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
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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.
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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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