7.4: The Vertebral Column
By the end of this section, you will be able to:
- Describe each region of the vertebral column and the number of bones in each region
- Discuss the curves of the vertebral column and how these change after birth
- Describe a typical vertebra and determine the distinguishing characteristics for vertebrae in each vertebral region and features of the sacrum and the coccyx
- Describe the structure of an intervertebral disc
- Determine the location of the ligaments that provide support for the vertebral column
The vertebral column is also known as the spinal column or spine (Figure \(\PageIndex{1}\)). It consists of a sequence of vertebrae (singular = vertebra), each of which is separated and united by an intervertebral disc . Together, the vertebrae and intervertebral discs form the vertebral column. It is a flexible column that supports the head, neck, and body and allows for their movements. It also protects the spinal cord, which passes down the back through openings in the vertebrae.
Regions of the Vertebral Column
The vertebral column originally develops as a series of 33 vertebrae, but this number is eventually reduced to 24 vertebrae, plus the sacrum and coccyx. The vertebral column is subdivided into five regions, with the vertebrae in each area named for that region and numbered in descending order. In the neck, there are seven cervical vertebrae, each designated with the letter “C” followed by its number. Superiorly, the C1 vertebra articulates (forms a joint) with the occipital condyles of the skull. Inferiorly, C1 articulates with the C2 vertebra, and so on. Below these are the 12 thoracic vertebrae, designated T1–T12. The lower back contains the L1–L5 lumbar vertebrae. The single sacrum, which is also part of the pelvis, is formed by the fusion of five sacral vertebrae. Similarly, the coccyx, or tailbone, results from the fusion of four small coccygeal vertebrae. However, the sacral and coccygeal fusions do not start until age 20 and are not completed until middle age.
An interesting anatomical fact is that almost all mammals have seven cervical vertebrae, regardless of body size. This means that there are large variations in the size of cervical vertebrae, ranging from the very small cervical vertebrae of a shrew to the greatly elongated vertebrae in the neck of a giraffe. In a full-grown giraffe, each cervical vertebra is 11 inches tall.
Curvatures of the Vertebral Column
The adult vertebral column does not form a straight line, but instead has four curvatures along its length (see Figure \(\PageIndex{1}\)). These curves increase the vertebral column’s strength, flexibility, and ability to absorb shock. When the load on the spine is increased, by carrying a heavy backpack for example, the curvatures increase in depth (become more curved) to accommodate the extra weight. They then spring back when the weight is removed. The four adult curvatures are classified as either primary or secondary curvatures. Primary curves are retained from the original fetal curvature, while secondary curvatures develop after birth.
During fetal development, the body is flexed anteriorly into the fetal position, giving the entire vertebral column a single curvature that is concave anteriorly. In the adult, this fetal curvature is retained in two regions of the vertebral column as the thoracic curve , which involves the thoracic vertebrae, and the sacrococcygeal curve , formed by the sacrum and coccyx. Each of these is thus called a primary curve because they are retained from the original fetal curvature of the vertebral column.
A secondary curve develops gradually after birth as the child learns to sit upright, stand, and walk. Secondary curves are concave posteriorly, opposite in direction to the original fetal curvature. The cervical curve of the neck region develops as the infant begins to hold their head upright when sitting. Later, as the child begins to stand and then to walk, the lumbar curve of the lower back develops. In adults, the lumbar curve is generally deeper in females.
Disorders associated with the curvature of the spine include kyphosis (an excessive posterior curvature of the thoracic region), lordosis (an excessive anterior curvature of the lumbar region), and scoliosis (an abnormal, lateral curvature, accompanied by twisting of the vertebral column).
Disorders of the...
Vertebral Column
Developmental anomalies, pathological changes, or obesity can enhance the normal vertebral column curves, resulting in the development of abnormal or excessive curvatures (Figure \(\PageIndex{2}\)). Kyphosis, also referred to as humpback or hunchback, is an excessive posterior curvature of the thoracic region. This can develop when osteoporosis causes weakening and erosion of the anterior portions of the upper thoracic vertebrae, resulting in their gradual collapse (Figure \(\PageIndex{3}\)). Lordosis, or swayback, is an excessive anterior curvature of the lumbar region and is most commonly associated with obesity or late pregnancy. The accumulation of body weight in the abdominal region results an anterior shift in the line of gravity that carries the weight of the body. This causes in an anterior tilt of the pelvis and a pronounced enhancement of the lumbar curve.
Scoliosis is an abnormal, lateral curvature, accompanied by twisting of the vertebral column. Compensatory curves may also develop in other areas of the vertebral column to help maintain the head positioned over the feet. Scoliosis is the most common vertebral abnormality among girls. The cause is usually unknown, but it may result from weakness of the back muscles, defects such as differential growth rates in the right and left sides of the vertebral column, or differences in the length of the lower limbs. When present, scoliosis tends to get worse during adolescent growth spurts. Although most individuals do not require treatment, a back brace may be recommended for growing children. In extreme cases, surgery may be required.
Excessive vertebral curves can be identified while an individual stands in the anatomical position. Observe the vertebral profile from the side and then from behind to check for kyphosis or lordosis. Then have the person bend forward. If scoliosis is present, an individual will have difficulty in bending directly forward, and the right and left sides of the back will not be level with each other in the bent position.
Interactive Link
Osteoporosis is a common age-related bone disease in which bone density and strength is decreased. Watch this video to get a better understanding of how thoracic vertebrae may become weakened and may fracture due to this disease. How may vertebral osteoporosis contribute to kyphosis?
General Structure of a Vertebra
Within the different regions of the vertebral column, vertebrae vary in size and shape, but they all follow a similar structural pattern. A typical vertebra will consist of a body, a vertebral arch, and seven processes (Figure \(\PageIndex{4}\)).
The body is the anterior portion of each vertebra and is the part that supports the body weight. Because of this, the vertebral bodies progressively increase in size and thickness going down the vertebral column. The bodies of adjacent vertebrae are separated and strongly united by an intervertebral disc.
The vertebral arch forms the posterior portion of each vertebra. It consists of four parts, the right and left pedicles and the right and left laminae. Each pedicle forms one of the lateral sides of the vertebral arch. The pedicles are anchored to the posterior side of the vertebral body. Each lamina forms part of the posterior roof of the vertebral arch. The large opening between the vertebral arch and body is the vertebral foramen , which contains the spinal cord. In the intact vertebral column, the vertebral foramina of all of the vertebrae align to form the vertebral (spinal) canal , which serves as the bony protection and passageway for the spinal cord down the back. When the vertebrae are aligned together in the vertebral column, notches in the margins of the pedicles of adjacent vertebrae together form an intervertebral foramen , the opening through which a spinal nerve exits from the vertebral column (Figure \(\PageIndex{5}\)).
Seven processes arise from the vertebral arch. Each paired transverse process projects laterally and arises from the junction point between the pedicle and lamina. The single spinous process (vertebral spine) projects posteriorly at the midline of the back. The vertebral spines can easily be felt as a series of bumps just under the skin down the middle of the back. The transverse and spinous processes serve as important muscle attachment sites. A superior articular process extends or faces upward, and an inferior articular process faces or projects downward on each side of a vertebrae. The paired superior articular processes of one vertebra join with the corresponding paired inferior articular processes from the next higher vertebra. These junctions form slightly moveable joints between the adjacent vertebrae. The shape and orientation of the articular processes vary in different regions of the vertebral column and play a major role in determining the type and range of motion available in each region.
Regional Modifications of Vertebrae
In addition to the general characteristics of a typical vertebra described above, vertebrae also display characteristic size and structural features that vary between the different vertebral column regions. Thus, cervical vertebrae are smaller than lumbar vertebrae due to differences in the proportion of body weight that each supports. Thoracic vertebrae have sites for rib attachment, and the vertebrae that give rise to the sacrum and coccyx have fused together into single bones.
Cervical Vertebrae
Typical cervical vertebrae , such as C4 or C5, have several characteristic features that differentiate them from thoracic or lumbar vertebrae (Figure \(\PageIndex{6}\)). Cervical vertebrae have a small body, reflecting the fact that they carry the least amount of body weight. Cervical vertebrae usually have a bifid (Y-shaped) spinous process. The spinous processes of the C3–C6 vertebrae are short, but the spine of C7 is much longer. You can find these vertebrae by running your finger down the midline of the posterior neck until you encounter the prominent C7 spine located at the base of the neck. The transverse processes of the cervical vertebrae are sharply curved (U-shaped) to allow for passage of the cervical spinal nerves. Each transverse process also has an opening called the transverse foramen . An important artery that supplies the brain ascends up the neck by passing through these openings. The superior and inferior articular processes of the cervical vertebrae are flattened and largely face upward or downward, respectively.
