CELL REPRODUCTION AND DIVISION
THE CELL
CYCLE
MITOSIS
MEIOSIS
THE CELL
CYCLE
Mitosis
is the division of the cell nucleus in which the chromosomes in the parent
cell divide into two identical sets. Mitosis is the process by which
body cells can increase the number of cells without changing the information
contained in the DNA or the amount of DNA in those cells. Mitosis
is part of the cell cycle. The cell cycle is the sequence of events
that occurs in a cell from one reproduction to the next. There are
five events that occur during the cell cycle; the G1 period, the
S period, the G2 period, mitosis, and finally cytokinesis. The first
three periods make up interphase, a period of cell growth and development
that precedes mitosis and follows cytokinesis. In the first phase,
the cell doubles in size and in the production of enzymes and organelles
usually double in number. During the S period the DNA replicates.
During the G2 period, the cell undergoes rapid growth, this prepares the
cell for mitosis.
PHASES
OF MITOSIS
Mitosis is
divided up into four different parts, prophase, metaphase, anaphase, and
telophase. Prophase is the first stage. During this phase the
chromatin coils into primary, secondary, and tertiary coils. Also
during this phase the nucleolus and the nuclear membrane disappear and
organelles called centrioles appear. These organelles move to opposite
poles of the cell and then spindle fibers begin to form. There are
two types of these fibers, kinetochore and polar. The difference
between the two is that kinetochore fibers connect the centrioles with
the centrosomes of the chromosomes and polar fibers connect the two centrioles.
The last step of prophase is the formation of asters, the protein fibers
that radiate from each centriole.
Metaphase
is the second stage of mitosis. During this stage the kinetochore
fibers move the chromosomes to the center of the cell. Anaphase is
the third phase of mitosis and is characterized by the division of the
chromatids, this is a quick and dramatic movement and the cause of it is
not fully understood. Telophase is the final phase of mitosis.
This phase can be described as a backwards prophase. First the centrioles
and the spindle fibers disintegrate. Next, the chromatids unwind
and stretch out to become known as chromatin. A nuclear membrane
and nucleolus then reform. The cells are now ready for cytokinesis,
the process of dividing cytoplasm up equally among the two cells.
MEIOSIS
Meiosis is
the process of nuclear division that reduces the numv\ber of chromosomes
by half. This is the process that takes place during sexual reproduction.
Meiosis is divided up into two parts, Meiosis I and Meiosis II. During
meiosis I- Prophase everything that occurs in mitosis occurs except for
one extra thing, in meiosis every chromosome lines up with it's homologue,
this process is called synapsis. During metaphase I tetrads (groups
of four chromatids that compose the two homologous chromosomes) are moved
by spindle fibers. In anaphase I the homologous chromosomes separate.
During telophase I the cytoplasm divides and forms two daughter cells.
In meiosis I the parent cell produces two daughter cells, each with one
member of each member of each pair of homologous chromosomes . Chromosome
number has been halved, but each chromosome, having been replicated earlier,
has twice as much of DNA.
Meiosis II
occurs in each cell formed during meiosis I and is not preceded by DNA
replication. In prophase II the chromosomes again coil and new spindle
fibers form. During metaphase II the chromosomes are moved to the
cell's center. In anaphase II the centromeres joining the chromatids divide
and one of each sister chromatid is moved toward an opposite pole by the
spindle fibers. AS the spindle dissolves, telophase II begins and a nuclear
membrane forms around the chromosomes in each daughter cell.
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