BGYA01 LECTURE 10 OCOTBER 16, 2007
Last class we looked at the cell cycle and at the different levels of chromsome packaging
that occur at the different times in the cell cycle.
We also discussed some of the terminology for Genetics and chromosomes
Today we want to consider cell division and get into the details of nuclear division.
THE PROCESS BY WHICH ONE CELL DIVIDES TO GIVE RISE TO TWO DAUGHTER CELLS IS
KNOWN AS CELL DIVISION.
Cell division is divided into two parts- division of the nucleus (either mitosis or meiosis)
and division of the cytoplasm (which is known as cytokinesis)
THERE ARE TWO TYPES OF M PHASES- MITOSIS AND MEIOSIS. WE’LL LOOK AT MITOSIS
We divide mitosis into four phases
PROPHASE, METAPHASE, ANAPHASE AND TELOPHASE. FIGURE 9.10, PAGE 190-191.
THERE ARE 6 KEY EVENTS IN PROPHASE
1. The chromosomes begin to thicken and shorten, as they accomplish the 4th & 5th level
of packaging. Transcription stops.
2. The nucleolus (a small round structure within the nucleus) disassembles. Why?
It is because RNA is such a major component of the nucleolus that without RNA
synthesis it falls apart.
3. Sisters chromatids become distinct and separate, except in the region of the
centromere. The pinched in region of the chromosome is the centromere region. Figure
9.11 (page 191)
4. Each sister chromatid organizes a structure known as the kinetochore at the centromere
region. The kinetochore can be seen it in figure 9.9, page 189. The kinetochore is a set
of proteins that form around the centromere region, and later will attract microtubules.
5. During prophase a region called the Microtubule Organizing center MTOC duplicates
and the two MTOCs move to opposite sides of the cell. Microtubules grow outward from
the MTOCs. [MTOC stands for MicroTubule Organizing Center]
Note: In animal cells you can see centrioles at the MTOC, but in plants the
MTOCs have no obvious visible structure.
6. At the end of prophase the nuclear envelope breaks up into vesicles. The nuclear
envelope’s break-up signals the end of prophase.
THE NEXT STAGE IS KNOWN AS METAPHASE.
By the end of metaphase, the chromosomes will be lined up in the middle of the cell
(called the metaphase plate, Figure 9.10, panel 4, page 191), with sister chromatids facing
opposite poles. Let’s see how this is accomplished.
When the nuclear envelope breaks down this allows the mitotic spindle to form. The
mitotic spindle is an organized collection of microtubules.
Microtubules start out from the MTOCs (which are at opposite poles) and grow towards
the middle of the cell.
Microtubules that meet other microtubules in the mid-region stabilize each other
and become the polar microtubules. (note figure 9.9 is inaccurate, they don’t show any
microtubules overlapping each other.) The polar microtubules interact to form an
elaborate support structure.
Other microtubules also starts out from the poles but get captured by the kinetochore
region of the chromosomes. We call these kinetochore microtubules. Each sister
chromatid has a kinetochore and each kinetochore will attach to microtubules Figure 9.9.
When one chromatid’s kinetochore captures a microtubule, the chromosome begins to
move to the pole, because the kinetochore has a motor protein (kinesin) on it that will try
to walk to the pole.
But the other chromatid (of the same chromosome) now faces the opposite pole and so
attaches to the opposite pole and tries to walk to the opposite pole. Since sister
chromatids are attached to each other, and are trying to move in opposite directions, the
forces are balanced.
Eventually all sister chromatids are attached to microtubules from opposite poles, and so
face opposite poles. figure 9.8 metaphase.
How do the chromosomes get to the center, to the metaphase plate?
Other microtubules that are pushing out from the poles, but do not meet other
microtubules nor kinetochores, become polar wind microtubules. This generates a force
from the poles to the mid-region (a region we call the metaphase plate).
We call this ‘pushing to the center force’, the polar wind.
Somehow the cell knows when all of the chromosomes are lined up at the metaphase
plate. Once all the chromosomes are lined up at the metaphase plate (with the
kinetochores of sister chromatids facing opposite poles) this triggers the next stage -
1. At anaphase the sister chromatids lose the connection in the centromere region as the
cohesions are removed from the centromere region Figure 9.11, page 191, and the polar
wind stops. Once the sister chromatids are no longer attached, each is called a
2. Since sister chromatids are no longer attached to each other, the kinesin motor protein
can now drag the daughter chromosomes to the poles.
Not only do the chromosomes walk to the pole, but the poles move away from each other
and so the chromosomes very quickly separate from each other.
When the chromosomes reach the poles that triggers the last stage of mitosis, telophase.
Telophase is in many ways, the reverse of prophase.
1. During telophase the chromosomes de-condense; they once again become very thin
and very long.
2. The kinetochores dis-assembled.
3. The nucleolus re-assembles.
4. The mitotic spindle dis-assembles.
5. The nuclear envelope re-forms.
You can see all of the stages in a real organism in Figure 9.10, pages 190-191.
In eukaryotes cell division is considered in two parts: (a) division of the nuclear
components and (b) division of the cytoplasmic components.
By the end of mitosis, the division of the nucleus is completed. But cell division is not
complete, the rest of cell division-- division of the cytoplasm and all its components
Separation of the cytoplasmic components of the cell is called cytokinesis.
In animals and single celled prokaryotes without a cell wall, cytokinesis occurs by a
pinching in rather like a balloon being squeezed in half. Figure 9.12a, page 192.
However in plants, fungi and protists with their cell walls another method must be used,
because it is not easy to ‘pinch in’ a wall. Instead a cell plate forms between the two new
cells. This cell plate will organize the new cell wall that will divide the two daughter
cells. Figure 9.12b