The first and second cervical vertebrae are further modified, giving each a distinctive appearance. The first cervical (C1) vertebra is also called the atlas , because this is the vertebra that supports the skull on top of the vertebral column (in Greek mythology, Atlas was the god who supported the heavens on his shoulders). The C1 vertebra does not have a body or spinous process. Instead, it is ring-shaped, consisting of an anterior arch and a posterior arch . The transverse processes of the atlas are longer and extend more laterally than do the transverse processes of any other cervical vertebrae. The superior articular processes face upward and are deeply curved for articulation with the occipital condyles on the base of the skull. The inferior articular processes are flat and face downward to join with the superior articular processes of the C2 vertebra.
The second cervical (C2) vertebra is called the axis , because it serves as the axis for rotation when turning the head toward the right or left. The axis resembles typical cervical vertebrae in most respects, but is easily distinguished by the dens (odontoid process), a bony projection that extends upward from the vertebral body. The dens joins with the inner aspect of the anterior arch of the atlas, where it is held in place by the transverse ligament.
Thoracic Vertebrae
The bodies of the thoracic vertebrae are larger than those of cervical vertebrae (Figure \(\PageIndex{7}\). The characteristic feature for a typical midthoracic vertebra is the spinous process, which is long and has a pronounced downward angle that causes it to overlap the next inferior vertebra. The superior articular facets of thoracic vertebrae face anteriorly and the inferior facets face posteriorly. These orientations are important determinants for the type and range of movements available to the thoracic region of the vertebral column.
Thoracic vertebrae have several additional articulation sites, each of which is called a facet , where a rib is attached. Most thoracic vertebrae have two facets located on the lateral sides of the body, each of which is called a costal facet (costal = “rib”). These are for articulation with the head (end) of a rib. An additional facet is located on the transverse process for articulation with the tubercle of a rib.
Lumbar Vertebrae
Lumbar vertebrae carry the greatest amount of body weight and are thus characterized by the large size and thickness of the vertebral body (Figure \(\PageIndex{9}\)). They have short transverse processes and a short, blunt spinous process that projects posteriorly. The articular processes are large, with the superior process facing medially and the inferior facing laterally.
Sacrum and Coccyx
The sacrum is a triangular-shaped bone that is thick and wide across its superior base where it is weight bearing and then tapers down to an inferior, non-weight bearing apex (Figure \(\PageIndex{10}\)). It is formed by the fusion of five sacral vertebrae, a process that does not begin until after the age of 20. On the anterior surface of the older adult sacrum, the lines of vertebral fusion can be seen as four transverse ridges. On the posterior surface, running down the midline, is the median sacral crest , a bumpy ridge that is the remnant of the fused spinous processes (median = “midline”; while medial = “toward, but not necessarily at, the midline”). Similarly, the fused transverse processes of the sacral vertebrae form the lateral sacral crest .
The sacral promontory is the anterior lip of the superior base of the sacrum. Lateral to this is the roughened auricular surface, which joins with the ilium portion of the hipbone to form the immobile sacroiliac joints of the pelvis. Passing inferiorly through the sacrum is a bony tunnel called the sacral canal , which terminates at the sacral hiatus near the inferior tip of the sacrum. The anterior and posterior surfaces of the sacrum have a series of paired openings called sacral foramina (singular = foramen) that connect to the sacral canal. Each of these openings is called a posterior (dorsal) sacral foramen or anterior (ventral) sacral foramen . These openings allow for the anterior and posterior branches of the sacral spinal nerves to exit the sacrum. The superior articular process of the sacrum , one of which is found on either side of the superior opening of the sacral canal, articulates with the inferior articular processes from the L5 vertebra.
The coccyx, or tailbone, is derived from the fusion of four very small coccygeal vertebrae (see Figure \(\PageIndex{10}\)). It articulates with the inferior tip of the sacrum. It is not weight bearing in the standing position, but may receive some body weight when sitting.
Intervertebral Discs and Ligaments of the Vertebral Column
The bodies of adjacent vertebrae are strongly anchored to each other by an intervertebral disc. This structure provides padding between the bones during weight bearing, and because it can change shape, also allows for movement between the vertebrae. Although the total amount of movement available between any two adjacent vertebrae is small, when these movements are summed together along the entire length of the vertebral column, large body movements can be produced. Ligaments that extend along the length of the vertebral column also contribute to its overall support and stability.
Intervertebral Disc
An intervertebral disc is a fibrocartilaginous pad that fills the gap between adjacent vertebral bodies (see Figure \(\PageIndex{5}\)). Each disc is anchored to the bodies of its adjacent vertebrae, thus strongly uniting these. The discs also provide padding between vertebrae during weight bearing. Because of this, intervertebral discs are thin in the cervical region and thickest in the lumbar region, which carries the most body weight. In total, the intervertebral discs account for approximately 25 percent of your body height between the top of the pelvis and the base of the skull. Intervertebral discs are also flexible and can change shape to allow for movements of the vertebral column.
Each intervertebral disc consists of two parts. The anulus fibrosus is the tough, fibrous outer layer of the disc. It forms a circle (anulus = “ring” or “circle”) and is firmly anchored to the outer margins of the adjacent vertebral bodies. Inside is the nucleus pulposus , consisting of a softer, more gel-like material. It has a high water content that serves to resist compression and thus is important for weight bearing. With increasing age, the water content of the nucleus pulposus gradually declines. This causes the disc to become thinner, decreasing total body height somewhat, and reduces the flexibility and range of motion of the disc, making bending more difficult.
The gel-like nature of the nucleus pulposus also allows the intervertebral disc to change shape as one vertebra rocks side to side or forward and back in relation to its neighbors during movements of the vertebral column. Thus, bending forward causes compression of the anterior portion of the disc but expansion of the posterior disc. If the posterior anulus fibrosus is weakened due to injury or increasing age, the pressure exerted on the disc when bending forward and lifting a heavy object can cause the nucleus pulposus to protrude posteriorly through the anulus fibrosus, resulting in a herniated disc (“ruptured” or “slipped” disc) (Figure \(\PageIndex{11}\)). The posterior bulging of the nucleus pulposus can cause compression of a spinal nerve at the point where it exits through the intervertebral foramen, with resulting pain and/or muscle weakness in those body regions supplied by that nerve. The most common sites for disc herniation are the L4/L5 or L5/S1 intervertebral discs, which can cause sciatica, a widespread pain that radiates from the lower back down the thigh and into the leg. Similar injuries of the C5/C6 or C6/C7 intervertebral discs, following forcible hyperflexion of the neck from a collision accident or football injury, can produce pain in the neck, shoulder, and upper limb.
Interactive Link
Watch this video to learn about a herniated disc.
Watch this second video to see one possible treatment for a herniated disc, removing the damaged portion of the disc. How could lifting a heavy object produce pain in a lower limb?
Ligaments of the Vertebral Column
Adjacent vertebrae are united by ligaments that run the length of the vertebral column along both its posterior and anterior aspects (Figure \(\PageIndex{12}\)). These serve to resist excess forward or backward bending movements of the vertebral column, respectively.
The anterior longitudinal ligament runs down the anterior side of the entire vertebral column, uniting the vertebral bodies. It serves to resist excess backward bending of the vertebral column. Protection against this movement is particularly important in the neck, where extreme posterior bending of the head and neck can stretch or tear this ligament, resulting in a painful whiplash injury. Prior to the mandatory installation of seat headrests, whiplash injuries were common for passengers involved in a rear-end automobile collision.
The supraspinous ligament is located on the posterior side of the vertebral column, where it interconnects the spinous processes of the thoracic and lumbar vertebrae. This strong ligament supports the vertebral column during forward bending motions. In the posterior neck, where the cervical spinous processes are short, the supraspinous ligament expands to become the nuchal ligament (nuchae = “nape” or “back of the neck”). The nuchal ligament is attached to the cervical spinous processes and extends upward and posteriorly to attach to the midline base of the skull, out to the external occipital protuberance. It supports the skull and prevents it from falling forward. This ligament is much larger and stronger in four-legged animals such as cows, where the large skull hangs off the front end of the vertebral column. You can easily feel this ligament by first extending your head backward and pressing down on the posterior midline of your neck. Then tilt your head forward and you will fill the nuchal ligament popping out as it tightens to limit anterior bending of the head and neck.
