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cell test #4- cell cycle review
Terms in this set (13)
What are the 4 phases of the cell cycle with regards to cell division, and define what happens in each phase?
G1 - Normal Growth or Growth arrest (Go)
S - DNA synthesis
G2 - Cell growth prior to mitosis
M - Mitosis - physical cell division process
What would happen if M phase started before S phase was completed? How does the cell ensure that this doesn't happen?
-If M phase started before S phase was complete then the cell would start to divide before the DNA was fully synthesized. This would probably result in chromosome abnormalities in the daughter cells.
-The cell regulates cell division through a balance of protein complexes which turn on genes at certain times. Proteins which prevent M phase from occurring will be present until they get degraded when S phase is complete. Other proteins go through step-by-step phosphorylation events which can be reversed if the cell is not ready to advance to the next phase.
Explain the Johnson and Rao experiment and the Sea Urchin egg experiment. What common conclusion was reached?
-Johnson and Rao fused frog egg cells together that were in different stages of cell cycle.
-Other researchers were removing the cytoplasm of a sea urchin egg in one phase of cell cycling to another egg in a different phase.
-Both researchers concluded that there were factors in the cytoplasm which directed a cell into a certain stage of the cell cycle.
If you set up a yeast temperature mutant assay, how would you know which mutants were likely to have their mutation involve a cell cycle protein.?
-The mutant with the cell cycle protein defect is the one whose colony cells were all in the same phase of the cell cycle when they died.
-This would imply that they could not make a certain protein that was needed to advance into the next phase and all the cells lived up until the stage and then died.
-These mutations are referred to as temperature sensitive.
Protein which is needed to activate Cyclin dependent kinase (CDKs). Cyclin levels fluctuate based on when they are needed for a particular phase of the cell cycle.
cyclin dependent kinase
proteins with enzymatic (kinase) function that phosphorylate proteins that can activate transcription factors which activate genes needed for the next cell cycle stage
degradation complex responsible for ubiquitin mediated degradation of Cyclins after a particular phase is complete
Control proteins which block the activation of the Cyclin-CDK dimers and inhibit cell cycle progression of a certain stage or of all stages. P-21 is the inhibitor covered in class
Describe the steps that occur to activate and then deactivate a cell-cycle complex. Hint: these are general steps, use the MPF activation steps but state in general terms what occurs.
-Transcription and expression of the cyclin needed for that stage
-Dimerization of a Cyclin and its CDK
-Phosphorylation events and dephosphorylation events which open the enzymatic pocket of the CDK
-Phosphatase activity to restore Cyclin and CDK to native forms
-Ubiquitin addition to the cyclin
-Degradation of cyclin by a proteasome and release of the CDK
7. Although we did not cover this (I bet you like questions that start like this) what do you think Cyclin A and CDK 2 might do, in other words what are their targets? Try to come up with several different things. Hint look at the picture in the slides that show what phase of the cycle they are expressed in and think what happens in this phase. The targets will be proteins involved in this phase. (I am not looking for specifics just general thoughts e.g. if the cycle is M phase: then upregulation of proteins which degrade nuclear membrane, upregulation of spindle apparatus proteins, etc, etc, etc)
Cyclin A and CDK 2 are active throughout S phase and into early G2 phase. Since this is the phase where DNA replication occurs this dimer would have something to do with activating DNA replication. For example the nucleus would need to have 2x as much histone proteins as are normally present so the transcription factors for the genes for histone 2A, 2B, 3, & 4 may be activated. In order to progress to the next cell phase (G2), there must be activation of Cyclin B during late S phase so it can be expressed in M phase. The regulatory region for the expression of Cyclin B would be another possible area that CyclinA CDK2 may target.
8. What is the complex that degrades the Cyclins and describe its structure?
Proteasomes degrade cyclins through a ubiquitin mediated pathway. The proteasome is a barrel shaped protein complex with enzymatic function. It consists of cap units at each end and a stacked ring structre. A protein targeted for destruction is unfolded using ATP hydrolysis and then fed into the proteasome. Each ring structure in the proteasome is able to cleave a different amino acid peptide bond combination. This allows for the protein to be degraded into small peptide pieces.
9. Explain how cells move into S-Phase. What genes get expressed to produce what proteins? Extra question
-The G1 proteins CyclinD and CDK4 activate the transcription factor E2F. E2F is activated by removing the inhibitor protein RB. CDKs are kinases which can phosphorylate other proteins. RB gets phosphorylated by the CyclinD-Cdk4 complex. This causes it to release the E2F transcription factor. When E2F is released it can transcribe the genes to drive the cell into S phase. This is done by producing the S phase Cyclin E.
-P21 can act as an inhibitor can hold the cell in G1 phase by preventing the CyclinD-Cdk4 complex from activating E2F.
What would happen if you had a mutation on the RB gene that produces an RB protein that binds to E2F and the DNA , but the phosphorylation site had a mutation which gave it abnormally strong phosphate binding?
If RB was mutated so that phosphate binding was abnormally strong. Then once phosphorylation occurred it would be harder for the cell to remove the phosphate from RB. When RB is phosphorylated, it releases E2F which transcribes genes promoting the cell cycle. Since RB can not bind back on to the E2F, these genes would be continually active leading to uncontrolled expression of S-phase and other pro-growth proteins. This could possibly lead to uncontrolled cell proliferation which is the hallmark of cancer.
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