Download File - Martin Ray Arcibal

Martin Ray A. Arcibal
February 15, 2012
AP Biology
Mrs. Delgado
Free Response: Cell Division
The mitotic phase of the cell cycle involves six stages: prophase, prometaphase,
metaphase, anaphase, telophase, and cytokinesis. In all of these stages, the main components of
the cell involved are the centrosome, the organelle responsible for the formation of microtubules,
chromosomes, which contain the information of replication and other metabolic processes that
the cell undergoes, the nuclear envelope and the nucleoli, which disappear as soon as mitosis is
initiated, and the mitotic spindle, derived from the microtubules formed by the centrosomes. In
prophase, the chromosomes coil more tightly together, condensing, and becoming more visible
through the use of the light microscope. The nucleolus, which is the part of the nucleus
responsible for ribosomal RNA formation, disappears. The centrosomes, including the replicated
one, move farther apart from one another, while creating the mitotic spindle. Asters, which
extend from the centrosomes, also become more visible. During prometaphase, the nuclear
envelope breaks down. The microtubules are now able to penetrate the nuclear region. The sister
chromatids now become more concentrated than before. Kinetochores are then produced by the
sister chromatids. Kinetochores are specialized proteins on the centromeres of the sister
chromatids that microtubules from the mitotic spindle attach to. Sister chromatids begin to move
back and forth from the poles due to their attachment to the kinetochore microtubules.
Nonkinetochore microtubules begin to interact and overlap one another. Metaphase, which is the
longest stage of mitosis (lasting about twenty minutes), then follows. At this point, the
centrosomes have already reached the opposite poles of the cell. The constant back and forth
movement of the sister chromatids after attachment to the kinetochore microtubules eventually
cause them to be oriented on the center of the cell, equidistant from both poles, which is often
referred to as the metaphase plate or the equatorial plane. Now, the entire group of sister
chromatids should be attached to kinetochore microtubules. Anaphase is the shortest stage of
mitosis. At this point, the cohesin proteins holding the sister chromatids together are cleaved by
specialized enzymes. The sister chromatids are now separated into individual chromosomes.
These chromosomes move towards their respective poles through the action of motor proteins.
As these proteins pass through the kinetochore microtubules, the microtubules are
depolymerized. The cell elongates as the nonkinetochore microtubules lengthen. At the time
when telophase begins, the chromosomes have already reached their respective poles. At this
stage, the nuclear envelope reforms for each of the new sets of chromosomes. The chromosomes
become less condensed and the nucleoli reforms in each newly established nucleus. Two
identical daughter nuclei are now formed. Often, the process of cytokinesis occurs while
telophase is underway.
Cytokinesis is the division of the cell nucleus. This process occurs in two different ways
for animal and plant cells. In animal cells, cytokinesis occurs in a process called cleavage. The
appearance of a cleavage furrow, a shallow groove in the cell surface near the metaphase plate,
signifies the beginning of this process. In the cytoplasmic side of the cleavage furrow, a
contractile ring of actin microfilaments begin interaction with the protein myosin. This
interaction causes the actin microfilaments to contract, deepening the cleavage furrow and
dividing the parent cell into two completely separated, identical, and fully equipped daughter
cells. Plant cells undergo a different type of cytokinesis. While telophase is occurring, vesicles
containing materials for the formation of the cell wall, such as cellulose, move towards the
middle of the cell. These vesicles converge to form a cell plate. This cell plate grows until its
membrane combines with the plasma membrane of the cell. The result is the appearance of two
identical daughter cells. Meanwhile, a new cell wall is formed from the contents of the vesicles
that formed the cell plate between the daughter cells.
After the production of the two daughter cells from the mitotic phase of the cell cycle,
each cell undergoes a series of growth phases, collectively known as interphase. In the G1 phase,
also known as the “first gap” or the “first growth,” the cell acquires all the necessary nutrients for
its survival. The cell grows as a result of this stage. Near the end of this stage is a checkpoint
known as the G1 checkpoint, also known as the restriction point. At this point, if the necessary
requirements were not completed, the cell will be in the G0 phase. In this phase, the cell is
amitotic, which means that it does not undergo cell division. Muscle cells and nerve cells are
examples of amitotic cells. Once the G1 phase is completed, DNA replication can proceed in the
S phase of the interphase. At this phase, the chromosomes of the cell are duplicated, making two
complete sets of DNA. After the completion of this stage of the interphase, the G2 phase occurs.
In the G2 phase, the organelles of the cell are replicated. It is because of this phase that the
daughter cells produced by mitosis contain identical sets of organelles. The G2 checkpoint occurs
after organelle replication is completed. Once all of the stages of interphase are completed, then
the cell can undergo mitosis. Near the anaphase stage of mitosis, there is again a checkpoint
called the M checkpoint that determines if all the necessities were completed. After mitosis, the
cell cycle continues once again with the newly formed daughter cells entering interphase.
As mentioned earlier, there are checkpoints in the cell cycle that determine whether or
not the cell will undergo mitosis. These checkpoints are regulated internally by the presence of
the “maturation-promoting factor.” MPFs form through the combination of the protein cyclin,
which grows in concentration as the cell cycle moves from interphase to the mitotic phase, and
the cyclin-dependent kinases, which mostly remain constant throughout the cycle. MPF activity
fluctuates as the cell moves from interphase to the mitotic phase, with the peak occurring during
mitosis and dropping during interphase. The cyclin in the MPF will then degrade as the process
moves on into the anaphase stage of mitosis, preparing the cyclin-dependent kinases for the next
cell cycle. There are two other ways in which the cell division of a cell is regulated: densitydependent inhibition and anchorage dependence. In density-dependent inhibition, cells stop
dividing as soon as the space available to them has been completely filled. In an experiment
performed many years ago, as part of a cell culture is removed, the cells bordering the empty
space once again performs cell division, and stops as all the vacant space become occupied.
Many animal cells also exhibit the latter of the regulation mechanisms, anchorage dependence.
In this regulation, cells require a substrate to attach to in order to divide. Like cell density, the
signals sent by plasma membrane proteins and elements of the cytoskeleton linked to them cause
this regulation.
The study of the division of cells is an important factor in the global fight against cancer.