Additional ligaments are located inside the vertebral canal, next to the spinal cord, along the length of the vertebral column. The posterior longitudinal ligament is found anterior to the spinal cord, where it is attached to the posterior sides of the vertebral bodies. Posterior to the spinal cord is the ligamentum flavum (“yellow ligament”). This consists of a series of short, paired ligaments, each of which interconnects the lamina regions of adjacent vertebrae. The ligamentum flavum has large numbers of elastic fibers, which have a yellowish color, allowing it to stretch and then pull back. Both of these ligaments provide important support for the vertebral column when bending forward.
Interactive Link
Use this tool to identify the bones, intervertebral discs, and ligaments of the vertebral column. The thickest portions of the anterior longitudinal ligament and the supraspinous ligament are found in which regions of the vertebral column?
Career Connection
Chiropractor
Chiropractors are health professionals who use nonsurgical techniques to help patients with musculoskeletal system problems that involve the bones, muscles, ligaments, tendons, or nervous system. They treat problems such as neck pain, back pain, joint pain, or headaches. Chiropractors focus on the patient’s overall health and can also provide counseling related to lifestyle issues, such as diet, exercise, or sleep problems. If needed, they will refer the patient to other medical specialists.
Chiropractors use a drug-free, hands-on approach for patient diagnosis and treatment. They will perform a physical exam, assess the patient’s posture and spine, and may perform additional diagnostic tests, including taking X-ray images. They primarily use manual techniques, such as spinal manipulation, to adjust the patient’s spine or other joints. They can recommend therapeutic or rehabilitative exercises, and some also include acupuncture, massage therapy, or ultrasound as part of the treatment program. In addition to those in general practice, some chiropractors specialize in sport injuries, neurology, orthopaedics, pediatrics, nutrition, internal disorders, or diagnostic imaging.
To become a chiropractor, students must have 3–4 years of undergraduate education, attend an accredited, four-year Doctor of Chiropractic (D.C.) degree program, and pass a licensure examination to be licensed for practice in their state. With the aging of the baby-boom generation, employment for chiropractors is expected to increase.


2.3.2: Internal Anatomy of the Primary Stem
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- Tom Buckley
- University of California, Davis
Learning Objectives
- Identify the structures representing each of the three tissue systems in stems.
- Compare the structure of solenosteles, eusteles, and atactosteles.
- Explain the arrangement of leaf traces, leaf trace gaps, branch traces, and branch trace gaps.
The primary stem refers to the herbaceous (non-woody) stem, which has not undergone secondary growth (the growth that produces bark and wood). Some species (all monocots and some eudicots) remain herbaceous for their entire lives, maintaining the primary stem. Other species of eudicots initially form a primary stem but later become woody, replacing the primary stem with the secondary stem. The anatomy of the stem (internal structure) can be examined through longitudinal sections (cutting the stem lengthwise) or in cross sections (cutting a slice of the stem perpendicular to the length).
All three tissue types are represented in the primary stem. The epidermis is the dermal tissue that surrounds and protects the stem. The epidermis typically consists of one layer of cells. A waxy cuticle on the outside of these cells limits water loss. Epidermal cells are the most numerous and least differentiated of the cells in the epidermis. Pores in the epidermis called stomata (singular: stoma) allow for gas exchange. Each stoma is bordered by a pair of guard cells , which regulate stomatal opening. While stomata are present in stems, they occur at higher densities in leaves. Trichomes are hair-like structures on the epidermal surface. They help to reduce transpiration (the loss of water by aboveground plant parts), increase solar reflectance, and store compounds that defend the leaves against predation by herbivores (Figure \(\PageIndex{1}\)).
Ground tissue fills much of the stem, forming the cortex directly within the epidermis and the pith (if present) in the center. The outermost portion of the cortex is usually a few layers of collenchyma cells. The remainder of the cortex and pith consist of parenchyma cells. The arrangement of vascular tissue in the stem depends on the species (see below).
Cross sections reveal three possible arrangements of vascular tissue ( steles ) in the stem. The first arrangement ( solenostele ) is present in a few eudicots, such as basswood ( Tilia ). In the solenostele, the vascular tissue appears as a continuous ring ( vascular cylinder ; Figure \(\PageIndex{2}\)). The interfascicular regions (pith rays) of parenchyma cells that separate vascular tissue are thus extremely narrow. The second arrangement ( eustele ) is present in most eudicots such as sunflower ( Helianthus ) and buttercup ( Ranunculus ). In the eustele, vascular tissue is clustered into distinct vascular bundles arranged in a ring, allowing for thicker interfascicular regions in between them (Figure \(\PageIndex{3-4}\)). In these solenosteles and eusteles, the vascular tissue separates ground tissue into an outer cortex and central pith. The third arrangement ( atactostele ) is present in most monocots, such as corn ( Zea mays ) and a few eudicots. In the atactostele, vascular bundles are scattered throughout the stem (Figure \(\PageIndex{3, 5}\)). While vascular bundles near the outside of the stem are packed more densely in this third arrangement, their distribution is somewhat disorderly. There is no distinction between the cortex and pith in the third arrangement.
The cells of embryonic tissue called the procambium (see Meristems) divides to produce primary xylem internally and primary phloem externally. In some vascular bundles, some procambial cells remain and form the fascicular cambium in the center of the vascular bundle. Once the stem has finished lengthening, sclerenchyma fibers called primary phloem fibers are produced just outside of the primary phloem. The primary phloem fibers of each vascular bundle are sometimes called phloem caps (bundle caps). If primary phloem fibers surrounded the entire vascular bundle, they form a bundle sheath (Figure \(\PageIndex{6}\)).
Vascular bundles connect leaves and stems. The strands of vascular tissue that connect the leaves to the stem are called leaf traces . Just above leaf traces are portions of stem without vascular tissue called leaf trace gaps . Similarly, branch traces connect axillary shoots to the main stem, leaving branch trace gaps just above them (Figure \(\PageIndex{7-8}\)).
Attributions
Curated and authored by Melissa Ha from the following sources:
- From 5.4: The Stem from Introduction to Botany by Alexey Shipunov (public domain)
- From 30.2 Stems from Biology 2e by OpenStax (licensed CC-BY )


7.25A: Inactivating and Marking Target Genes with Transposons
- Describe the utility of experimentally introduced Transposons
A transposable element (TE) is a DNA sequence that can change its relative position (self-transpose) within the genome of a single cell. The mechanism of transposition can be either “copy and paste” or “cut and paste. ” Transposition can create phenotypically significant mutations and alter the cell’s genome size. Barbara McClintock’s discovery of these jumping genes early in her career earned her a Nobel prize in 1983.
Transposons in bacteria usually carry an additional gene for function other than transposition—often for antibiotic resistance. In bacteria, transposons can jump from chromosomal DNA to plasmid DNA and back, allowing for the transfer and permanent addition of genes such as those encoding antibiotic resistance (multi-antibiotic resistant bacterial strains can be generated in this way). When the transposable elements lack additional genes, they are known as insertion sequences. Transposons are semi-parasitic DNA sequences that can replicate and spread through the host ‘s genome. They can be harnessed as a genetic tool for analysis of gene and protein function. The use of transposons is well-developed in Drosophila (in which P elements are most commonly used) and in Thale cress (Arabidopsis thaliana) and bacteria such as Escherichia coli (E. coli ).
Synthetic DNA transposon system are constructed to introduce precisely defined DNA sequences into the chromosomes of vertebrate animals for the purposes of introducing new traits and to discover new genes and their functions (e.g. by establishing a loss-of-function phenotype or gene inactivation). Transposition is a precise process in which a defined DNA segment is excised from one DNA molecule and moved to another site in the same or different DNA molecule or genome.
Insertional inactivation is a technique used in recombinant DNA engineering where a plasmid (such as pBR322) is used to disable the expression of a gene. A gene is inactivated by inserting a fragment of DNA into the middle of its coding sequence. Any future products from the inactivated gene will not work because of the extra codes added to it. An example is the use of pBR322, which has genes that respectively encode polypeptides that confer resistance to ampicillin and tetracyclin antibiotics. As a result, when a genetic region is interrupted by integration of pBR322, the gene function is lost but new gene function (resistance to specific antibiotics) is gained. An alternative strategy for insertional mutagenesis has been used in vertebrate animals to find genes that cause cancer. In this case a transposon, e.g. Sleeping Beauty, is designed to interrupt a gene in such a way that it causes maximal genetic havoc. Specifically, the transposon contains signals to truncate expression of an interrupted gene at the site of the insertion and then restart expression of a second truncated gene. This method has been used to identify oncogenes.
