cell cycle regulation Flashcards
Why is there a need for checkpoint controls?
To ensure the faithful replication of the cells genome during S phase, and to ensure the proper allocation of replicated DNA to daughter cells during M phase.
What checkpoint controls are in place in each phase of the cell cycle?
At the G1 to S phase transition, there is a DNA damage checkpoint. The cell will not continue into S phase until DNA is appropriately repaired. During S phase there is another DNA damage check, DNA replication is halted until damaged DNA is repaired. In G2, the transition into mitosis is blocked if DNA replication of the entire genome has not been completed. During mitosis, anaphase is blocked if chromosomes have not appropriately assembled on the mitotic spindle.
What happens to a cells sensitivity to mitogens and cell growth inhibitors beyond the restriction point?
The cell loses sensitivity. Once in S phase the cell will continue through the rest of the cell cycle, even if growth factors were removed.
What is the restriction point?
The point where the cell either commits to quiescence or to undergo DNA replication and cellular division.
Describe another cell cycle check at the G1 to S phase transition. How does this differ between normal and cancerous cells.
Following the advance through the R point, cells must attach appropriately to the ECM before entering into S phase. This is generally mediated by integrins. If this attachment does not occur then progression is halted until attachment is achieved. If this isn’t possible then the cell may undergo anoikis. Tumour cells have lost their anchorage dependence, avoiding this checkpoint altogether. This is because oncogenic ras/src are able to mislead the cell into thinking that full anchorage has been achieved.
What is a CDK
Cyclin dependent kinase
What is a cyclin
A regulatory subunit associated with CDKs. They increase the catalytic activity of CDKs (up to 400,000 fold) by inducing a conformational change in the T loop, moving it out of the substrate binding cleft). They also direct CDKs to their appropriate substrate.
What kind of kinase’s are CDKs
Serine/ threonine kinase’s
What ensures that the cell cycle is a unidirectional process.
Due to the rapid degredation of cyclins following the advance through the checkpoints that they control. This is triggered by the action of highly coordinated ubiquitin ligases. These attach polyubiquitin chains leading to cyclin degredation at the proteosome.
Which cyclin CDK complex’s are responsible for the G1 - S phase transition
CDK4/6-cyclin D, CDK2-cyclin E
Which cyclin CDK complex is responsible for mitosis
CDK1-Cyclin B
How does phosphorylation of cyclins increase their activity?
Cyclin binding alone not sufficient for complete T-loop displacement and subsequent CDK activation. Phosphorylation of a specific theonine residue in the T loop ensures complete displacement. It is mediated by CDK activating kinases.
What is the CDK activating kinase in mammalian cells?
CDK7 - Cyclin H complexes.
What inhibitory phosphorylation does cyclin B experience and how is this overcome.
During interphase when cyclin B synthesis has begun, CDK1-Cyclin B complexes are kept inactive due to phosphorylation of two adjacent residues (Thr14 and Tyr15). These are mediated by the dual specific kinases Wee1 and Myt1. At the G2 to M transition, Myt1 and Wee1 are inactivated, whilst the dual specific phosphatase CDC25 becomes active. This removes the phosphates from Thr14/Tyr15, allowing for activation of CDK1-Cyclin B complexes and subsequent entry into mitosis.
What is meant by a dual specificity kinase/phophatase
Able to phos/dephosphorylate tyrosine residues as welll as serine/threonine residues.
How do D cyclin expression levels differ from the other cyclins>? How does this help identify its role in the cell cycle?
They do not vary dramatically as the cell advances through the cell cycle. The levels of D cyclins depend on the extracellular cues the cell is receiving, specifically by mitogenic growth factors. As a result D cyclins act to convey extracellular signals to the cell cycle clock. This is because the concentration of D cyclins fluctuates alongside the levels of extracellular mitogens, so it is constantly informing the cell of the extracellular environment surrounding the cell.
What group of CDK inhibitors target CDK4/6 complexes and how do they inhibit them.
The INK4 proteins, p16, 15, 18 and 19; these bind to the catalytic subunits of the CDKs preventing their ability to bind to cyclin D.
Which CDK inhibitors act on the CDK cyclin complexes not found in the early stages of mitosis? How do they inhibit their action?
p57kip2, p27kip1, p21cip1. These bind all the CDK cyclin complexes other than CDK4/6-Cyclin D. They bind to both subunits of the complex, and are able to block their kinase activity as a result.
