The Cell Cycle
The cell cycle is the sequence of stages through which a cell passes between one cell
division and the next. The length of time it takes a cell to complete the cell cycle
varies from one cell to another. Some cells complete the entire cycle in a few
minutes, and other cells spend their entire life frozen in a particular phase.
Most of the cell cycle is spent in interphase, as shown in the figure below.
Interphase consists of three major parts: G1, S, G2. During the G 1 phase of
interphase, the cell grows in size. In the S phase, replication of the DNA-containing
the genetic material_ occurs. In the G2 phase, the cell prepares for mitosis by
replicating organelles and increasing the amount of cytoplasm.
Mitosis
All of the cells in the body, with the exception of reproductive cells are called
somatic cells. Some examples are heart cells, liver cell and skin cells. Somatic cells
undergo a process called mitosis. Mitosis is a type of cell division that generates two
daughter cells. Both new cells are identical to the mother cell. The stages of mitosis
are prophase, metaphase, anaphase and telophase. During these phases, DNA is
organized into threadlike material called chromosomes, which are then divided and
separated.
The daughter cells that result from mitotic cell division are identical to each other as
well as to the parent cell. The daughter cells have the same (diploid) number of
chromosomes as the parent cell. Mitosis is the mechanism for asexual reproduction,
which only requires one parent. Mitosis also allows multicellular organisms to grow
and replace damaged or worn out cells. The stages of mitosis are: prophase,
metaphase, anaphase and telophase.
Prophase: The nucleus of the cell organizes the chromatin material into threadlike structures called chromosomes.
Metaphase: The chromosomes attached at the centromeres line up at the center of
the cell
Anaphase: Chromosomes separate at the center and are pulled toward either end
of the cell. A nuclear membrane forms around the chromosomes as they
disorganize.
Telophase: Chromatin again forms from the chromosomes, and a cell membrane
begins to grow across the center between the two new nuclei.
Cytokinesis
Cytokinesis, the division of the cell cytoplasm, usually follows mitosis. Cytokinesis
generally begins during the telophase of mitosis. It finalizes the production of two
new daughter cells, each with approximately half of the cytoplasm and organelles as
well as one of the two nuclei formed during mitosis. The processes of mitosis and
cytokinesis are together called cell division.
Section Review Questions
1. Define the following:
a. Cell cycle
b. Somatic cells
c. Mitosis
d. Asexual reproduction
e. Prophase
f. Metaphase
g. Anaphase
h. Telophase
i. Cytokinesis
j. Cell division
2. A type of nuclear division that takes place in somatic cell is
a. Meiosis
b. Cytokinesis
c. Interphase
d. Mitosis
3. The length of time it takes for a cell to complete the cell cycle is
a. Around two hours
b. Different for each cell
c. The same for each kind of cell
d. Around two minutes
4. The two distinct phases of the cell cycle are
a. Prophase and metaphase
b. Interphase and mitosis
c. Interphase and telophase
d. Anaphase and cytokinesis
5. What type of reproduction is accomplished by mitosis?
a. Sexual
b. Asexual
c. Immediate
d. indeterminate
6. What phase of mitosis separates the chromosomes?
a. Prophase
b. Metaphase
c. Anaphase
d. telophase
7. Name and describe the stages of mitosis.