CHAPTER OUTLINE
5.1 The Cell Cycle
Cell division increases the number of somatic or body cells. Apoptosis or programmed cell death decreases the number of cells.
The Cell Cycle
The cell cycle is an orderly set of stages that takes place between the time a cell divides and the time the resulting cells also divide.
The Stages of Interphase
Most of the cell cycle is spent in interphase. This is when the cell carries on its usual functions. Interphase is divided into three stages: The G1 stage occurs before DNA synthesis, the S stage includes DNA synthesis, and the G2 stage occurs after DNA synthesis.
The Mitotic Stage
Following interphase, the cell enters the mitotic (M) stage. When complete, two daughter cells are present.
Apoptosis
During apoptosis, the cell progresses through a typical series of events that bring about its destruction.
5.2 Control of the Cell Cycle and Cancer
Eukaryotic cells have evolved a complex system of both internal and external signals for regulation of the cell cycle.
Proto-oncogenes and Tumor Suppressor Genes
Proto-oncogenes encode proteins that promote the cell cycle and prevent apoptosis. Tumor suppressor genes encode proteins that stop the cell cycle and promote apoptosis. Carcinogenesis, the development of cancer, is a multistage process involving disruption of normal cell division and behavior, typically due to mutations in either proto-oncogenes or tumor suppressor genes.
5.3 Mitosis: Maintaining the Chromosome Number
Eukaryotic chromosomes are composed of chromatin, a combination of both DNA and protein. Each species has a characteristic chromosome number called the diploid (2n) number because it contains two (a pair) of each type of chromosome. Half the diploid number, called the haploid (n) number of chromosomes, contains only one of each kind of chromosome.
Overview of Mitosis
Mitosis is nuclear division in which the chromosome number stays constant. A 2n nucleus divides to produce two daughter nuclei that are also 2n. Before nuclear division takes place, DNA replication occurs. A duplicated chromosome is composed of two sister chromatids, which are genetically identical, held together at a region called the centromere.
Mitosis in Detail
Mitosis is nuclear division that produces two daughter nuclei, each with the same number and kinds of chromosomes as the parental nucleus. During mitosis, a spindle brings about an orderly distribution of chromosomes to the daughter cell nuclei.
Mitosis in Animal Cells
Mitosis is a continuous process that is arbitrarily divided into four phases:
Prophase
The chromosomes are now visible, spindle fibers appear and the nuclear envelope begins to fragment as the nucleolus begins to disappear.
Metaphase
The chromosomes are attached to centromeric spindle fibers and line up at the metaphase plate.
Anaphase
The sister chromatids separate, becoming daughter chromosomes that move toward the opposite poles of the spindle.
Telophase
The spindle disappears and the nuclear envelope components reassemble around the daughter chromosomes, which become more diffuse as the nucleolus appears in each daughter nucleus.
Mitosis in Plant Cells
Exactly the same phases occur in plant cells as in animal cells. Although plant cells have a centrosome and spindle, there are no centrioles or asters during cell division.
Cytokinesis in Animal and Plant Cells
Cytokinesis, or cytoplasmic cleavage, usually accompanies mitosis, but they are separate processes.
Cytokinesis in Animal Cells
A cleavage furrow, an indentation of the membrane between the two daughter nuclei, forms as anaphase ends. A contractile ring forms a constriction between the two daughter cells until they separate.
Cytokinesis in Plant Cells
The rigid cell wall does not permit cytokinesis by furrowing. Instead, cytokinesis in plant cells involves building a new cell wall between the daughter cells.
5.4 Meiosis: Reducing the Chromosome Number
Meiosis occurs in any life cycle that involves sexual reproduction and reduces the chromosome number in such a way that the daughter nuclei receive only one of each kind of chromosome.
Overview of Meiosis
When a cell is diploid, the chromosomes occur in pairs and the members of a pair are called homologous chromosomes. Meiosis is divided into meiosis I and meiosis II.
Meiosis I
During meiosis I, the homologous chromosomes come together as a pair at the metaphase plate.
Meiosis II and Fertilization
During meiosis II, the sister chromatids separate. In some life cycles, the daughter cells mature into gametes or sex cells—sperm and egg that fuse during fertilization. Fertilization restores the diploid number of chromosomes.
Meiosis in Detail
Meiosis requires two nuclear divisions and results in four daughter nuclei each with half the number of chromosomes as the parental cell.
First Division
Meiosis I has four phases:
Prophase I
During prophas I synapsis, homologous chromosomes coming together and lining up side by side, occurs followed by crossing-over.
Metaphase I and Anaphase I
During metaphase I, the homologues align at the metaphase plate. Independent assortment occurs when these pairs separate from each other during anaphase I.
Telophase I
In some species, telophase I occurs at the end of meiosis, the nuclear envelopes re-form and nucleoli reappear.
Interkinesis
The period of time between meiosis I and meiosis II is called interkinesis. No replication of DNA occurs.
Second Division
During the second division of meiosis, sister chromatids separate from each other. Four daughter cells result.
The Importance of Meiosis
Meiosis produces haploid cells that are no longer identical to the diploid parent cell. Crossing-over and independent assortment result in different combinations of chromosomes than the parental cell. Upon fertilization, the combining of chromosomes from genetically different gametes helps ensure that offspring are not identical to their parents.
5.5 Comparison of Meiosis with Mitosis
DNA replication takes place only once prior to either meiosis or mitosis, but meiosis requires two nuclear divisions, as opposed to one in mitosis. Meiosis and subsequent cytokinesis results in four daughter cells that are haploid and not identical to one another or the parent cell. Mitosis and associated cytokinesis results in two identical diploid daughter cells that are also identical to the parent cell.
Occurrence
Meiosis occurs only at certain times in the life cycle and in certain organs of sexually reproducing organisms. Mitosis occurs almost continuously in all tissues during growth and repair.
Processes
Tables 5.1 and 5.2 compare meiosis I and meiosis II to mitosis.
Comparison of Meiosis I to Mitosis
During meiosis I, but not mitosis, homologous chromosomes pair, undergo crossing-over, and separate.
Comparison of Meiosis II to Mitosis
The events of meiosis II are just like mitosis, except that in meiosis II, the nuclei contain the haploid number of chromosomes.
5.6 The Human Life Cycle
The human life cycle requires both meiosis and mitosis.
Spermatogenesis and Oogenesis in Humans
Spermatogenesis occurs in the testes of males and produces sperm. Oogenesis occurs in the ovaries of females and produces eggs.
Spermatogenesis
Primary spermatocytes, which are diploid, divide via meiosis to produce four spermatids, which mature into sperm.
Oogenesis
Primary oocytes, which are diploid, begin meiosis but become arrested in prophase I. Following puberty one primary oocyte begins to complete meiosis, finishing the first meiotic division as two cells, each of which is haploid. One of these cells, the secondary oocyte, receives almost all the cytoplasm, the other is a polar body, a nonfunctioning cell. If the secondary oocyte is fertilized by a sperm, it completes the second meiotic division in which it again divides unequally, forming an egg and a second polar body. If the secondary oocyte is not fertilized, it disintegrates and passes out of the body.
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