Key Points
- Transposons contain signals to truncate expression of an interrupted gene, thus inactivating it.
- Transposons are widely used tools in biology, frequently utilized for insertion mutagenesis, large-scale gene disruption studies, and gene tagging.
- Transposon-mediated gene disruption experiments and promoter traps rely on promiscuous, undirected, and pseudo-random insertion of the transposon.
Key Terms
- transposable : Able to be transposed (in any sense).
- plasmid : A circle of double-stranded DNA that is separate from the chromosomes, which is found in bacteria and protozoa.


4.13: HESIOD’S WORKS AND DAYS
(*Spellings have been adapted to American English)
Invocation of the Muses
(ll. 1-10) Muses of Pieria who give glory through song, come hither, tell of Zeus your father and chant his praise. Through him mortal men are famed or un-famed, sung or unsung alike, as great Zeus wills. For easily he makes strong, and easily he brings the strong man low; easily he humbles the proud and raises the obscure, and easily he straightens the crooked and blasts the proud,—Zeus who thunders aloft and has his dwelling most high.
Attend thou with eye and ear, and make judgments straight with righteousness. And I, Perses [the audience], would tell of true things.
The Two Strifes
(ll. 11-24) So, after all, there was not one kind of Strife alone, but all over the earth there are two. As for the one, a man would praise her when he came to understand her; but the other is blameworthy: and they are wholly different in nature. For one fosters evil war and battle, being cruel: her no man loves; but perforce, through the will of the deathless gods, men pay harsh Strife [Eris] her honor due. But the other is the elder daughter of dark Night, and the son of Cronos who sits above and dwells in the aether, set her in the roots of the earth: and she is far kinder to men. She stirs up even the shiftless to toil; for a man grows eager to work when he considers his neighbor, a rich man who hastens to plough and plant and put his house in good order; and neighbor vies with his neighbor as he hurries after wealth. This Strife is wholesome for men. And potter is angry with potter, and craftsman with craftsman, and beggar is jealous of beggar, and minstrel of minstrel.
(ll. 25-41) Perses, lay up these things in your heart, and do not let that Strife who delights in mischief hold your heart back from work, while you peep and peer and listen to the wrangles of the court-house. Little concern has he with quarrels and courts who has not a year’s victuals laid up betimes, even that which the earth bears, Demeter’s grain. When you have got plenty of that, you can raise disputes and strive to get another’s goods. But you shall have no second chance to deal so again: nay, let us settle our dispute here with true judgment divided our inheritance, but you seized the greater share and carried it off, greatly swelling the glory of our bribe-swallowing lords who love to judge such a cause as this. Fools! They know not how much more the half is than the whole, nor what great advantage there is in mallow [marsh grass] and asphodel [lily].
The Source of Human Struggles
(ll. 42-53) For the gods keep hidden from men the means of life. Else you would easily do work enough in a day to supply you for a full year even without working; soon would you put away your rudder over the smoke, and the fields worked by ox and sturdy mule would run to waste. But Zeus in the anger of his heart hid it, because Prometheus the crafty deceived him; therefore he planned sorrow and mischief against men. He hid fire; but that the noble son of Iapetus stole again for men from Zeus the counselor in a hollow fennel-stalk, so that Zeus who delights in thunder did not see it.  But I will give men as the price for fire an evil thing in which they may all be glad of heart while they embrace their own destruction.’
(ll. 60-68) So said the father of men and gods, and laughed aloud. And he bade famous Hephaestus make haste and mix earth with water and to put in it the voice and strength of humankind, and fashion a sweet, lovely maiden-shape, like to the immortal goddesses in face; and Athene to teach her needlework and the weaving of the varied web; and golden Aphrodite to shed grace upon her head and cruel longing and cares that weary the limbs.  And they obeyed the lord Zeus, the son of Cronos. Forthwith the famous Lame God molded clay in the likeness of a modest maid, as the son of Cronos purposed. And the goddess bright-eyed Athene girded and clothed her, and the divine Graces and queenly Persuasion put necklaces of gold upon her, and the rich-haired Hours crowned her head with spring flowers. And Pallas Athene bedecked her form with all manners of finery. Also the Guide, the Slayer of Argus, contrived within her lies and crafty words and a deceitful nature at the will of loud thundering Zeus, and the Herald of the gods put speech in her. And he called this woman Pandora, because all they who dwelt on Olympus gave each a gift, a plague to men who eat bread.
(ll. 83-89) But when he had finished the sheer, hopeless snare, the Father sent glorious Argos-Slayer, the swift messenger of the gods, to take it to Epimetheus as a gift. And Epimetheus did not think on what Prometheus had said to him, bidding him never take a gift of Olympian Zeus, but to send it back for fear it might prove to be something harmful to men. But he took the gift, and afterwards, when the evil thing was already his, he understood.
(ll. 90-105) For ere this the tribes of men lived on earth remote and free from ills and hard toil and heavy sickness which bring the Fates upon men; for in misery men grow old quickly. But the woman took off the great lid of the jar with her hands and scattered all these and her thought caused sorrow and mischief to men. Only Hope remained there in an unbreakable home within under the rim of the great jar, and did not fly out at the door; for ere that, the lid of the jar stopped her, by the will of Aegis-holding Zeus who gathers the clouds. But the rest, countless plagues, wander amongst men; for earth is full of evils and the sea is full. Of themselves diseases come upon men continually by day and by night, bringing mischief to mortals silently; for wise Zeus took away speech from them. So is there no way to escape the will of Zeus.
(ll. 106-108) Or if you will, I will sum you up another tale well and skilfully—and do you lay it up in your heart,—how the gods and mortal men sprang from one source.
The Age of Gold
(ll. 109-120) First of all the deathless gods who dwell on Olympus made a golden race of mortal men who lived in the time of Cronos when he was reigning in heaven. And they lived like gods without sorrow of heart, remote and free from toil and grief: miserable age rested not on them; but with legs and arms never failing they made merry with feasting beyond the reach of all evils. When they died, it was as though they were overcome with sleep, and they had all good things; for the fruitful earth unforced bare them fruit abundantly and without stint. They dwelt in ease and peace upon their lands with many good things, rich in flocks and loved by the blessed gods.
The Age of Silver
(ll. 121-139) But after earth had covered this generation—they are called pure spirits dwelling on the earth, and are kindly, delivering from harm, and guardians of mortal men; for they roam everywhere over the earth, clothed in mist and keep watch on judgments and cruel deeds, givers of wealth; for this royal right also they received;—then they who dwell on Olympus made a second generation which was of silver and less noble by far. It was like the golden race neither in body nor in spirit.  But when they were full grown and were come to the full measure of their prime, they lived only a little time in sorrow because of their foolishness, for they could not keep from sinning and from wronging one another, nor would they serve the immortals, nor sacrifice on the holy altars of the blessed ones as it is right for men to do wherever they dwell.
The Age of Bronze
(ll. 140-155) But when earth had covered this generation also—they are called blessed spirits of the underworld by men, and, though they are of second order, yet honor attends them also—Zeus the Father made a third generation of mortal men, a brazen race, sprung from ash-trees; and it was in no way equal to the silver age, but was terrible and strong. They loved the lamentable works of Ares and deeds of violence; they ate no bread, but were hard of heart like adamant, fearful men. Great was their strength and unconquerable the arms which grew from their shoulders on their strong limbs. Their armor was of bronze, and their houses of bronze, and of bronze were their implements: there was no black iron. These were destroyed by their own hands and passed to the dank house of chill Hades, and left no name: terrible though they were, black Death seized them, and they left the bright light of the sun.
The Age of Heroes
(ll. 156-169b) But when earth had covered this generation also, Zeus the son of Cronos made yet another, the fourth, upon the fruitful earth, which was nobler and more righteous, a god-like race of hero-men who are called demi-gods, the race before our own, throughout the boundless earth. Grim war and dread battle destroyed a part of them, some in the land of Cadmus at seven-gated Thebe when they fought for the flocks of Oedipus, and some, when it had brought them in ships over the great sea gulf to Troy for rich-haired Helen’s sake: there death’s end enshrouded a part of them. But to the others father Zeus the son of Cronos gave a living and an abode apart from men, and made them dwell at the ends of earth. And they live untouched by sorrow in the islands of the blessed along the shore of deep swirling Ocean, happy heroes for whom the grain-giving earth bears honey-sweet fruit flourishing thrice a year, far from the deathless gods, and Cronos rules over them; for the father of men and gods released him from his bonds. And these last equally have honor and glory.
