BIOB11H3 Lecture Notes - Lecture 17: Cyclin-Dependent Kinase, Microtubule Organizing Center, Spindle Pole Body

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Published on 21 Apr 2013
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Last time we talked about the cell cycle and mitosis and the events that go on during
mitosis. Today we will review that and discuss two main things. The first of those are some
aspects of chromosome condensation and the second is chromosomal movements. When
the nuclear membrane breaks down in prophase, the chromosomes might find themselves
all over the interior of the cell. They have to move to the middle in conjunction with each
other to lineup on the metaphase plate. Then, once they are in line, they have to move in
opposite the direction. We will see how this can take place.
This is a review of the last slide from last lecture. Figure 14 11- Keep in mind what is
already taking place. The chromosomes have duplicated in S-phase, they have gone through
G2 and they got a signal for anaphase to begin. Then what happens? During prophase, the
chromosomes are very visible. During interphase, they were very decondensed and you
would not be able to see a strand of chromatin through a microscope. Now as they enter
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mitosis, chromatin condensation takes place and the sister chromatids become visible
using a microscope. Some dramatic changes take place in the cytoplasm as well. The
normal cytoskeletal array of microtubules disassembles and is replaced by a different type
of microtubule organizing center called the spindle apparatus. Microtubules from the
spindle apparatus attach to the chromosomes and pull them to opposite poles. The nuclear
membrane then breaks down. We will review cytoskeletal MTOCs or microtubule
organizing centers. The chromosomes are captured by microtubules, the nuclear
membrane is no longer in place, the microtubules invade the space of the nucleus and
capture the chromosomes at the kinetochores and begin the movement towards the
equator or the middle of the cell. This is Prometaphase.
When they arrive at the middle of the cell, they are said to lie at the metaphase plate. You
can see that the pairs of sister chromatids exist there and that they have connections
emanating through both opposite spindle poles. The cell needs to conduct a check point, to
make sure that both pairs of chromosomes are aligned at the metaphase plate and that they
are attached to microtubules coming from opposite spindle poles. The cell needs to be able
to ensure itself so it conducts a check point to make sure that those pairs of chromosomes
are aligned at the metaphase plate and they are attached microtubules coming from
opposite spindle poles. They do this by monitoring the tension from each pair of poles, that
is, if they are being pulled to the left or to the right. When that tension is equivalent, it tells
the cell that the chromosomes are aligned in the middle.* See blackboard animations on
checkpoints.* Once they pass the checkpoint, the cell is told that all the chromosomes are at
the middle of the cell attached to opposite poles and now the cell initiates anaphase.
Anaphase is the poleward movement of the sister chromatids. Sister chromatids lose their
affinity for one another and get pulled in opposite directions. The poles also move apart. So
there are two different movements that act called anaphase A and B. When the
chromosomes arrive at opposite poles, they begin to decondense. The activities that
require them to be condensed that is the separation of chromosomes don't need that
anymore. The chromosomes decondense so they are more accessible to enzymes like RNA
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polymerase and normal types of transcription and translation will begin again. Since you
have separated the two chromosome bodies from each other, this is a eukaryotic cell; to
begin the next step of the cell cycle compartmentalization is needed. You need to reform
the nuclear membrane around each of the two sets of chromosomes. Later, the cytokinesis
will divide the cytoplasm and the cell has duplicated the chromosomes, segregated them
efficiently and correctly to opposite poles, built two new nuclear membranes, divided the
cytoplasm, and turned one cell into two identical cells (at least usually).
This slide was shown last time, but today we will talk about some of the events that cyclin
dependent kinases mediate in cell cycle transitions. Figure 14 5 shows that there are two
primary transition points for a cell. One of those is the decision to go from G1 to initiate a
new cell cycle by approving new replication. That is, the transition from G1 to S-phase is
mediated by a complex in yeast called START. Later, a different complex called MPF
(maturation promoting factor) is going to mediate the transition from G2 to M phase.
Remember that the complexes exist or consist of a couple of subunits. One of the subunits
are a cyclin dependent kinase kinase is an enzyme that phosphorylates other proteins
but not just any proteins, they are specific targets of the cyclin dependent kinases and now
think about what those targets might be. What things have to happen in order to move
from G1 to S-phase using the start transition or from G2 to M phase is in the MPF trigger.
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Document Summary

Last time we talked about the cell cycle and mitosis and the events that go on during mitosis. Today we will review that and discuss two main things. The first of those are some aspects of chromosome condensation and the second is chromosomal movements. When the nuclear membrane breaks down in prophase, the chromosomes might find themselves all over the interior of the cell. They have to move to the middle in conjunction with each other to lineup on the metaphase plate. Then, once they are in line, they have to move in opposite the direction. We will see how this can take place. This is a review of the last slide from last lecture. Figure 14 11- keep in mind what is already taking place. The chromosomes have duplicated in s-phase, they have gone through. G2 and they got a signal for anaphase to begin.

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