation, topoisomerase also prevents the overwinding of the DNA double helix ahead of the replication fork as the DNA is opening up; it does so by causing temporary nicks in the DNA helix and then resealing it. As synthesis proceeds, the RNA primers are replaced by DNA. The primers are removed by the exonuclease activity of DNA polymerase I, and the gaps are filled in. The nicks that remain between the newly synthesized DNA (that replaced the RNA primer) and the previously synthesized DNA are sealed by the enzyme DNA ligase that catalyzes the formation of covalent phosphodiester linkage between the 3’-OH end of one DNA fragment and the 5’ phosphate end of the other fragment, stabilizing the sugar-phosphate backbone of the DNA molecule.
Termination
Once the complete chromosome has been replicated, termination of DNA replication must occur. Although much is known about initiation of replication, less is known about the termination process. Following replication, the resulting complete circular genomes of prokaryotes are concatenated, meaning that the circular DNA chromosomes are interlocked and must be separated from each other. This is accomplished through the activity of bacterial topoisomerase IV, which introduces double-stranded breaks into DNA molecules, allowing them to separate from each other; the enzyme then reseals the circular chromosomes. The resolution of concatemers is an issue unique to prokaryotic DNA replication because of their circular chromosomes. Because both bacterial DNA gyrase and topoisomerase IV are distinct from their eukaryotic counterparts, these enzymes serve as targets for a class of antimicrobial drugs called quinolones.
| Enzyme or Factor | Function |
|---|---|
| DNA pol I | Exonuclease activity removes RNA primer and replaces it with newly synthesized DNA |
| DNA pol III | Main enzyme that adds nucleotides in the 5’ to 3’ direction |
| Helicase | Opens the DNA helix by breaking hydrogen bonds between the nitrogenous bases |
| Ligase | Seals the gaps between the Okazaki fragments on the lagging strand to create one continuous DNA strand |
| Primase | Synthesizes RNA primers needed to start replication |
| Single-stranded binding proteins | Bind to single-stranded DNA to prevent hydrogen bonding between DNA strands, reforming double-stranded DNA |
| Sliding clamp | Helps hold DNA pol III in place when nucleotides are being added |
| Topoisomerase II (DNA gyrase) | Relaxes supercoiled chromosome to make DNA more accessible for the initiation of replication; helps relieve the stress on DNA when unwinding, by causing breaks and then resealing the DNA |
| Topoisomerase IV | Introduces single-stranded break into concatenated chromosomes to release them from each other, and then reseals the DNA |
Exercise \(\PageIndex{2}\)
- Which enzyme breaks the hydrogen bonds holding the two strands of DNA together so that replication can occur?
- Is it the lagging strand or the leading strand that is synthesized in the direction toward the opening of the replication fork?
- Which enzyme is responsible for removing the RNA primers in newly replicated bacterial DNA?
DNA Replication in Eukaryotes
Eukaryotic genomes are much more complex and larger than prokaryotic genomes and are typically composed of multiple linear chromosomes (Table \(\PageIndex{2}\)). The human genome, for example, has 3 billion base pairs per haploid set of chromosomes, and 6 billion base pairs are inserted during replication. There are multiple origins of replication on each eukaryotic chromosome (Figure \(\PageIndex{5}\)); the human genome has 30,000 to 50,000 origins of replication. The rate of replication is approximately 100 nucleotides per second—10 times slower than prokaryotic replication.
The essential steps of replication in eukaryotes are the same as in prokaryotes. Before replication can start, the DNA has to be made available as a template. Eukaryotic DNA is highly supercoiled and packaged, which is facilitated by many proteins, including histones (see Structure and Function of Cellular Genomes ). At the origin of replication, a prereplication complex composed of several proteins, including helicase, forms and recruits other enzymes involved in the initiation of replication, including topoisomerase to relax supercoiling, single-stranded binding protein, RNA primase, and DNA polymerase. Following initiation of replication, in a process similar to that found in prokaryotes, elongation is facilitated by eukaryotic DNA polymerases. The leading strand is continuously synthesized by the eukaryotic polymerase enzyme pol δ, while the lagging strand is synthesized by pol ε. A sliding clamp protein holds the DNA polymerase in place so that it does not fall off the DNA. The enzyme ribonuclease H (RNase H), instead of a DNA polymerase as in bacteria, removes the RNA primer, which is then replaced with DNA nucleotides. The gaps that remain are sealed by DNA ligase.
Because eukaryotic chromosomes are linear, one might expect that their replication would be more straightforward. As in prokaryotes, the eukaryotic DNA polymerase can add nucleotides only in the 5’ to 3’ direction. In the leading strand, synthesis continues until it reaches either the end of the chromosome or another replication fork progressing in the opposite direction. On the lagging strand, DNA is synthesized in short stretches, each of which is initiated by a separate primer. When the replication fork reaches the end of the linear chromosome, there is no place to make a primer for the DNA fragment to be copied at the end of the chromosome. These ends thus remain unpaired and, over time, they may get progressively shorter as cells continue to divide.