The Age of Iron
(ll. 169c-169d) And again far-seeing Zeus made yet another generation, the fifth, of men who are upon the bounteous earth.
(ll. 170-201) Thereafter, would that I were not among the men of the fifth generation, but either had died before or been born afterwards. For now truly is a race of iron, and men never rest from labor and sorrow by day, and from perishing by night; and the gods shall lay sore trouble upon them. But, notwithstanding, even these shall have some good mingled with their evils. And Zeus will destroy this race of mortal men also when they come to have gray hair on the temples at their birth. The father will not agree with his children, nor the children with their father, nor guest with his host, nor comrade with comrade; nor will brother be dear to brother as aforetime. Men will dishonor their parents as they grow quickly old, and will carp at them, chiding them with bitter words, hard-hearted they, not knowing the fear of the gods. They will not repay their aged parents the cost their nurture, for might shall be their right: and one man will sack another’s city. There will be no favor for the man who keeps his oath or for the just or for the good; but rather men will praise the evil-doer and his violent dealing. Strength will be right and reverence will cease to be; and the wicked will hurt the worthy man, speaking false words against him, and will swear an oath upon them. Envy, foul-mouthed, delighting in evil, with scowling face, will go along with wretched men one and all. And then Aidos [goddess of shame and modesty] and Nemesis [goddess of retribution], with their sweet forms wrapped in white robes, will go from the wide-pathed earth and forsake mankind to join the company of the deathless gods: and bitter sorrows will be left for mortal men, and there will be no help against evil.
A Fable for Princes
(ll. 202-211) And now I will tell a fable for princes who themselves understand. Thus said the hawk to the nightingale with speckled neck, while he carried her high up among the clouds, gripped fast in his talons, and she, pierced by his crooked talons, cried pitifully. To her he spoke disdainfully: ‘Miserable thing, why do you cry out? One far stronger than you now holds you fast, and you must go wherever I take you, songstress as you are. And if I please I will make my meal of you, or let you go. He is a fool who tries to withstand the stronger, for he does not get the mastery and suffers pain besides his shame.’ So said the swiftly flying hawk, the long-winged bird.
(ll. 212-224) But you, Perses, listen to right and do not foster violence; for violence is bad for a poor man. Even the prosperous cannot easily bear its burden, but is weighed down under it when he has fallen into delusion. The better path is to go by on the other side towards justice; for Justice beats Outrage when she comes at length to the end of the race. But only when he has suffered does the fool learn this. For Oath keeps pace with wrong judgments. There is a noise when Justice is being dragged in the way where those who devour bribes and give sentence with crooked judgments, take her. And she, wrapped in mist, follows to the city and haunts of the people, weeping, and bringing mischief to men, even to such as have driven her forth in that they did not deal straightly with her.
(ll. 225-237) But they who give straight judgments to strangers and to the men of the land, and go not aside from what is just, their city flourishes, and the people prosper in it: Peace, the nurse of children, is abroad in their land, and all-seeing Zeus never decrees cruel war against them. Neither famine nor disaster ever haunt men who do true justice; but light-heartedly they tend the fields which are all their care. The earth bears them victual in plenty, and on the mountains the oak bears acorns upon the top and bees in the midst. Their wooly sheep are laden with fleeces; their women bear children like their parents. They flourish continually with good things, and do not travel on ships, for the grain-giving earth bears them fruit.
(ll. 238-247) But for those who practice violence and cruel deeds far-seeing Zeus, the son of Cronos, ordains a punishment. Often even a whole city suffers for a bad man who sins and devises presumptuous deeds, and the son of Cronos lays great trouble upon the people, famine and plague together, so that the men perish away, and their women do not bear children, and their houses become few, through the contriving of Olympian Zeus. And again, at another time, the son of Cronos either destroys their wide army, or their walls, or else makes an end of their ships on the sea.
(ll. 248-264) You princes, mark well this punishment you also; for the deathless gods are near among men and mark all those who oppress their fellows with crooked judgments, and reck not the anger of the gods. For upon the bounteous earth Zeus has thrice ten thousand spirits, watchers of mortal men, and these keep watch on judgments and deeds of wrong as they roam, clothed in mist, all over the earth. And there is virgin Justice, the daughter of Zeus, who is honored and reverenced among the gods who dwell on Olympus, and whenever anyone hurts her with lying slander, she sits beside her father, Zeus the son of Cronos, and tells him of men’s wicked heart, until the people pay for the mad folly of their princes who, evilly minded, pervert judgment and give sentence crookedly. Keep watch against this, you princes, and make straight your judgments, you who devour bribes; put crooked judgments altogether from your thoughts.


1.5: Anglo-Saxon Riddles
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- Bonnie J. Robinson & Laura Getty
- University of North Georgia via University of North Georgia Press
Dates unknown (probably seventh to eighth century)
Collections of riddles in Latin date back to the fourth century. Most of the authors are unknown, although the names of a few Anglo-Saxon authors survive: Aldhelm, Tatwine, and Eusebius. The first two were born in the seventh century and died in the eighth century, and the third may have been eighth century. The riddles written in Latin usually had the answer as the title, and they were considered enigmas: logic games that could be quite sophisticated and detailed. The most famous collection of riddles written in Anglo-Saxon are found in the Exeter Book; these riddles, in contrast to the Latin ones, often ask the reader to guess the answer, and no answer is provided. While the topics of the riddles range from animals and natural phenomena to weapons and writing, there are some riddles that contain double meanings (one of them obscene) that demonstrate an earthy sense of humor. Riddles appear to have occupied an important place in Anglo-Saxon culture and beyond. The concept of riddles as a standard game can be found in J.R.R. Tolkien’s The Hobbit (which has numerous Anglo-Saxon borrowings), with the riddle competition between Bilbo Baggins and Gollum.
1.10.1 Selections from Old English Poems
I. A Storm
What man is so clever, so crafty of mind,
As to say for a truth who sends me a-traveling?
When I rise in my wrath, raging at times,
Savage is my sound. Sometimes I travel,
Go forth among the folk, set fire to their homes
And ravage and rob them; then rolls the smoke
Gray over the gables; great is the noise,
The death-struggle of the stricken. Then I stir up the woods
And the fruitful forests; I fell the trees,
I, roofed over with rain, on my reckless journey,
Wandering widely at the will of heaven.
I bear on my back the bodily raiment,
The fortunes of folk, their flesh and their spirits,
Together to sea. Say who may cover me,
Or what I am called, who carry this burden?
II. A Storm
At times I travel in tracks undreamed of,
In vasty wave-depths to visit the earth,
The floor of the ocean. Fierce is the sea
. . . . . . . the foam rolls high;
The whale-pool roars and rages loudly;
The streams beat the shores, and they sling at times
Great stones and sand on the steep cliffs,
With weeds and waves, while wildly striving
Under the burden of billows on the bottom of ocean
The sea-ground I shake. My shield of waters
I leave not ere he lets me who leads me always
In all my travels. Tell me, wise man,
Who was it that drew me from the depth of the ocean
When the streams again became still and quiet,
Who before had forced me in fury to rage?
III. A Storm
At times I am fast confined by my Master,
Who sendeth forth under the fertile plain
My broad bosom, but bridles me in.
He drives in the dark a dangerous power
To a narrow cave, where crushing my back
Sits the weight of the world. No way of escape
Can I find from the torment; so I tumble about
The homes of heroes. The halls with their gables,
The tribe-dwellings tremble; the trusty walls shake,
Steep over the head. Still seems the air
Over all the country and calm the waters,
Till I press in my fury from my prison below,
Obeying His bidding who bound me fast
In fetters at first when he fashioned the world,
In bonds and in chains, with no chance of escape
From his power who points out the paths I must follow.
Downward at times I drive the waves,
Stir up the streams; to the strand I press
The flint-gray flood: the foamy wave Lashes the wall. A lurid mountain
Rises on the deep; dark in its trail
Stirred up with the sea a second one comes,
And close to the coast it clashes and strikes
On the lofty hills. Loud soundeth the boat,
The shouting of shipmen. Unshaken abide
The stone cliffs steep through the strife of the waters,
The dashing of waves, when the deadly tumult
Crowds to the coast. Of cruel strife
The sailors are certain if the sea drive their craft
With its terrified guests on the grim rolling tide;
They are sure that the ship will be shorn of its power,
Be deprived of its rule, and will ride foam-covered
On the ridge of the waves. Then ariseth a panic,
Fear among folk of the force that commands me,
Strong on my storm-track. Who shall still that power?
