Control of the cell cycle

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  • 9 Control of the cell cycle
    • Checkpoints
      • Cells grow between stages of division so there is enough cytoplasm for cell division
      • 1)G1 phase; nutritional and environmental checking. Stage maintains cell size
      • 2)Restriction point; what cells have to pass through to enter the S phase. In yeast this is the start
      • S phase; DNA replication. Check if environment is favourable.
      • G2; more growth, more cell checks
      • At the end of each stage the cells are checked for errors
      • Mitosis; check environment favourable and if DNA replicated. Anaphase; check if chromosomes are properly attached to spindle
      • Work though negative signalling. The difference in positive signal intensity between 45 and 46 chromosomes would be hard to differentiate leading to fatal errors
        • Any unattached chromosomes therefore send negative signals that inhibit the cycle. As soon as all are attached, a positive signal is sent
          • Unattached chromosomes leads to inhibition of the anaphase promoting complex and anaphase arrest (APC activates the UPS)
            • Inhibition of cyclin degradation (if APC is inactivated) results in anaphase arrest
        • Nature of the signals are phosphorylations and proteolysis
    • Detection of S phase
      • Fluorescent dna dyes used to determine DNA content in a flow cytometer or fluorescence-activated sorter (FAC's)
      • 3H-thymidine  incorporation revealed by autoradiography
      • Bromo-deoxyuridine (BrdU) incorporation visualised by anti-BrdU antibodies
    • Mitosis
      • 1)Prophase; chromosomes visible 2)Prometaphase; chromosomes move to cell centre 3)Metaphase;chromosomes attach to microtubules in cell centre
      • 4)Anaphase; chromosomes pull apart 5)Telophase; spindle detaches 6)Cytokinesis; cytoplasm divides
      • Takes about an hour in humans. Interphase can last 16 hours to a lifetime
      • In frogs eggs cell growth happens without dividing and then divides rapidly
    • Cdk's (cyclically dependent kinases)
      • Requires cyclin for activation. CDk further regulated by phosphorylation/dephosphorylation. They are relatively small proteins (34 to 40KDa)
      • Binds to a regulatory protein called a cyclin. Only cyclin-CDK is an active kinase
        • CDK levels relatively constant. Cyclin is synthesised and degraded (by the ups) throughout cell cycle
          • Mitotis cyclins accumulate during G2. Once at a high conc they can bind to cdk's. When multiple ones bind, the resulting complex is known as mitosis-promoting factor (MRF). Signal for G2 to enter mitosis.
            • Mitosis positive feedback further activated in late G2 by phosphatases
    • Errors in cell cycle
      • DNA Damage, lagging chromosomes (aneuploidy)
      • Unchecked division can lead to cell death, hyperplasia, cancer
      • Withdrawal from the cycle. Senescence (ceases to divide) and differentiation


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