Download CHAPTER 3: CELL STRUCTURE AND FUNCTION

CHAPTER 5: CELL DIVISION
LECTURE OUTLINE
5.1 Cell Increase and Decrease
Cell division increases the number of somatic or body cells. Cell division consists of mitosis and
cytokinesis. Apoptosis or programmed cell death decreases the number of cells.
The Cell Cycle
The cell cycle is an orderly set of stages that take 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 stage. When complete, two
daughter cells are present.
Control of the Cell Cycle
The cell cycle is controlled by internal and external signals. These signals ensure
that the stages follow one another in the normal sequence and that each stage is
properly completed before the next stage begins.
Apoptosis
During apoptosis, the cell progresses through a typical series of events that bring about its
destruction.
5.2 Maintaining the Chromosome Number
At the time of cell division, the chromatin condenses to form highly compacted structures called
chromosomes. Each species has a characteristic chromosome number called the diploid or 2n
number because it contains two (a pair) of each type 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 held together at a region called the centromere.
Mitosis in Detail
During mitosis, a spindle brings about an orderly distribution of chromosomes to the
daughter cell nuclei.
Mitosis in Animal Cells
Mitosis is arbitrarily divided into four phases:
Prophase
The chromosomes are now visible. Spindle fibers appear and the nuclear
envelope begins to disappear.
Metaphase
The chromosomes are attached to 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, the nuclear envelope reforms, and the
chromosomes decondense.
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 usually accompanies mitosis, but is a separate process.
Cytokinesis in Animal Cells
A cleavage furrow, an indentation of the membrane between the two daughter
nuclei, forms during anaphase. A contractile ring pinches the cleavage furrow
separating the two cells.
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.
Cell Division in Prokaryotes
Prokaryotes divide by a process called binary fission.
5.3 Reducing the Chromosome Number
Meiosis 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. 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 separate.
Meiosis II and Fertilization
During meiosis II, the sister chromatids separate. 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. During meiosis I genetic recombination
of the parental genes occurs through two key events: crossing-over and independent
assortment.
Prophase I
Meiosis I has four phases called prophase I, metaphase I, anaphase I, and
telophase I. In prophase I, synapsis occurs followed by crossing-over.
Metaphase I
During metaphase I, the homologues align at the metaphase plate. Independent
assortment occurs when these pairs separate from each other, generating cells
with different combinations of maternal and paternal chromosomes.
First Division: All Phases
Homologous chromosomes separate, and each chromosome still consists of two
chromatids. Cytokinesis may or may not occur.
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.
Nondisjunction
Nondisjunction is a failure of homologous chromosomes (anaphase I) or sister
chromatids (anaphase II) to separate such that gametes contain one more or one less than
the correct number of chromosomes.
Genetic Recombination
Crossing-over and independent assortment result in different combinations of
chromosomes than the parental cell. Upon the fertilization, the combining of
chromosomes from genetically different gametes helps ensure that offspring are not
identical to their parents.
5.4 Comparison of Meiosis with Mitosis
Figure 5.14 compares meiosis to mitosis.
Occurrence
Meiosis occurs only at certain times in the life cycle 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 the nuclei contain the
haploid number of chromosomes.
5.5 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 divide in meiosis I to produce one secondary
oocyte which receives almost all the cytoplasm and 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 the egg
and a second polar body. If the secondary oocyte is not fertilized, it disintegrates
and passes out of the body.