 It is the greatest of noises,
Of clamoring crowds, of crashes the loudest,
When clouds as they strive in their courses shall strike
Edge against edge; inky of hue
In flight o’er the folk bright fire they sweat,
A stream of flame; destruction they carry
Dark over men with a mighty din.
Fighting they fare.  In battle they strive,
The awful army; anguish arises,
Terror of mind to the tribes of men,
Distress in the strongholds, when the stalking goblins,
The pale ghosts shoot with their sharp weapons.
The fool alone fears not their fatal spears;
But he perishes too if the true God send
Straight from above in streams of rain,
Whizzing and whistling the whirlwind’s arrows,
The flying death. Few shall survive
Whom that violent guest in his grimness shall visit.
I always stir up that strife and commotion;
Then I bear my course to the battle of clouds,
Powerfully strive and press through the tumult,
Over the bosom of the billows; bursteth loudly
The gathering of elements. Then again I descend
In my helmet of air and hover near the land,
And lift on my back the load I must bear,
Minding the mandates of the mighty Lord.
So I, a tried servant, sometimes contend:
Now under the earth; now from over the waves
I drive to the depths; now dropping from heaven,
I stir up the streams, or strive to the skies,
Where I war with the welkin. Wide do I travel,
Swift and noisily. Say now my name,
Or who raises me up when rest is denied me,
Or who stays my course when stillness comes to me?
V. A Shield
A lonely warrior, I am wounded with iron,
Scarred with sword-points, sated with battle-play,
Weary of weapons. I have witnessed much fighting,
Much stubborn strife. From the strokes of war
I have no hope for help or release
Ere I pass from the world with the proud warrior band.
With brands and billies they beat upon me;
The hard edges hack me; the handwork of smiths
In crowds I encounter; with courage I endure
Ever bitterer battles. No balm may I find,
And no doctor to heal me in the whole field of battle,
To bind me with ointments and bring me to health,
But my grievous gashes grow ever sorer
Through death-dealing strokes by day and night.
VII.
At times I am lifted o’er the lodgings of men
By the aid of my trappings and the air above.
The strength of the clouds then carries me far,
Bears me on its bosom. My beautiful ornament,
My raiment rustles and raises a song,
Sings without tiring. I touch not the earth
But wander a stranger over stream and wood.
VIII.
An old evening-scop, to earls I bring
Solace in cities; when, skillful in music,
My voice I raise, restful at home
They sit in silence. Say what is my name,
That call so clearly and cleverly imitate
The song of the scop, and sing unto men
Words full welcome with my wonderful voice.
XIV. A Horn
I was once an armed warrior. Now the worthy youth
Gorgeously gears me with gold and silver,
Curiously twisted. At times men kiss me.
Sometimes I sound and summon to battle
The stalwart company. A steed now carries me
Across the border. The courser of the sea
Now bears me o’er the billows, bright in my trappings.
Now a comely maiden covered with jewels
Fills my bosom with beer. On the board now I lie
Lidless and lonely and lacking my trappings.
Now fair in my fretwork at the feast I hang
In my place on the wall while warriors drink.
Now brightened for battle, on the back of a steed
A war-chief shall bear me. Then the wind I shall breathe,
Shall swell with sound from someone’s bosom.
At times with my voice I invite the heroes,
The warriors to wine; or I watch for my master,
And sound an alarm and save his goods,
Put the robber to flight. Now find out my name.
XV. A Badger
My throat is like snow, and my sides and my head
Are a swarthy brown; I am swift in flight.
Battle-weapons I bear; on my back stand hairs,
And also on my cheeks. O’er my eyes on high
Two ears tower; with my toes I step
On the green grass. Grief comes upon me
If the slaughter-grim hunter shall see me in hiding,
Shall find me alone where I fashion my dwelling,
Bold with my brood. I abide in this place
With my strong young children till a stranger shall come
And bring dread to my door. Death then is certain.
Hence, trembling I carry my terrified children
Far from their home and flee unto safety.
If he crowds me close as he comes behind,
I bare my breast. In my burrow I dare not
Meet my furious foe (it were foolish to do so),
But, wildly rushing, I work a road
Through the high hill with my hands and feet.
I fail not in defending my family’s lives;
If I lead the little ones below to safety,
Through a secret hole inside the hill,
My beloved brood, no longer need I
Fear the offense of the fierce-battling dogs.
Whenever the hostile one hunts on my trail,
Follows me close, he will fail not of conflict,
Of a warm encounter, when he comes on my war-path,
If I reach, in my rage, through the roof of my hill
And deal my deadly darts of battle
On the foe I have feared and fled from long.
XXIII. A Bow
My name is spelled AGOB with the order reversed.
I am marvelously fashioned and made for fighting.
When I am bent and my bosom sends forth
Its poisoned stings, I straightway prepare
My deadly darts to deal afar.

Not any man shall make his escape,
Not one that I spoke of shall speed from the fight,
If there falls on him first what flies from my belly.
He pays with his strength for the poisonous drink,
For the fatal cup which forfeits his life.
Except when fettered fast, I am useless.
Unbound I shall fail. Now find out my name.
XXVI.  Then the hard edge
Of the keen knife cut me and cleansed me of soil;
Then fingers folded me. The fleet quill of the bird
With speedy drops spread tracks often
Over the brown surface, swallowed the tree-dye,
A deal of the stream, stepped again on me,
Traveled a black track. With protecting boards
Then a crafty one covered me, enclosed me with hide,
Made me gorgeous with gold. Hence I am glad and rejoice
At the smith’s fair work with its wondrous adornments.
Now may these rich trappings, and the red dye’s tracings,
And all works of wisdom spread wide the fame
Of the Sovereign of nations! Read me not as a penance!
If the children of men will cherish and use me,
They shall be safer and sounder and surer of victory,
More heroic of heart and happier in spirit,
More unfailing in wisdom. More friends shall they have,
Dear and trusty, and true and good,
And faithful always, whose honors and riches
Shall increase with their love, and who cover their friends
With kindness and favors and clasp them fast
With loving arms. I ask how men call me
Who aid them in need. My name is far famed.
I am helpful to men, and am holy myself.
XLV. Dough
In a corner I heard a curious weak thing
Swelling and sounding and stirring its cover.
On that boneless body a beautiful woman
Laid hold with her hands; the high-swelled thing
She covered with a cloth, the clever lord’s daughter.
XLVII. A Bookworm
A moth ate a word. To me that seemed
A curious happening when I heard of that wonder,
That a worm should swallow the word of a man,
A thief in the dark eat a thoughtful discourse
And the strong base it stood on. He stole, but he was not
A whit the wiser when the word had been swallowed.
LX. A Reed
I stood on the strand to the sea-cliffs near,
Hard by the billows. To the home of my birth
Fast was I fixed. Few indeed are there
Of men who have ever at any time
Beheld my home in the hard waste-land.
In the brown embrace of the billows and waves
I was locked each dawn. Little I dreamed
That early or late I ever should
With men at the mead-feast mouthless speak forth
Words of wisdom. It is a wondrous thing,
And strange to the sight when one sees it first
That the edge of a knife and the active hand
And wit of the earl who wields the blade
Should bring it about that I bear unto thee
A secret message, meant for thee only,
Boldly announce it, so that no other man
May speak our secrets or spread them abroad.
1.10.2 Reading and Review Questions
- If there were no answers in the titles, which riddles would be the most difficult to guess, and why?
- What do we learn about Anglo-Saxon culture from the riddles? What details and ideas do the riddles expect the audience to know?
- Why do you think that the answers for the Anglo-Latin riddles are in the titles? What does that tell us about the intended audience?
- Which of the riddles has a double meaning? Compare it to another of the “obscene” Anglo-Saxon riddles (easily found online). What seems to be to most common theme?
- Look at the riddles used during the riddle game in Tolkien’s The Hobbit. How do they compare to the riddles found here?


19.1B: Population Genetics
- Define a population gene pool and explain how the size of the gene pool can affect the evolutionary success of a population
Population Genetics
A gene for a particular characteristic may have several variations called alleles. These variations code for different traits associated with that characteristic. For example, in the ABO blood type system in humans, three alleles (I A , I B , or i) determine the particular blood-type protein on the surface of red blood cells. A human with a type I A allele will display A-type proteins (antigens) on the surface of their red blood cells. Individuals with the phenotype of type A blood have the genotype I A I A or I A i, type B have I B I B or I B i, type AB have I A I B , and type O have ii.