The ends of the linear chromosomes are known as telomeres and consist of noncoding repetitive sequences. The telomeres protect coding sequences from being lost as cells continue to divide. In humans, a six base-pair sequence, TTAGGG, is repeated 100 to 1000 times to form the telomere. The discovery of the enzyme telomerase (Figure \(\PageIndex{6}\)) clarified our understanding of how chromosome ends are maintained. Telomerase contains a catalytic part and a built-in RNA template. It attaches to the end of the chromosome, and complementary bases to the RNA template are added on the 3’ end of the DNA strand. Once the 3’ end of the lagging strand template is sufficiently elongated, DNA polymerase can add the nucleotides complementary to the ends of the chromosomes. In this way, the ends of the chromosomes are replicated. In humans, telomerase is typically active in germ cells and adult stem cells; it is not active in adult somatic cells and may be associated with the aging of these cells. Eukaryotic microbes including fungi and protozoans also produce telomerase to maintain chromosomal integrity. For her discovery of telomerase and its action, Elizabeth Blackburn (1948–) received the Nobel Prize for Medicine or Physiology in 2009.
| Property | Bacteria | Eukaryotes |
|---|---|---|
| Genome structure | Single circular chromosome | Multiple linear chromosomes |
| Number of origins per chromosome | Single | Multiple |
| Rate of replication | 1000 nucleotides per second | 100 nucleotides per second |
| Telomerase | Not present | Present |
| RNA primer removal | DNA pol I | RNase H |
| Strand elongation | DNA pol III | pol δ, pol ε |
Exercise \(\PageIndex{3}\)
- How does the origin of replication differ between eukaryotes and prokaryotes?
- What polymerase enzymes are responsible for DNA synthesis during eukaryotic replication?
- What is found at the ends of the chromosomes in eukaryotes and why?
DNA Replication of Extrachromosomal Elements: Plasmids and Viruses
To copy their nucleic acids, plasmids and viruses frequently use variations on the pattern of DNA replication described for prokaryote genomes. For more information on the wide range of viral replication strategies, see The Viral Life Cycle .
Rolling Circle Replication
Whereas many bacterial plasmids (see Unique Characteristics of Prokaryotic Cells ) replicate by a process similar to that used to copy the bacterial chromosome, other plasmids, several bacteriophages, and some viruses of eukaryotes use rolling circle replication (Figure \(\PageIndex{7}\)). The circular nature of plasmids and the circularization of some viral genomes on infection make this possible. Rolling circle replication begins with the enzymatic nicking of one strand of the double-stranded circular molecule at the double-stranded origin (dso) site. In bacteria, DNA polymerase III binds to the 3’-OH group of the nicked strand and begins to unidirectionally replicate the DNA using the un-nicked strand as a template, displacing the nicked strand as it does so. Completion of DNA replication at the site of the original nick results in full displacement of the nicked strand, which may then recircularize into a single-stranded DNA molecule. RNA primase then synthesizes a primer to initiate DNA replication at the single-stranded origin (sso) site of the single-stranded DNA (ssDNA) molecule, resulting in a double-stranded DNA (dsDNA) molecule identical to the other circular DNA molecule.
Exercise \(\PageIndex{4}\)
Is there a lagging strand in rolling circle replication? Why or why not?
Key Concepts and Summary
- The DNA replication process is semiconservative , which results in two DNA molecules, each having one parental strand of DNA and one newly synthesized strand.
- In bacteria, the initiation of replication occurs at the origin of replication , where supercoiled DNA is unwound by DNA gyrase , made single-stranded by helicase , and bound by single-stranded binding protein to maintain its single-stranded state. Primase synthesizes a short RNA primer , providing a free 3’-OH group to which DNA polymerase III can add DNA nucleotides.
- During elongation , the leading strand of DNA is synthesized continuously from a single primer. The lagging strand is synthesized discontinuously in short Okazaki fragments , each requiring its own primer. The RNA primers are removed and replaced with DNA nucleotides by bacterial DNA polymerase I , and DNA ligase seals the gaps between these fragments.
- Termination of replication in bacteria involves the resolution of circular DNA concatemers by topoisomerase IV to release the two copies of the circular chromosome.
- Eukaryotes typically have multiple linear chromosomes, each with multiple origins of replication. Overall, replication in eukaryotes is similar to that in prokaryotes.
- The linear nature of eukaryotic chromosomes necessitates telomeres to protect genes near the end of the chromosomes. Telomerase extends telomeres, preventing their degradation, in some cell types.
- Rolling circle replication is a type of rapid unidirectional DNA synthesis of a circular DNA molecule used for the replication of some plasmids.