1.What cellular mechanism(s) ensure that passage through the cell cycle is unidi
ID: 215428 • Letter: 1
Question
1.What cellular mechanism(s) ensure that passage through the cell cycle is unidirectional and irreversible? What molecular machinery underlies these mechanism(s)?
2. What is the functional definition of START? Cancer cells typically lose START control. Explain how the following mutations, which are found in some cancer cells, lead to a bypass of START controls: (a) overexpression of cyclin D, (b) loss of Rb function, (c) loss of p16 function, (d) hyperactive E2F.
3. The Rb protein has been called the “master brake” of the cell cycle. Describe how the Rb protein acts as a cell cycle brake. How is the brake released in mid- to late G1 to allow the cell to proceed to the S phase?
4. A common feature of cell cycle regulation is that the events of one phase ensure progression into a subsequent phase. In S. cerevisiae, G1 and G1/S phase CDKs promote S-phase entry. Name two ways in which they promote the activation of S phase.
Please explain in detail.
Explanation / Answer
1. There are different molecular mechanisms which ensure that cell cycle goes in forward direction. One type of molecular mechanism, proteolysis, ensures that the proteins of the previous phase are not present which prevents the cycle cycle to go backwards. For example, CKI inhibitor protein present in G1 phase is rapidly degraded when cycle moves to S phase. Similarly, Wee1 kinase is degraded when cycle moves from G2 to M phase. Also, once M phase finishes cohesion rings are cleaved by separase.
Proteolysis is performed by proteasomes. The proteins, which are to be degraded, are marked by addition of certain molecules for identification and degradation by proteasome.
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