A diploid organism can only carry two alleles for a particular gene. In human blood type, the combinations are composed of two alleles such as I A I A or I A I B . Although each organism can only carry two alleles, more than those two alleles may be present in the larger population. For example, in a population of fifty people where all the blood types are represented, there may be more I A alleles than i alleles. Population genetics is the study of how selective forces change a population through changes in allele and genotypic frequencies.
Allele Frequency
The allele frequency (or gene frequency) is the rate at which a specific allele appears within a population. In population genetics, the term evolution is defined as a change in the frequency of an allele in a population. Frequencies range from 0, present in no individuals, to 1, present in all individuals. The gene pool is the sum of all the alleles at all genes in a population.
Using the ABO blood type system as an example, the frequency of one of the alleles, for example I A , is the number of copies of that allele divided by all the copies of the ABO gene in the population, i.e. all the alleles. Allele frequencies can be expressed as a decimal or as a percent and always add up to 1, or 100 percent, of the total population. For example, in a sample population of humans, the frequency of the I A allele might be 0.26, which would mean that 26% of the chromosomes in that population carry the I A allele. If we also know that the frequency of the I B allele in this population is 0.14, then the frequency of the i allele is 0.6, which we obtain by subtracting all the known allele frequencies from 1 (thus: 1 – 0.26 – 0.14 = 0.6). A change in any of these allele frequencies over time would constitute evolution in the population.
Population Size and Evolution
When allele frequencies within a population change randomly with no advantage to the population over existing allele frequencies, the phenomenon is called genetic drift. The smaller a population, the more susceptible it is to mechanisms such as genetic drift as alleles are more likely to become fixed at 0 (absent) or 1 (universally present). Random events that alter allele frequencies will have a much larger effect when the gene pool is small. Genetic drift and natural selection usually occur simultaneously in populations, but the cause of the frequency change is often impossible to determine.
Natural selection also affects allele frequency. If an allele confers a phenotype that enables an individual to better survive or have more offspring, the frequency of that allele will increase. Because many of those offspring will also carry the beneficial allele and, therefore, the phenotype, they will have more offspring of their own that also carry the allele. Over time, the allele will spread throughout the population and may become fixed: every individual in the population carries the allele. If an allele is dominant but detrimental, it may be swiftly eliminated from the gene pool when the individual with the allele does not reproduce. However, a detrimental recessive allele can linger for generations in a population, hidden by the dominant allele in heterozygotes. In such cases, the only individuals to be eliminated from the population are those unlucky enough to inherit two copies of such an allele.
The Founder Effect
The founder effect occurs when part of a population becomes isolated and establishes a separate gene pool with its own allele frequencies. When a small number of individuals become the basis of a new population, this new population can be very different genetically from the original population if the founders are not representative of the original. Therefore, many different populations, with very different and uniform gene pools, can all originate from the same, larger population. Together, the forces of natural selection, genetic drift, and founder effect can lead to significant changes in the gene pool of a population.
Key Points
- A gene pool is the sum of all the alleles (variants of a gene) in a population.
- Allele frequencies range from 0 (present in no individuals) to 1 (present in all individuals); all allele frequencies for a given gene add up to 100 percent in a population.
- The smaller a population, the more susceptible it is to mechanisms like natural selection and genetic drift, as the effects of such mechanisms are magnified when the gene pool is small.
- The founder effect occurs when part of an original population establishes a new population with a separate gene pool, leading to less genetic variation in the new population.
Key Terms
- allele : one of a number of alternative forms of the same gene occupying a given position on a chromosome
- gene pool : the complete set of unique alleles that would be found by inspecting the genetic material of every living member of a species or population
- founder effect : a decrease in genetic variation that occurs when an entire population descends from a small number of founders


8.1: Drama as a Genre
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Like fiction, drama features characters caught up in a plot. In fact,
some plays have been based on novels, and novels on plays. Yet, whereas
the narrator of a novel can spend pages painting a picture of the story’s
circumstances for the reader, a play is restricted to the space of the stage
and the time frame of a couple of hours. What strategies are available to
the playwright to ensure that the play successfully conveys its intended
effects and themes?
To provide the story’s setting, a play requires sets. If you’ve ever been
involved with a play, you know that the set can be made up of detailed
backdrops, specifically designed props, strategic lighting, and sometimes
even background noise. A set, along with the characters’ subtle indications
of the scene, can generate a full setting in the audience’s imagination.
Another difference between fiction and drama is that usually a play’s
plot is primarily forwarded through dialogue and action. Although a novel’s
narrator can describe in detail the thoughts and impressions of its characters,
a play’s effects depend much more heavily on what the characters say and
do. A play is a performance, a spectacle, rather than words on paper. Some
plays do include a narrator or a chorus, to introduce the scene or set the tone
of the play, but the bulk of the production’s effect is generated through the
dialogue and its visual devices, and since the play’s script dictates what
the characters will say and often, through stage direction, its production
strategies as well, the script is crucial to a successful performance.


14.5: Activities
Suppose \(G = (V,E)\) is a graph. Decide the truth of the following statement.
Every pair of a subset \(V' \subseteq V\) and a subcollection \(E' \subseteq E\) defines a subgraph \(G' = (V',E')\) of \(G\text{.}\)
Draw a graph where the nodes are students present in today's class. Draw edges between pairs of students that are in the same group today. Additionally, draw an edge between a member of your group and another student if that pair was in a group together last class.
For each of the following graphs, write out its formal definition as either a (regular) graph, a weigthed graph, or a directed graph, as appropriate.
Consider the website Facebook as a graph where vertices are profiles and edges represent “friendship”.
- Should this graph be a directed graph? Why or why not?
- Is this graph simple? complete? Justify your answers.
- What does the degree of a vertex represent?
- Could this graph have isolated vertices?
- Suppose the following graph is a subgraph of the Facebook graph.
- What is the largest party one of these people could throw where each party-goer is Facebook friends with every other party-goer? Justify your answer.
- Assume all of the people in this graph live in the same geographic area. Which pair of non-friends are most likely to become friends in the future? Which pair of non-friends are least likely to become friends in the future? Justify your answers.


4.1: Introduction
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This chapter applies evolutionary concepts to the understanding of human origins and explains the biological variation seen in our ancestors across time. Chapters 4, Biological Evolution and Early Human Evidence and Chapter 5, The Genus Homo and the Emergence of Us, represent a field of study that is probably the most dynamic, controversial and highly debated subfield of anthropology. Perspectives and opinions vary not only within the mindset of the general public but also amongst scientists and anthropologists alike. As the human fossil puzzle begins to fill in with new discoveries, we find ourselves gaining valuable insights into what makes us human and the ways in which we are a part of, not separate from, the natural world. Despite our advances in the field, we also have to be prepared for the possibility we may end up with more questions than answers! It is these very reasons that explain why so many of us find this such a fascinating field and why so many of us take it so personally. It is after all a journey into the discovery of who we are and where we came from; and that should be of interest to all of us as members of the Homo genus.


4.4: The Function of Digestive Organs
By the end of this section, you should be able to:
- 4.3.1 Describe the function of digestive organs in ingestion, digestion, absorption, and elimination.
- 4.3.2 Describe the pathophysiology associated with digestive organs.
- 4.3.3 Identify the impact of aging on digestive organs.
Normal Function of the Digestive Organs
Before food ever enters the body, the sensory system starts the digestive process . The sight, smell, feel, and taste of food influences the desire to eat. These senses trigger the salivary glands to begin releasing saliva in the anticipation of eating, which helps break down food once it enters the oral cavity. When food enters the mouth, chewing, also known as mastication , starts the breakdown of food. The tongue secretes mucins and lingual lipase to help break down lipids. At the same time, the parotid, sublingual, and submandibular salivary glands release a mixture of buffers, glycoproteins known as mucins , and salivary amylase to lubricate the food for swallowing and to begin the digestion of complex carbohydrates and starches (National Institute of Diabetes and Digestive and Kidney Diseases, 2022; Ogobuiro, Gonzales, & Tuma, 2022).
Food then passes to the pharynx. To swallow food into the esophagus and not the trachea, the swallowing center in the brain stem of the central nervous system triggers certain actions:
- The nasopharynx is closed off by the rise of the soft palate when the tongue initiates swallowing by pushing food to the back of the mouth.
- The larynx is closed in the pharynx to prevent food from entering the trachea (Mealie, Ali, & Manthey, 2022; Ogobuiro, Gonzales, & Tuma, 2022).
- Once this swallowing reflex is initiated, it cannot be stopped until completed.
