Control of the Cell Cycle
One of the characteristics of living things is the ability to replicate and pass on genetic
information to the next generation. Cell division in individual bacteria and archaea usually
occurs by binary fission. Mitochondria and chloroplasts also replicate by binary fission,
which is evidence of the evolutionary relationship between these organelles and
prokaryotes.
Cell division in eukaryotes is more complex. It requires the cell to manage a complicated
process of duplicating the nucleus, other organelles, and multiple chromosomes. This
process, called the cell cycle, is divided into three parts: interphase, mitosis, and cytokinesis
(Figure 1).
1. Summarize what you see occurring in each of the following phases:
G1:
S:
G2:
Mitosis:
Cytokinesis:
2. In the following phases, record if chromosomes are unduplicated (single-stranded,
shaped like an I) or duplicated (double-stranded, shaped like an X):
G1:
S:
G2:
Mitosis:
Cytokinesis
3. How many chromosomes does the cell in G1 have present? How many chromosomes
are present in each cell at cytokinesis?
4. From this, what is the POINT of mitosis and cytokinesis, in terms of number of cells
produced and how the daughter cells relate to the parent cell?
5. Mitosis uses several checkpoints to ensure that cells are not going through abnormal
division. Where in the cell cycle would it be logical to have such checkpoints?
Explain!
During this activity, you will look at control of the cell cycle as it occurs in normal and
abnormal cells through diagram examination, video, and an online game.
Cell division is tightly controlled by complexes
made of several specific proteins. These complexes
contain enzymes called cyclin-dependent kinases
(CDKs), which turn on or off the various processes
that take place in cell division. CDK partners with a
family of proteins called cyclins. One such complex
is mitosis-promoting factor (MPF), sometimes
called maturation-promoting factor, which contains
cyclin A or B and cyclin-dependent kinase (CDK).
(See Figure 2a.)
1. In figure 2a, what is the relationship between
cyclin levels and MPF activity in cells? What occurs
to both levels after mitosis?
2. In figure 2b, describe what happens at step 1.
What is combining to form MPF?
3. After the cell passes through the G2 checkpoint, what happens to CDK? To cyclin?
4. According to the diagram above, which varies in the cell cycle, cyclins or CDK?
Figure 3 above shows the various CDKs that regulate the cell’s progression through the cell
cycle. Describe how the levels of each CDK varies throughout the cell cycle.
Cyclins and CDKs do not allow
the cell to progress through its
cycle automatically. There are
three checkpoints a cell must
pass through: the G1 checkpoint,
G2 checkpoint, and the Mspindle checkpoint (Figure 4). At
each of the checkpoints, the cell
checks that it has completed all
of the tasks needed and is ready
to proceed to the next step in its
cycle.
1. What causes cells to pass
through the G1 checkpoint?
2. External growth factors;
for example, platelet-derived
growth factor (PDGF) stimulates
cells near a wound to divide so
that they can repair the injury.
Why would it be beneficial to
cells to stimulate cell growth in advance of proceeding through DNA replication and
mitosis?
3. What does the G2 checkpoint check for?
4. Why is it in the cell’s best interest to check if there is DNA damage and prevent the
cell from going into mitosis? If a cell’s DNA is damaged beyond a certain point, the
cell may trigger apoptosis, programmed cell death. Why is it a organism’s best
interest to trigger apoptosis if cell has significant DNA damage?
5. What does the M-spindle (metaphase) checkpoint check for? Why would it be
beneficial for a cell to check for spindle attachment, rather than check for proper
separation at the end of mitosis?
Cell Cycle Animations
1. Go to: http://outreach.mcb.harvard.edu/animations.htm. Click on the unit that says
“The Biology of Cancer” on the sidebar.
2. Click the ‘Breast Cancer’ video. It should pop up. Note: If it does not pop up, enable the
pop-up mechanism in your browser. Also, none of these videos have sound.
a. View the section on Anatomy. Describe how and where the milk glands in the
breast produce milk.
b. Click on ‘Normal Cell Activity.’ What is the role of apoptosis in the formation
and maintenance of the lumen in milk glands?
c. Click on ‘Breast Cancer.” How do mutations affect apoptosis?
d. How do benign tumors form?
e. How do cancer cells enter the bloodstream?
f. Where can cancer metastasize to in the body?
3. Return to the ‘Biology of Cancer’ videos.
a. Click on the video for the Cell Cycle. Use this video to annotate/take notes to
supplement Figure 1 and/or your notes on mitosis.
4. Click on the “Checkpoints and Cell Cycle Control” video.
a. What are the ‘rules’ that normal cells follow?
b. What are the two necessary checkpoints for normal cell division?
c. View the ‘normal cell’ undergoing mitosis.
d. What is the G1 checkpoint? The G2 checkpoint? The spindle checkpoint?
e. What, normally, encases sister chromatids during cell division?
f. How do the centrioles play ‘tug of war’ with the chromatids?
g. What cuts the protein hoops and allows chromosomes to separate?
h. View ‘abnormal cell division.’
i. What do the blinking traffic lights represent?
j. What makes these abnormal cells likely to become cancerous?
5. Click on the “Biochemical Pathways of Normal and Cancer Cells” video.
a. Click on ‘Skin Cells in Vitro.” (The first part of the video is a repeat of the
earlier cell cycle video.)
b. What normally triggers cells to divide? What is growth factor?
c. What is apoptosis? What does apoptosis trigger in surrounding cells?
d. Pick a cell to follow the biochemical pathway in normal cells, mutated cells,
and cells undergoing apoptosis.
e. For normal cells, describe the role of growth factor and growth factor
receptors; kinases; Ras; and PTEN.
f. What differs between normal and apoptotic cells?
g. What differs between normal and cancerous cells?
Cell Cycle Game
1. Go to: http://nobelprize.org/educational_games/medicine/2001/
2. Click “Play” (the arrow as part of the animation) and wait for the game to load. You
do not need sound on for the game to work. If your laptop has sound, please turn it
off.
3. While you are waiting for the game to load, click on the cell picture to see the cell go
through one phase of mitosis.
4. Enter the game once it’s loaded. Click through and answer the following questions.
Use the forward and back arrows to navigate within the game.
5. When do cells perform mitosis?
6. How does cell division vary between tissue types?
7. What signals the process of mitosis to begin?
8. What two molecules are responsible for regulating the cell cycle?
9. Navigate to the next page and click the doorway to enter the game.
10. What prompts the cell to undergo division?
11. Navigate through the game by clicking the action the cell should take and read the
reasoning behind why the cell should take that action.
12. Record the cycle the cell goes through, including the various checkpoints.
13. After the game: What role do CDK and cyclin play in the cell cycle?
14. How can tumors develop?