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Chapter15: The Eukaryotic Cell Cycle, Mitosis, and Meiosis

Page history last edited by Derek Weber 13 years, 5 months ago


Learning Objectives  

15.1 The Eukaryotic Cell Cycle
•  Describe the structure of eukaryotic chromosomes.
•  Distinguish between homologous chromosomes and sister chromatids.
•  Name the four phases in a eukaryotic cell's life cycle, and briefly describe the events occurring in each phase.
•  Describe the events that take place during interphase.
•  Explain the importance of checkpoints in the cell cycle.
•  Describe the functional relationship between cyclins and CDKs and the importance of these proteins in the regulation of the cell cycle.

 

15.2 Mitotic cell Division
•  Outline the steps involved in the mitotic phase, culminating in the production of two daughter cells.
•  Compare cytokinesis in plants and animals.
•  Explain why mitotic cell division results in two genetically identical daughter cells.

 

15.3 Meiosis and Sexual reproduction
•  Describe the events in prophase I that lead to the physical recombination of maternal and paternal genes.
•  Compare and contrast the events of metaphase I and anaphase I of meiosis with metaphase and anaphase of mitosis.
•  Explain what is meant by “reduction division.”
•  Compare and contrast the means by which gametes are formed by plants, animals, and fungi.

 

15.4 Variation in Chromosome Structure and Number
•  List several different types of alterations in normal chromosome number, and describe some medical conditions that occur as a result.

Chapter Summary

Eukaryotic cell division is more complicated than that of prokaryotic cells because the eukaryotic genome is larger and more complex. Eukaryotic chromosomes are linear structures composed of chromatin, mostly DNA and protein with a small amount of RNA. Eukaryotic DNA is a long double-stranded fiber. Every 200 nucleotides it coils around a core of eight histone polypeptides forming a nucleosome. The string of nucleosomes is further wrapped into supercoils. Heterochromatin is highly condensed chromatin while euchromatin is relatively uncondensed. Some portions of the DNA are permanently heterochromatic to prevent DNA expression; the remainder is uncondensed at the proper time to facilitate transcription.

 

The number of chromosomes in eukaryotic organisms varies widely from species to species. Human cells possess a diploid complement of 23 homologous pairs of chromosomes each with a characteristic appearance. Prior to cell division each homologue replicates producing two identical sister chromatids joined by a common centromere. The process of growth and division in a typical eukaryotic cell is called the cell cycle and is composed of five phases. The G1 phase is the cell’s primary growth phase while the genome is replicated during the S phase. During the G2 phase, various organelles are replicated, the chromosomes start to condense, and microtubules are synthesized. All of these are preparatory for mitosis or M phase. Actual cell division occurs in the final C phase, cytokinesis.

 

Cell cycle control is based on a check-point feedback system. When certain conditions at a checkpoint are met, the cell proceeds to the next stage of activity or division. Cyclin-dependent kinases (Cdk’s) and cyclins are intimately associated with these control processes. Unicellular organisms make independent decisions on whether or not to divide. Multicellular organisms must limit independent cell proliferation to maintain the integrity of the whole. Eukaryotes utilize various growth factors to do this. Disruption of these control mechanisms is characteristic of cancer.

 

Mitosis is a continuous process that is divided into four stages for ease of examination: prophase, metaphase, anaphase, and telophase. Much of the preparation for mitosis occurs during

interphase, a collective stage that includes G1, S, and G2. Preparations include chromosome replication, centriole replication (in animals only), and tubulin synthesis. Chromatin condensation begins near the end of interphase and continues through prophase when individual chromosomes become visible. At the same time, the nuclear envelope breaks down and the centrioles of animal cells move apart. One set of microtubules assembles between the nucleolar organizing regions while another set grows outward from each centromere toward the poles. Metaphase begins when the pairs of sister chromatids align across the center of the cell at the metaphase plate. The end of this phase is signalled by the division of the centromeres. During anaphase, each chromatid moves toward the pole to which it is attached. Separation occurs when the central spindle fibers slide past one another, moving the poles farther apart. The chromatids also move toward the poles as the microtubules to which they are attached shorten. The nucleus begins to reform around the uncoiling chromosomes during telophase. The spindle apparatus breaks down and the nucleolus reappears as rRNA genes are again expressed.

 

There are significant differences in cytokinesis in animals and plants. Animal cells are pinched in two by a belt of constricting microfilaments at the cleavage furrow. Rigid plant cells are not easily deformed and divide from the inside outward. This expanding partition is called the cell plate. The final addition of cellulose to either side of the membrane results in two separate cells.

 

Meiosis and syngamy constitute a cycle of sexual reproduction. Fertilization would double the chromosome number of each subsequent generation except that the gametes possess only a haploid complement of DNA. Thus the resultant zygote inherits genetic material from both its father and its mother, in the case of humans, twenty-three chromosomes from each. Sexual reproduction produces offspring that are genetically different from either parent while asexual reproduction produces progeny that are genetically identical to the parent cell. The specific events of sexual reproduction varies from kingdom to kingdom. For example, in most unicellular eukaryotes, the individual cells function directly as gametes. In plants, specific haploid cells are produced by meiosis, these cells then divide by mitosis to form a multicellular haploid phase which further produces eggs and/or sperm. In animals special gamete-producing cells differentiate from the other somatic cells early on in development. Only these cells are able to undergo meiosis to create haploid eggs or sperm.

 

Gamete-producing cells differentiate from somatic cells early in development. While they themselves are diploid, their products are haploid as a result of meiosis. Although meiosis and mitosis share many features, including microtubule formation, meiosis is unique for three reasons: synapsis, homologous recombination, and reduction division. During synapsis homologous chromosomes physically pair along their length. In homologous recombination genetic exchange, called crossing over, occurs between the homologues. Reduction division is the two separate rounds of nuclear division that occur in the remainder of the process. In the first division, homologous chromosomes pair, exchange material, and separate. No genetic replication occurs before the second division when the non-identical sister chromatids separate into individual gametes. Each division is composed of prophase, metaphase, anaphase, and telophase, additionally labeled I or II.

 

Some of the most important events of meiosis occur during prophase I. The ends of the sister chromatids attach to specific sites on the nuclear envelope. The attachment sites for the two homologues are near one another ensuring that each chromosome associates closely with its homologue. Each gene corresponds with its partner forming the synaptonemal complex. Certain genes are exchanged between homologues, an event called crossing over. The homologues are released from the membrane but remain tightly connected to one another. The homologues line up along the central plate of the cell during metaphase I. Only one face of each centromere is accessible to microtubule attachment, thus each homologue attaches to only one polar spindle fiber. The microtubules shorten at anaphase I and pull the homologues apart to opposite ends of the cell. Each pole ends up with a complete set of haploid chromosomes. Telophase I finishes division I, cytokinesis may or may not occur.

 

Meiosis II is essentially a mitotic process. During metaphase II, the still connected sister chromatids line up along their new metaphase plate with spindle fibers from each pole attached to each centromere. During anaphase II, the centromeres split and the sister chromatids are drawn to opposite poles. The result is four cells containing a haploid complement of genetic material.

 

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