THE CELL CYCLE AND MITOTIC CELL DIVISION
The life of a cell
begins when a parent cell divides to produce the new cell. The new cell then
goes through maintenance and growth processes until it matures and ultimately
divides to produce another generation of two cells. The life of a cell, from
its beginning until it divides to produce the new generation of cells, is
called the cell cycle.
Mitosis (Gr. mitos, thread) is the distribution of chromosomes
between two daughter cells, and cytokinesis (Gr. kytos, hollow vessel 1
kinesis, motion) is the partitioning of the cytoplasm between the two daughter
cells.
Interphase (L. inter, between) is the time between the end of
cytokinesis and the beginning of the next mitotic division. It is a time of
cell growth, DNA synthesis, and preparation for the next mitotic division.
The G1 (first growth or gap) phase represents the early
growth phase of the cell. During the S (DNA synthesis) phase, growth continues,
but this phase also involves DNA replication.
The G2 (second growth or gap) phase prepares the cell for
division. It includes replication of the mitochondria and other organelles, synthesis of microtubules and
protein that will make up the mitotic spindle fibers, and chromosome
condensation.
Interphase:
copy,
or sister, at a point of constriction called a centromere. The
centromere is a specific DNA sequence of about 220 nucleotides and has a
specific location on any given chromosome. Bound to each centromere is a disk
of protein called a kinetochore, which eventually is an attachment site for the
microtubules of the mitotic spindle. The final stage of interphase is gap phase
2 (G2). As the cell cycle moves into the G2 phase, the chromosomes begin
condensation.
During
the G2 phase, the cell also begins to assemble the structures that it will
later use to move the chromosomes to opposite poles (ends) of the cell. For
example, centrioles replicate, and there is extensive synthesis of the proteins
that make up the microtubules. The time spent by a cell in interphase varies
greatly depending on the cell. Rapidly dividing embryonic cells move very
quickly through G1 to S, and again quickly through G2 to M. The entire cell
cycle may occur within a few minutes time. Rapidly dividing cells produce a
many-celled embryo from a single fertilized egg within hours. On the other hand,
maturing cells spend relatively more time in G1 because they are growing and
taking on functions of adult cells. Many adult cells are not dividing. Mature
bone, muscle, and nerve cells enter G1 and pause. They may remain in this G0
phase indefinitely or until cell division is required, for example, to repair
an injury.
M-Phase:
Mitosis is divided into five phases: prophase, prometaphase, metaphase, anaphase, and telophase. In a dividing cell, however, the process is actually continuous, with each phase smoothly flowing into the next. The first phase of mitosis, prophase (Gr. pro, before 1 phase), begins when chromosomes become visible with the light microscope as threadlike structures. The nucleoli and nuclear envelope begin to break up, and the two centriole pairs move apart. By the end of prophase, the centriole pairs are at opposite poles of the cell. The centrioles radiate an array of microtubules called asters (L. aster, little star), which brace each centriole against the plasma membrane. Between the centrioles, the microtubules form a spindle of fibers that extends from pole to pole. The asters, spindle, centrioles, and microtubules are collectively called the mitotic spindle (or mitotic apparatus). Prometaphase follows the break-up of the nuclear envelope. A second group of microtubles attach at one end to the kinetochore of each chromatid and to one of the poles of the cell at the other end of the microtuble. This bipolar attachment of spindle fibers to chromatids is critical to the movement of the chromatids of each chromosome to opposite poles of the cell in subsequent phases of mitosis.
As the dividing cell
moves into metaphase (Gr.
meta, after 1 phase), the chromatids (replicated chromosomes) begin to
align in the center of the cell, along the spindle equator. Toward the end of
metaphase, the centromeres divide and detach the two sister chromatids from
each other, although the chromatids remain aligned next to each other. After
the centromeres divide, the sister chromatids are considered fullfledged
chromosomes (called daughter chromosomes).
During anaphase (Gr. ana, back again
1 phase), the shortening of the microtubules in the mitotic spindle,
and perhaps the activity of motor
proteins of the kinetochore, pulls each daughter chromosome apart from its copy
and moves it toward its respective pole. Anaphase ends when all the daughter
chromosomes have moved to the poles of the cell. Each pole now has a complete,
identical set of chromosomes.
Telophase (Gr. telos, end 1 phase) begins once the daughter
chromosomes arrive at the opposite poles of the cell. During telophase, the
mitotic spindle disassembles. A nuclear envelope re-forms around each set of
chromosomes, which begin to uncoil for gene expression, and the nucleolus is
resynthesized. The cell also begins to pinch in the middle. Mitosis is over,
but cell division is not.
