Medical Biology
Lec -4-
Shorooq Wessam
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 daughter
cells also divide. The cell cycle is controlled by internal and
external signals. A signal is a molecule that stimulates or inhibits
a metabolic event. For example, growth factors are external
signals received at the plasma membrane that cause a
resting cell to undergo the cell cycle. When blood platelets release
a growth factor, skin fibroblasts in the vicinity finish the
cell cycle, thereby repairing an injury. Other signals ensure
that the stages follow one another in the normal sequence
and that each stage of the cell cycle is properly completed before
the next stage begins.
Cell Cycle Stages
The cell cycle has two major portions: interphase and the mitotic stage.
Interphase
During interphase, the cell carries on its regular activities,
and it also gets ready to divide if it is going to complete the
cell cycle. For these cells, interphase has three stages, called G1
phase, S phase, and G2 phase.
G1 Phase Early microscopists named the phase before DNA
replication G1, and they named the phase after DNA replication
G2. G stood for “gap.” Now that we know how metabolically
active the cell is, it is better to think of G as standing for growth .
protein synthesis is very much a part of these growth phase .
During G1, a cell doubles its organelles (such as mitochondria
and ribosomes) and accumulates materials that will be used for DNA synthesis.
S Phase Following G1, the cell enters the S (for “synthesis”)
phase. During the S phase, DNA replication occurs. At
the beginning of the S phase, each chromosome is composed
of one DNA double helix, which is equal to a chromatid.
At the end of this phase, each chromosome has two
identical DNA double helix molecules, and therefore is
composed of two sister chromatids. Another way of expressing
these events is to say that DNA replication has resulted in duplicated
chromosomes.
G2 Phase During this phase, the cell synthesizes proteins that
will assist cell division, such as the protein found in microtubules.
Also, chromatin condenses, and the chromosomes become visible.
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Mitotic Stage
Following interphase, the cell enters the M (for mitotic) stage.
This cell division stage includes mitosis (division of the nucleus)
and cytokinesis (division of the cytoplasm). During
mitosis, daughter chromosomes are distributed to two daughter
nuclei. When cytokinesis is complete, two daughter cells are present .
Events During the Mitotic Stage
The mitotic stage of the cell cycle consists of mitosis and cytokinesis.
By the end of interphase , the
centrioles have doubled and the chromosomes are becoming
visible. Each chromosome is duplicated—it is composed of
two chromatids held together at a centromere. As an aid in describing
the events of mitosis, the process is divided into four
phases: prophase, metaphase, anaphase, and telophase. The parental cell is the
cell that divides, and the daughter cells are the cells that result .
Prophase
Several events occur during prophase that visibly indicate the
cell is about to divide. The two pairs of centrioles outside the
nucleus begin moving away from each other toward opposite ends of the
nucleus. Spindle fibers appear between the separating
centriole pairs, the nuclear envelope begins to fragment,
and the nucleolus begins to disappear.
The chromosomes are now fully visible. Although humans
have 46 chromosomes, . Spindle fibers attach to the centromeres as the
chromosomes continue to shorten and thicken. During prophase,
chromosomes are randomly placed in the nucleus.
Structure of the Spindle At the end of prophase, a cell has
a fully formed spindle. A spindle has poles, asters, and fibers.
The asters are arrays of short microtubules that radiate from
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the poles, and the fibers are bundles of microtubules that stretch between the
poles. Centrioles are located in centrosomes,
which are believed to organize the spindle.
Metaphase
During metaphase, the nuclear envelope is fragmented, and
the spindle occupies the region formerly occupied by the nucleus.
The chromosomes are now at the equator (center) of
the spindle. Metaphase is characterized by a fully formed
spindle, and the chromosomes, each with two sister chromatids, are aligned at
the equator.
Anaphase
At the start of anaphase, the sister chromatids separate. Once
separated, the chromatids are called chromosomes. Separation of
the sister chromatids ensures that each cell receives a copy of
each type of chromosome and thereby has a full complement
of genes. During anaphase, the daughter chromosomes move
to the poles of the spindle. Anaphase is characterized by the
movement of chromosomes toward each pole.
Function of the Spindle The spindle brings about chromosome movement.
Two types of spindle fibers are involved in the movement of chromosomes
during anaphase. One type extends from the poles to the equator of the spindle;
there, they overlap. As mitosis proceeds, these fibers increase
in length, and this helps push the chromosomes apart. The chromosomes
themselves are attached to other spindle fibers that simply extend
from their centromeres to the poles. These fibers get shorter and shorter
as the chromosomes move toward the poles. Therefore, they pull
the chromosomes apart . Spindle fibers, as stated earlier, are composed
of microtubules. Microtubules can assemble and disassemble by the
addition or subtraction of tubulin (protein) subunits. This is
what enables spindle fibers to lengthen and shorten, and it ultimately
causes the movement of the chromosomes.
Telophase and Cytokinesis
Telophase begins when the chromosomes arrive at the poles. During telophase,
the chromosomes become indistinct chromatin again. The spindle disappears as
nucleoli appear, and nuclear envelope components reassemble in each cell.
Telophase is characterized by the presence of two daughter nuclei.
Cytokinesis is division of the cytoplasm and organelles. In
human cells, a slight indentation called a cleavage furrow
passes around the circumference of the cell. Actin filaments
form a contractile ring, and as the ring gets smaller and smaller,
the cleavage furrow pinches the cell in half. As a result, each cell
becomes enclosed by its own plasma membrane.
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