Lecture 23 – Cell Cycle and G1 Phase – Flashcards
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Experiments with autoradiography (pulse chase) charted the pattern of DNA replication of growing cells and 4 stages of the cell cycle were able to be characterized by following uptake of radioactive phosphate
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How was the cell cycle "discovered"?
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The time between mitosis and DNA replication when cells grow and check for DNA damage and sample environment to make pivotal decision to replicate genomes and progress through cell cycle
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What is the G1 phase?
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10 hours
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How long does the G1 phase last?
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10 hours
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How long does the G1 phase last?
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When the genome is replicated
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What is the S phase?
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7.5 hours
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How long does the S phase last?
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The gap between DNA replication and mitosis in which cells check for completion of DNA replication before entering mitosis
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What is the G2 phase?
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3.5 hours
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How long does the G2 phase last?
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When mitosis and cytokinesis separate replicated genome into 2 daughter cells
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What is the M phase?
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1 hour
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How long does the M phase last?
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When most differentiated cells withdraw from cell cycle
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What is the Go Phase?
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Liver, muscle, nerve cells
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What cells spend most of their time in the Go phase?
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Transitions between the cell cycle phases
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What are check points?
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The transition at which the cell monitors its size, environmental conditions and interactions w/ neighbors and EC matrix
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What is the G1 restriction point?
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When DNA is checked for damage and stops the cell cycle if DNA damage is detected
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What is the G1/S restriction point?
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A protein kinase cascade halts the cell cycle until all DNA is replicated
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What is the G2/M restriction point?
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It delays separation of daughter chromatids until all are attached to mitotic spindle
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What is the metaphase spindle assembly checkpoint?
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Cell division cycle proteins that are required for cell cycle progression
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What are Cdc proteins?
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Biochemical experiments on protein synthesis in early embryos found proteins (cyclins) that are synthesized/destroyed in synchrony w/ cell cycle stages
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How were cyclins discovered?
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A factor that pushes interphase cells into mitosis
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What is the M phase promoting factor?
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By cell fusion and cytoplasm injection experiments
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How was the MPF found?
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A protein product of Cdc2/Cdk1 which is a protein complex (cyclin + protein kinase)
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What did purification of MPF yield?
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More than 10 Cdk genes and 16 cyclin genes
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How many cyclin dependent kinases (cdk) genes and cyclin genes do humans have?
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G1/S, S/G2, G2/M
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What transitions are regulated by Cdks?
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-Phosphorylation of a a threonine in a T-loop (activation loop) stimulates Cdk activity and phosphorylation of another threonine that promotes binding to cyclins
- Active Cdk autophosphorylates to set up a positive feedback loop
-Cyclin binding activates Cdk by changing conformation of active site
- Active Cdk autophosphorylates to set up a positive feedback loop
-Cyclin binding activates Cdk by changing conformation of active site
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How are Cdks positively regulated?
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Particular cyclins are synthesized and degraded for each cell cycle transition
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How do Cyclin levels regulate cell cycle transitions?
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-Phosphorylation of tyrosine + threonine inhibits Cdk activity (this P can be removed by Cdc25 tyrosine phosphatase to promote activity)
- Protein inhibitors such as CKIs (cyclin dependent kinase inhibitors) and INKs help to control Cdk activity in G1 and Go phases
- Protein inhibitors such as CKIs (cyclin dependent kinase inhibitors) and INKs help to control Cdk activity in G1 and Go phases
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How are Cdks negatively regulated?
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Structural proteins and cyclins
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What cell cycle proteins are targeted by ubiquitin ligases?
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APC/C and SCF
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What are examples of cell cycle ubiquitin ligase?
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A large protein complex with E3 activity (ubiquitin ligase that initiates a rxn between E2-ubiquitin-conjugating enzyme and protein substrate) that triggers degradation of mitotic cyclins at metaphase/anaphase transition
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What is APC/C?
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A G1/S E3 ubiquitin ligase that is dependent on phosphorylation to become active
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What is SCF?
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They are to be degraded by proteasomes
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What is the fate of these ubiquitinated proteins?
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-APC ubiquinates securin, a chaperone that keeps separase inactive. Active separase can then degrade cohesin and allow sister chromatids to separate
- APC/C degrades the cyclins required for mitotic Cdk activity to enter a growing or non-cycling Go phase
- APC/C degrades the cyclins required for mitotic Cdk activity to enter a growing or non-cycling Go phase
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When can the cell exit from the M phase into the G1?
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-Differentiation signals such as TGFB can arrest cell cycle by stimulating Cdk inhibitor activity
-Adhesion to neighboring cells via cadherins mediates "contact inhibition of growth" - probably through B-catenin pathway which can regulate gene expression
-Adhesion to neighboring cells via cadherins mediates "contact inhibition of growth" - probably through B-catenin pathway which can regulate gene expression
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What arrests the cell in the Go phase?
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When stimulated by growth factors
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When can cells in G0 enter cell cycle?
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Fibroblast proliferation
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What is an example of growth factors initiating cell cycle entry?
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1) Growth factors from serum initiate signaling cascade
2) PDGF (platelet derived growth factor) turns on genes required to synthesize mRNA + proteins
3) EFG + insulin turn on house-keeping and cell growth genes as well as genes for TFs required to express proteins used to advance cell to G1 restriction point
4) Cell continues through cell cycle
2) PDGF (platelet derived growth factor) turns on genes required to synthesize mRNA + proteins
3) EFG + insulin turn on house-keeping and cell growth genes as well as genes for TFs required to express proteins used to advance cell to G1 restriction point
4) Cell continues through cell cycle
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How is fibroblast proliferation activated?
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Stem cells
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What is a prime example of Go cells?
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Pluripotent stem cells that give rise to all tissues of organism
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What are ESCs?
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A few cells set aside in most tissues that can reproduce themselves and give rise to differentiated cells
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What are tissue stem cells?
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Blood cells, skin cells, skeletal muscle cells, meristematic stem cells in plants
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What are examples of tissue stem cells?
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1) Asymmetric division - one stem cell remains associated w/ niche cell and one committed cell differentiates
2) Symmetrical division - two stem cells are produced
2) Symmetrical division - two stem cells are produced
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How do stem cells divide?
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Adequate cell size, no DNA damage, growth factors/nutrients from EC
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How can cells pass through the G1 restriction point?
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E2F/DPA, pRb and histone deacetylase targets/represses genes required for cell cycle progression via chromatin compaction
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What happens if there are no external growth factors to promote passing the G1 checkpoint?
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1) GFs stimulate receptor tyrosine kinase, integrins or 7 helix receptors
2) These receptors activate MAP kinase pathway
3) MAP kinase pathway leads to synthesis of cyclin D
4) Cdk4/Cyclin D phosphorylates pRb, dissociating it from the repressive complex
5) Acetylation of local chromatin is now allowed
6) Expression of genes required for cell cycle progression
2) These receptors activate MAP kinase pathway
3) MAP kinase pathway leads to synthesis of cyclin D
4) Cdk4/Cyclin D phosphorylates pRb, dissociating it from the repressive complex
5) Acetylation of local chromatin is now allowed
6) Expression of genes required for cell cycle progression
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What happens if GFs are available and stimulate the cell to progress past G1?
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TFs that bind regulatory elements for expression of many cell cycle proteins such as Cdk1 and Cyclin A
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What is E2F/DP1?
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A tumor supressor gene
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What is pRb?