Cytokinesis:
The
final phase of cell division is cytokinesis, in which the cytoplasm divides.
Cytokinesis usually starts sometime during late anaphase or early telophase. A
contracting belt of microfilaments called the contractile ring pinches the
plasma membrane to form the cleavage furrow. The furrow deepens, and two new,
genetically identical, daughter cells form.
MEIOSIS:
THE BASIS OF SEXUAL REPRODUCTION
Sexual reproduction requires a genetic contribution from
two different sex cells. Egg and sperm cells are specialized sex cells called gametes (Gr. gamete, wife; gametes, husband). In animals, a male gamete
(sperm) unites with a female gamete (egg) during fertilization to form a single
cell called a zygote (Gr. zygotos, yoked together). The zygote is the first
cell of the new animal. The fusion of nuclei within the zygote brings together
genetic information from the two parents, and each parent contributes half of
the genetic information to the zygote.
Somatic
cells
“To maintain a constant number of chromosomes
in the next generation, animals that reproduce sexually must produce gametes
with half the chromosome number of their ordinary body cells (called somatic
cells)”.
All of the cells in the bodies of most animals, except
for the egg and sperm cells, have the diploid (2N) number of chromosomes.
Gametes are produced by cells set aside for that purpose early in development.
These cells are called germ-line cells and eventually undergo a type of cell
division called meiosis (Gr. meiosis, diminution). Meiosis occurs in
germ-line cells of the ovaries and testes and reduces the number of chromosomes
to the haploid (1N) number. The nuclei of the two gametes combine during
fertilization and restore the diploid number. Meiosis begins after the G2 phase
in the cell cycle— after DNA replication. Two successive nuclear divisions,
designated meiosis I and meiosis II, take place. The two nuclear divisions of
meiosis result in four daughter cells, each with half the number of chromosomes
of the parent cell. Moreover, these daughter cells are not genetically
identical. Like mitosis, meiosis is a continuous process, and biologists divide
it into the phases that follow only for convenience.
The First Meiotic Division
new combinations of genes on the various chromatids in
homologous pairs. Thus, each chromatid ends up with new combinations of
instructions for a variety of traits. Crossingover is a form of genetic
recombination and is a major source of genetic variation in a population of a
given species. In metaphase I, the microtubules form a spindle apparatus just
as in mitosis. However, unlike mitosis, where homologous chromosomes do not
pair, each pair of homologues lines up
in the center of the cell, with centromeres on each side of the spindle
equator. Anaphase I begins when homologous chromosomes separate and begin to
move toward each pole. Because the orientation of each pair of homologous
chromosomes in the center of the cell is random, the specific chromosomes that
each pole receives from each pair of homologues are also random. This random
distribution of members of each homologous pair tothe poles of the cell, along
with the genetic recombination between homologous chromosomes that occurs
during crossing-over (prophase I), means
that no two daughter cells produced by meiotic cell division will be identical.
Meiotic telophase I is similar to mitotic telophase. The transition to the
second nuclear division is called inter-kinesis. Cells proceeding through
interkinesis do not replicate their DNA. After a varying time period, meiosis
II occurs.
The
Second Meiotic Division
The second meiotic division (meiosis II) resembles an
ordinary mitotic division (see figure 3.6b), except that the number of
chromosomes has been reduced by half. The phases are prophase II, metaphase II,
anaphase II, and telo-phase II. At the end of telo-phase II and cytokinesis, the
final products of these two divisions of meiosis are four new “division
products.” In most animals, each of these “division products” is haploid and
may function directly as a gamete (sex cell).
Spermatogenesis and Oogenesis
The result of meiosis in most animals is the formation of
sperm and egg cells. Spermatogenesis produces mature sperm cells and follows
the sequence previously described. All four products of meiosis often acquire a
flagellum for locomotion and a cap like structure that aids in the penetration
of the egg. Oogenesis produces a mature
ovum or egg. It differs from spermatogenesis in that only one of the four
meiotic products develops into the functional gamete. The other products of
meiosis are called polar bodies and eventually disintegrate. In some animals
the mature egg is the product of the first
meiotic division and only completes meiosis if it is fertilized by a
sperm cell.
meiotic and cytoplasmic divisions during which homologous
pairs of chromosomes undergo synapsis, including crossingover, followed by the
separation of members of each pair into gametes that have one-half the number
of chromosomes of the parental cells. Fertilization restores the diploid (2N)
chromosome number in the zygote.
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