Notes
Cell Cycle Control
Sections
4 checkpoints
• Ensure major events occur at correct times
G1 checkpoint
- Restriction point to enter S phase
- Checks for DNA damage & favorable conditions
- Nutrient availability in yeast; growth factors in humans
- G1 checkpoint can direct cell into quiescence (G0) if conditions are not favorable
S checkpoint
- Checks for DNA damage before/during replication
- Prevents reduplication of DNA
G2 checkpoint
- Allows entry into mitosis
- Checks for DNA damage
- Ensures DNA is duplicated
Spindle-assembly (M) checkpoint
- During mitosis: allows entry to anaphase
- Ensures all chromosomes aligned at metaphase plate & attached to spindle
Cyclin-cdk complexes
- Active cyclin-CDK complexes control passage through checkpoints by phosphorylating other proteins
- CDK has enzymatic activity, cyclin does not
- Cyclin concentrations rise gradually throughout interphase --> peak during mitosis
- Cyclin degraded at end of mitosis by proteasome
CDK active when:
- Associated with a cyclin
- Phosphorylated at activation site
- Dephosphorylated at its 2 inhibition sites
- Inactive if fully dephosphorylated or phosphorylated
CLINICAL CORRELATIONS
Stomach cells
• Rarely enter G0: rapidly divide
Red blood cells
• Mostly in G0: never divide
Skin fibroblast cells
• Remain in G0 until stimulated to divide
Full-Length Text
- Here, we will learn about cell cycle control in human cells – cell cycle regulation has been studied in a number of model organisms, from yeast to sea urchins.
- We will specifically study cell cycle checkpoints and cell cycle regulation via cyclin-CDK activity.
- To begin, start a table.
- Denote that cell cycle control includes:
- Checkpoints, which ensure that major events in the cell cycle occur at the correct times.
- Cyclin and cyclin-dependent kinases (CDK's), which are intracellular proteins that fluctuate in activity and drive cells through the cycle.
Now, let's illustrate them both. We'll start with checkpoints.
- First, draw a pie chart to represent the cell cycle.
- Demarcate the following periods in interphase:
- G1
- S
- G2
- Finally, demarcate the M-phase (mitosis); we leave out its discreet phases for clarity.
Now, let's add the major checkpoints.
- Show that the cell must pass through the following:
- G1 checkpoint to enter the S phase.
- S checkpoint to begin and complete the S phase.
- G2 checkpoint to enter mitosis.
- Spindle-assembly checkpoint in mitosis to enter anaphase.
Let's learn how each of these checkpoints prevent errors in the cycle.
- Indicate that the G1 checkpoint is also called the Restriction Point. I
- n yeast, this checkpoint is known as START.
- Show that the G1 checkpoint can direct the cell in one of two ways depending on whether growth factors are present:
- If they are, the cell progresses to the S phase.
- If they are not, the cell enters a quiescent phase called G0.
Write that cells remain metabolically active in G0 but do not proliferate. - In yeast cells, progression through the restriction point (START) depends on the presence of nutrients, mating factors and cell size. In animals, however, extracellular growth factors stimulate cell division.
- As a clinical correlation, denote that stomach cells rarely enter the quiescent phase because they have short lifespans and must divide continuously.
- On the other hand, red blood cells spend most of their time in the quiescent phase and do not divide frequently.
- Skin fibroblast cells divide more frequently than red blood cells, but they do not proliferate until extracellular signals, such as injured tissue, stimulate them to divide. Until then, they remain metabolically active in G0.
Now, let's return to the G1 checkpoint.
- Indicate that the G1 checkpoint is also a DNA damage checkpoint.
- If DNA is damaged at a DNA damage checkpoint, the cell cycle pauses for repair (an event known as cell cycle arrest) before entering the following phase (in this case, the S phase).
- Indicate that the S and G2 checkpoints are also DNA damage checkpoints.
- The S checkpoint checks for DNA damage before DNA replication begins.
- The G2 checkpoint checks for DNA damage before mitosis begins.
- Show that the S-checkpoint monitors DNA throughout the replication process to prevent and repair errors.
- We draw the S-checkpoint in the middle of the S phase because it acts throughout the duration of this phase: before, after and during replication.
- Indicate that it also prevents reduplication of DNA.
- Show that the G2 checkpoint checks that DNA is fully replicated before the cell enters mitosis.
- Finally, indicate that the spindle-assembly checkpoint makes sure that all chromosomes attach to the spindle and align on the metaphase plate, so that each daughter cell receives a full set of chromosomes.
- Each of these checkpoints prevents errors by ensuring that later events in the cycle cannot begin until earlier events occur.
Now, that we've drawn the cell cycle's major checkpoints, let's illustrate the role of cyclin and cyclin-dependent kinases in cell cycle regulation.
- Draw cyclin, which is a regulatory protein.
- Cyclin is a protein with no enzymatic activity.
- Show that it binds Cdk, which is an enzyme.
- Cyclin binds to Cdk enzymes to activate them.
- Thus, Cdk activity is "cyclin-dependent."
- Now, draw a graph to illustrate cyclin and Cdk activity during the cell cycle.
- Show that the x-axis represents multiple cell cycles, each of which comprises interphase and mitosis.
- Next, indicate that the y-axis measures the concentration of cyclin in the cell during each of these periods of the cell cycle.
- Illustrate that cyclin concentrations rise gradually throughout interphase and peak during mitosis.
- Show that they drop to zero again at the onset of the following interphase.
- Cyclins are so named because their concentration varies "cyclically" throughout the cell cycle.
- Next, draw the fluctuation in cyclin concentration for the next cycle.
Now, let's add Cdk activity to our graph.
- First, illustrate that Cdk activity peaks whenever the cyclin concentration peaks.
- As cyclin concentration increases, the number of active Cyclin-Cdk complexes increases.
- Now, write that cyclin-Cdk complexes drive entry into mitosis.
Finally, let's learn how cyclins activate Cdk's.
- Return to our diagram of cyclin bound to Cdk.
- First, write that our cyclin-Cdk complex is inactive.
- The cyclin-Cdk complex must be phosphorylated at one site and dephosphorylated at two other sites in order to be active.
- Now, draw another cyclin-Cdk complex.
- Show that one activating and two inhibitory protein phosphates bind to it.
- Use an arrow to show that protein kinases phosphorylate the complex.
- Indicate that this complex is also inactive.
- Next, use an arrow to show that protein phosphatases remove the two inhibitory phosphatases.
- Then, draw a cyclin-Cdk complex with only an activating phosphate on it.
- Label this cyclin-Cdk complex as active.
- Indicate that it promotes mitosis.
- Next, illustrate that proteasomes degrade cyclins at the end of mitosis.
- Let's remind ourselves that cyclin concentrations rise throughout interphase and fall rapidly at the end of mitosis.
- Write that active cyclin-Cdk complexes phosphorylate other proteins that function in cell cycle events.
- These events range from chromosome condensation during interphase to spindle formation during mitosis.
- Next, draw a Cdk protein without cyclin and indicate that it is inactive.
- Then, show that another phosphatase removes the final phosphate from the Cdk protein.
- Finally, illustrate that another cyclin binds Cdk and the cycle begins again.