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Cellular Reproduction In Eukaryotic Cells OBJECTIVE: By the end of the exercise you should be able to: 1
* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project
Cellular Reproduction In Eukaryotic Cells OBJECTIVE: By the end of the exercise you should be able to: 1. Describe the events associated with the cell cycle. 2. Describe the events associated with mitosis. 3. Distinguish the phases of mitosis on prepared slides of mitotic cells. 4. Outline the events of each stage of meiosis. 5. Compare and contrast meiosis with mitosis. 6. Discuss the relevance of meiosis to sexual reproduction. 7. Discuss the relationship of meiosis and gametogenesis. 8. Describe the events of spermatogenesis and oogenesis. INTRODUCTION: The remarkable diversity of form and function that eukaryotic cells assume is even more remarkable when you consider that a multicellular organism begins life as a single cell, the zygote. The egg and sperm (the gametes), though usually unequal in size, give an equal number of chromosomes to the zygote: each contributes the haploid number of chromosomes. The developing organism, therefore, contains a diploid number of chromosomes - a haploid set from each parent. For example, the diploid complement of a human zygote (and subsequently all other body cells) is 46 chromosomes; the haploid complement of each gamete is 23 chromosomes. After fertilization, the zygote gives rise to all the cells that make up the organism by repeated cell divisions, called mitosis. In multicellular organisms, mitosis permits growth and repair of tissues. In eukaryotic unicellular organisms, mitosis is a form of asexual reproduction. During the formation of the eggs and sperm, the chromosome number is reduced from diploid to haploid by a type of cell division called meiosis. We will examine this form of cell division later in this laboratory exercise. A. THE CELL CYCLE: The complex series of events that encompasses the life span of an actively dividing cell is termed the cell cycle, which includes an interphase during which copy of the DNA) occurs, as well as the synthesis of RNA and proteins. Note that replication of DNA occurs only during a period of interphase called the S phase (S for "synthesis"). The doubling of DNA during the S phase provides a full complement of DNA for the 2 daughter cells that will result from the ensuing mitotic division. During interphase, there are also 2 phases called G1 and G2. (G for "gap"). The G1 phase, which precedes DNA replication, includes a variety of growth processes and synthesis of compounds other than DNA. During the G2 phase, which immediately follows DNA replication, the molecules. and structures directly involved with mitosis are synthesized, assembled. The combination of G1, S, G2, and M phases make up the cell cycle (mitotic cycle). For the convenience of scientists, the process of mitosis is divided into 4 distinct stages: prophase, metaphase, anaphase, and telophase. The mitotic process usually occupies less than 10% of the total time taken by the cell cycle. Thus, mitosis is only part of the overall cell cycle. Mitosis is essentially the same in all organisms but, just as plant and animal cells differ to some extent structurally, there are some differences between them in this process. It is the objective of this part of the lab exercise to examine the essential steps in mitosis and to characterize the similarities and differences in this process between plant and animal cells. 1. MITOSIS IN PLANTS CELLS The onion (Allium) root tip is one of the most widely used materials for the study of mitosis because it is readily available, preparation of the dividing cells is easy, and the chromosomes are large and few in number. Root tips of plants contain meristems, which are localized areas of rapid cell division; therefore, chances are good that in a specimen of such tissues, one can find every stage of mitosis. Obtain a slide of onion root tips, and note a series of dark streaks on it. Each streak is a very thin longitudinal section through an onion root tip. Place the slide on the stage of your compound microscope, and locate 1 of the sections under low power ( I0X). It is often possible to determine whether a given section shows good examples of mitotic stages. Because each section is very thin, not all will be equally good for study. After your preliminary examination under low power, change to high power (40X or 43X), being careful not to break the slide. Keep in mind the sequence in which the stages occur, but do not try to find them in sequence. Thus, if you happen to find an anaphase first, study it before proceeding to another stage. Because cells remain in interphase and prophase longer than in the other stages, chances are that most of the cells will be in interphase; many will be in prophase; and only a few will be in the other stages. a. Interphase The interphase cell, so named because early biologists thought it was a resting phase, is actively undergoing cellular respiration and the synthesis of DNA, RNA, and protein. Interphase cells are characterized by a distinct nucleus bounded by a nuclear envelope. One or more nucleoli may be visible. b. Prophase During prophase, the DNA has condensed enough for the chromosomes to become distinguishable in the nucleus. The nuclear envelope begins to break down, and the chromosomes are then distribute