Once food is swallowed, it enters the esophagus by passing through the upper esophageal sphincter. The food travels down the esophagus body involuntarily by way of a series of muscular contractions known as peristalsis . It then exits the esophagus through the lower esophageal sphincter into the stomach (National Institute of Diabetes and Digestive and Kidney Diseases, 2022).
While in the stomach, food is temporarily stored while mechanical and chemical processes break it down further into chyme . The stomach uses its three muscular layers (the inner oblique, middle circular, and external longitudinal layers) to contract and relax in a mixing and churning action to help mechanically break down food. To assist with the chemical breakdown of food, the stomach releases hydrochloric acid secreted by its parietal cells, enzymes such as pepsin , and hormones such as somatostatin and gastrin (Ogobuiro, Gonzales, & Tuma, 2022). The stomach is also responsible for secreting an intrinsic factor (a glycoprotein that is necessary for the absorption of vitamin B 12 ) from the parietal cells.
The food, in the form of chyme, exits the stomach through the pyloric sphincter into the first of the three segments of the small intestine , the duodenum. While in the duodenum , the pancreas (by way of the pancreatic duct) and liver (by way of the gallbladder and biliary tree) empty enzymes into the duodenum through the sphincter of Oddi . The liver secretes bile that is stored and concentrated in the gallbladder before it is released to aid in digestion (National Institute of Diabetes and Digestive and Kidney Diseases, 2022; Ogobuiro, Gonzales, & Tuma, 2022). The pancreas’s exocrine glands generate various enzyme precursors and enzymes that play a crucial role in the process of digestion (Ogobuiro, Gonzales, & Tuma, 2022).
The food continues to the second segment of the small intestine, the jejunum, by way of peristalsis. This is where the bulk of chemical digestion and nutrient absorption will occur before it moves into the last segment, the ileum, which has some digestive and absorptive duties as well (Ogobuiro, Gonzales, & Tuma, 2022). Absorption in the small intestine is done through nutrient exchange in extensive capillary networks that are located on the walls of the small intestine. The surface area of the small intestine is increased to promote increased absorption through the use of villi and microvilli that cover the walls and are home to the extensive capillary networks. There are also multiple exocrine and endocrine glands that assist in the absorptive process by producing and releasing hormones, alkaline mucinous materials, and enzymes.
Undigested food materials that pass through to the end of the small intestine through the ileocecal valve end up as feces in the cecum, which is the first of the five segments of the large intestine (Ogobuiro, Gonzales, & Tuma, 2022). While peristalsis moves the food through the large intestine, water and electrolytes are absorbed from the feces. Final digestion of undigested food particles in the feces is completed by microbes that live in the large intestine. Nutrients like vitamin K are produced and absorbed in the large intestine as well (Cleveland Clinic, 2022a; Ogobuiro, Gonzales, & Tuma, 2022). The feces travel through the cecum, ascending colon, transverse colon, descending colon, and sigmoid colon before moving into the rectum , where stretch receptors signal the brain to defecate.
From the rectum, the stool exits the body through the anal canal by passing through the internal and then external anal sphincters.
Pathophysiology Associated with Digestive Organs and Function
Many causes of diseases and disorders that impact the gastrointestinal system are linked with nutrition, as are many of their treatments. Table 4.2 lists several organs impacted by nutritional deficit and the corresponding disease(s).
| Organ | Nutritional Link | Disease or Disorder |
|---|---|---|
| Salivary gland | Deficiency of vitamin B 12 (cobalamin) | Salivary gland tumors |
| Gums | Deficiency of vitamin B 12 | Gingivitis |
| Mouth | Deficiency of vitamin B 12 | Periodontal disease / cancers |
| Lips | Maternal deficiency of vitamin B 12 | Cleft lip |
| Oral palate | Maternal deficiency of vitamin B 12 | Cleft palate |
| Esophagus | Food allergies | Eosinophilic esophagitis |
| Stomach | Deficiency of vitamin B 12 /lack of intrinsic factor | Pernicious anemia |
| Intestine | Low fiber intake , low water intake , and high dairy intake | Constipation |
| Anus /rectum | Low fiber intake, low water intake, and high dairy intake | Hemorrhoids , fissures , and fistulas |
| Gastrointestinal system | Low fiber intake, low water intake, high fat intake , and high cholesterol intake | Cancers |
| Intestine | Gluten | Celiac disease |
| Stomach/esophagus | Worsened by food that contains caffeine , spicy foods , acidic foods , and dairy products | Gastroesophageal reflux disease (GERD) |
Aging of the Digestive System
The nurse should be aware of considerations for aging clients in connection with the digestive system and the role of nutrition. Older adults tend to have a decrease in appetite over younger adults due to comorbidities and/or medications that decrease the desire to eat, cause nausea, or create dietary food restrictions due to drug interactions. Older adults may experience the following issues:
- Appetite can decrease due to a loss in smell or taste or changes in the release of the hormones that control appetite (e.g., ghrelin, peptide tyrosine, tyrosine, cholecystokinin, insulin, and leptin) (Nigam & Knight, 2017).
- Food may appear less attractive or appetizing due to changes made to food consistency for safety reasons (Bartel, 2022; Nigam & Knight, 2017).
- The feeling of being full may happen faster or last longer due to the stomach wall losing elasticity, resulting in a loss of hydrochloric acid secretion into the stomach, making food accommodation more difficult and gastric emptying slower.
- Esophageal reflux may occur, and food may backflow more easily into the esophagus (Austin Gastroenterology, 2019).
- Hyposecretion of hydrochloric acid and protective mucus that occurs with age can result in chronic atrophic gastritis , increased peptic ulcer disease , and decreased vitamin B 12 absorption.
- An increase in the bacterial populations in the small intestine may lead to increased bloating and abdominal pain as well as decreased absorption of calcium , folic acid , and iron (Bartel, 2022; Nigam & Knight, 2017).
- Decreases in digestive enzymes from the pancreas result in a reduction in the digestion of proteins and fats.
- The liver decreases bile production, rate of protein synthesis and metabolism , and its ability to detoxify substances (Nigam & Knight, 2017).
- Risk for constipation increases because in the large intestine, there are reductions in neurotransmitters and neuroreceptors, atrophies of mucosa and muscle layers, formations of diverticula, declines in the rate of cell division in the digestive epithelium, and increases in inflammatory conditions due to the increase in facultative anaerobes. The increase in constipation increases the risk for hemorrhoid development.
Biological changes due to age make it important for an older client to be educated so that they eat nutrient-dense options for adequate nutritional intake. With a lower caloric intake demand due to a slower metabolic rate from aging, older adults ’ need for calories will likely decrease, so the nutritional value of the foods they ingest is important.


23.2: The V7 and Ger+6 as Pivot Chords
23.2 The V7 and Ger+6 as Pivot Chords
In this first type of enharmonic modulation, the pivot to the new key will consist of the VV7 being enharmonically reinterpreted as a GerGer+6, or the GerGer+6 being enharmonically reinterpreted as VV7. Secondary dominants, like VIVV7/IV, VVV7/V, etc., will also be reinterpreted harmonically as GerGer+6 chords (and vice versa) in enharmonic modulations.
As we saw in the previous section, the dominant seventh chord and the GerGer+6chord have the same sound but are spelled differently. This involves a fairly straightforward enharmonic respelling. Most commonly, the top note of the chord is enharmonically respelled to envision the enharmonic alternative.
Each chord above implies a key based on its spelling: the dominant seventh chord occurs diatonically only on the 5^ scale degree in major, while the GerGer+6chord most commonly occurs with the lowest note on the lowered 6^ scale degree.
Less commonly, the bottom three notes could be respelled and the uppermost note could be retained as a common tone.
Remember, you will not see this respelling in the music you are analyzing. You must be able to visualize the enharmonic respelling in order to analyze the enharmonic modulation correctly.
The examples below illustrate some of the ways this enharmonic modulation occurs in pieces from the literature
In the first example from Tchaikovsky’s Nutcracker ballet, a DD7 is spelled on the staff as D–F♯–A–B♯ so that it sounds like a VIVV7/IV in D major but is spelled and resolves as a GerGer+6 in the key of F♯ minor.
In the following example from the second movement of Beethoven’s Fifth Symphony, the same pivot chords are used as in the example above—VIVV7/IV in A♭ major is enharmonically respelled and resolves as a GerGer+6 in C major.
In the following example from Les Miserables, a GerGer+6 in E minor is spelled as a VV7chord in F major and resolves to the I chord F major.
In the next section we will examine how the fully diminished seventh chord can be enharmonically reinterpreted.