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Questions and Answers
Cyclin E expression is promoted by the action of E2F.
True
P53 is primarily responsible for promoting the cell cycle progression.
False
The G1/S checkpoint is regulated by the activity of cyclin-CDK complexes.
True
ATM/ATR are kinases that are activated in response to DNA damage.
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The anaphase-promoting complex (APC) inhibits degradation of mitotic cyclins to ensure proper chromosome segregation.
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Proto-oncogenes can become oncogenes through mutations that increase protein activity.
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CHK1 and CHK2 are involved in the activation of the G1/S checkpoint.
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Inhibiting CDK1 activity stops the cell cycle at the G1/S transition.
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The length of the eukaryotic cell cycle varies based solely on the type of cell.
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Cyclins must be bound to cyclin-dependent kinases (cdks) to activate their kinase activity.
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Cdks maintain fluctuating levels throughout the cell cycle.
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The G1/S checkpoint is activated by pro-mitotic extracellular signals.
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The restriction point (R) indicates that a cell can stop responding to extracellular signals.
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Oncogenes are known for functioning as tumor suppressors in cellular processes.
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Permanent entry into the G0 phase occurs in senescent and differentiated cells.
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Cyclin A/B-CDC2 is primarily involved in the G1 phase of the cell cycle.
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Study Notes
Cell Cycle Control
- The cell cycle includes events that must proceed in an orderly manner, requiring effective control mechanisms.
- Cyclin-dependent kinases (Cdks) and cyclins are central components of the cell cycle control system.
- Cdks have kinase activity when cyclin is tightly bound.
- Cdk protein levels are almost constant throughout the cell cycle.
- Different cyclins are found in cells and their protein levels change cyclically.
- Different cyclin-cdk complexes trigger different cell-cycle events.
Checkpoints
- There are three main checkpoints in the cell cycle: G1/S, G2/M, and the metaphase-anaphase checkpoint (spindle assembly checkpoint).
G1/S Checkpoint
- G1 cyclin-CDK complexes become active upon receiving a pro-mitotic extracellular signal.
- These complexes (cyclin D-CDK4/6 in human cells) promote the expression of transcription factors (E2F).
- Transcription factors promote DNA replication by inhibiting RB protein (retinoblastoma protein, an inhibitor of E2F).
G2/M Checkpoint
- Mitotic cyclin-CDK complexes (cyclin A/B-CDK1 in human cells) promote the initiation of mitosis.
- These complexes are synthesized but inactivated during S and G2 phases.
- Damaged or unreplicated DNA activates the G2/M checkpoint by inactivating CDK1 activity.
- ATM/ATR/CHK1/CHK2 kinases are activated in response to DNA damage.
- Wee1 phosphorylates and inactivates CDK1.
Metaphase-Anaphase Checkpoint (Spindle Assembly Checkpoint)
- A critical complex activated during this phase is the anaphase-promoting complex (APC).
- APC is a ubiquitin ligase that promotes degradation of structural proteins associated with the chromosomal kinetochore (i.e., cohesin).
- APC targets the mitotic cyclin (cyclin B in human cells) for degradation to allow anaphase onset.
- The mitotic checkpoint complex (MCC) consists of Bub3, BubR1, Mad2, and Cdc20.
Securin
- Securin inhibits separase, which is necessary for sister chromatid separation.
- Separase is a protease that cleaves cohesin.
Consequences of a Dysfunctional Cell Cycle Control System
- A dysfunctional cell cycle control system can contribute to cancer.
- p53, a tumor suppressor, is activated if DNA is damaged to inhibit cell cycle progression by:
- Cell cycle arrest
- Activation of enzymes to repair DNA
- Programmed cell death
Proto-Oncogenes
- An oncogene is a gene with the potential to cause cancer.
- A proto-oncogene is a normal gene that could become an oncogene due to mutations or increased expression.
- Three ways a proto-oncogene can become an oncogene:
- A mutation increases protein activity and loss of negative regulation.
- Increased gene expression.
- Chromosomal rearrangements.
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Description
This quiz explores the intricate processes involved in cell cycle control, focusing on cyclin-dependent kinases and their roles during various checkpoints. You will learn about the significance of G1/S, G2/M, and spindle assembly checkpoints in maintaining order during cell division. Test your knowledge on how different cyclin-CDK complexes interact and regulate critical events.
