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Cell cycle clock
Operates in the cell nucleus.
Is the master regulator that integrates all of the cytoplasmic signals and ultimately decides whether or not to divide.
Mainly made up of cyclin-CDK complexes
Cell cycle phases
- G0 - quiescent state
- G1 - time b/t generation of a daughter cell and DNA synthesis (~12-15 hrs)
- S - DNA synthesis or replication (~6-8 hrs, varies)
- G2 - Prepares for entrance into mitosis
- M - Mitosis
Time between different mitoses. Includes G1, S, and G2
Phases of mitosis
Prophase - Chomosomal condensation, centromeres begin to assemble at poles of the cell
Metaphase - Chromosomal allignment and attachment to the microtubules of the mitotic spindle; Breakdown of nuclear membrane
Anaphase - Chromatid separation
Telophase - Chromosomal decondensation; New nuclear membrane formation around each set of chromatids; Cytokinesis at the end
Surveillance mechanisms that monitor each step of the cell cycle.
Allow cell to proceed to the next phase of the cell cycle only if each previous step has been sucessfuly completed.
Often blocked/inactivated by oncogenes and mutant tumor suppressor genes.
Examples of checkpoints
- G1 - DNA damage checkpoint - entrance into S blocked if genome damaged
- S - DNA damage checkpoint - DNA replication halted if genome damaged
- G2 - entrance into M blocked if DNA replication not completed
- M - anaphase blocked if chromatids no properly assembled on mitotic spindle
- prevents re-replication of DNA during S
- A) Wild type - Vast majoirty of cells in G1
- B) Treatment 1 - High % of cells in G2/M, indicative of cell arrest
- C) Treatment 2 - High % of cells in sub-G1 fraction (i.e. not a full genome), indicative of cell death
Point at which cells decide to proceed through cell cycle or enter G0
- Before: cells responsive to mitogenic GFs and TGF-beta
- After: cells no longer able to respond to environmental signals
Deregulation of R point signaling is present in virtually ALL type of cancer cells
Cyclin-dependent kinases (CDKs)
Catalytic activity only active when boud to partner cyclin proteins
Cyclin-CDK complexes drive actual execution of cell cycle (make up the cell cycle clock)
CDK4/6, CDK2, CDC2
Regulatory subunits of CDKs
Aid in identification of substrates for CDKs
A, B, D, E
How are cyclin/CDK complexes regulated? Example
- Altered concentration of cyclin proteins - CDK levels rarely change.
- Example, from Xenopus and sea urchin embryos: B cyclins increase in expression in anticipation of M, drop rapidly during M.
Loss of cyclin protins due to rapid degradation triggered by ubiquitin ligases
Rapid degradation forces cells to move in ONE direction through cell cycle, ensures, they can't move backwards into a previous phase
- Cyclin/CDK complexes in later part of cell cycle will inhibit activation of cyclin/CDK complexes before them.
- Ex: A/CDK2 inactivates transcription factoar leading to E expression, contributes to the loss of E expression
Cyclin-CDK complexes and timing
-R point => D
transition => E
to end of G2
- End of G2 => B cyclins, CDC2; triggers cell to undergo mitosis
A levels - closely assciated w/ entrance and exit of S
E levels - rise rapidly after R point, decline throughout S
B levels - increase before M, drop rapidly during M
D levels - not predictably elevated at certain points of cell cycle
- Regulated by presence or absence of mitogenic growth factors: removal of GF => rapid degradation
- (Explains why cells are responsive to GF only up to R point)
Synthesized in cytoplasm, migrates to nucleus
Induced by a variety of signaling pathways (Wnt, Ras MEK, etc.)
Cells will not respond to extracellular signals after the R point due to:
phosphorylation of CDC2 [DOUBLE CHECK]