PROBLEM SET 7 MCB2210 Spring 2015 1 In the assay pictured above an extract is made from frog eggs When sperm nuclei are added to the extract the DNA replicates the chromosomes condense the nuclear membrane breaks down and the cyclin is destroyed Chromosomes then de condense and the nuclear membrane reforms In 20 minutes the same steps repeat just as they do in a fertilized frog egg If you treat the extract with RNase to destroy all mRNA in the extract panel b nothing happens when you add the sperm nuclei How is this explained a The cyclin protein is needed in the extract and is destroyed by the RNase so the cell cycle does not start b The cyclin protein is present in the extract but it is not active because it needs to associate with an mRNA from the extract to be active After destroying the mRNA the cyclin cannot activate the cell cycle c CDK protein and cyclin mRNA are initially present in the extract and cyclin protein translation occurs following the addition of the nuclei if you destroy the mRNA the extract cannot make new cyclin protein and the cell cycle does not start d The cyclin protein is present in the extract but the RNase destroys the CDK mRNA Since the CDK cannot be synthesized the cell does not enter the cell cycle e The destruction of mRNA by the RNase destroys both the CDK and cyclin mRNAs and so neither protein can be made Because both CDK and cyclin proteins are absent from the extract the cell cycle does not start 1 2 If a yeast cell in G1 is moved from rich medium to a low nutrient medium the cell will likely a Enter into S phase immediately b Enter into M phase immediately c Stay in G1 longer until it reaches the normal size to enter S phase d Stay in G1 for the same amount of time as if it were left in rich medium and enter S phase when the bud is smaller than normal e Enter into G2 phase immediately 3 Which is the most accurate statement about what controls the phosphorylation events that regulate the cell cycle a the action of a kinase that is present only during mitosis b the action of a kinase that requires bound cyclin for its activity c the action of cyclins that are present only during mitosis d the action of a phosphatase that requires cyclin for its activity e the action of a phosphatase that is only present during one phase of the cell cycle 4 Once the signal that directs a cell to move from G1 to S phase has been generated what does the cell do a The cell replicates its RNA b The cell grows larger c The cell replicates its DNA d The cell degrades its RNA e The cell condenses its chromatin 5 When G1 phase cells and S phase cells are fused together the G1 nucleus begins to replicate DNA What conclusion would you draw from this result a G1 phase cytoplasm contains factors that prevent a cell from going into S phase b G1 phase cytoplasm contains diffusible factors that stimulate the initiation of DNA synthesis c S phase nuclei contain non diffusible factors bound to the DNA that stimulate the initiation of DNA synthesis d S phase cytoplasm contains diffusible factors that stimulate the initiation of DNA synthesis e Fusion of any two cells will always causes S phase 6 As concentrations of cyclin increase in a cell a CDK binds the cyclin subunit and is inactivated b cyclin phosphorylates the CDK to activate it c cyclin de phosphorylates the CDK to activate it d cyclin independently signals the cell to progress in the cell cycle e CDK binds the cyclin subunit and is activated 7 What do CDKs do just before the end of G2 to initiate the events leading to mitosis a They phosphorylate substrates needed for the cell to enter mitosis b They de phosphorylate substrates needed for the cell to enter mitosis c They polyubiquitinate proteins to allow the cell cycle to continue d They add acetyl groups to histones activating the genes with which they are associated e They degrade proteins that are inhibiting progress toward mitosis 2 8 In addition to binding cyclin what else contributes to CDK activation a Phosphorylation at an activating site on the Cdk b Degradation of Cyclin Activating Kinase c Removal of an inhibitory phosphate group from Cdk d Phosphorylation of an activating site on cyclin e a and c 9 What circumstances lead to a halt in the progress of the cell cycle a When chromosomal DNA is damaged b When chromosomal DNA is not damaged c When DNA replication is not completed d When the proteasome is activated e a and c 10 What kind of interaction attaches polyubiquitin to the target protein a Hydrophobic interactions b Ionic interactions c Covalent interactions d Van der waals interactions e Hydrophobic and ionic interacations 11 What would be likely to happen to yeast cells in G1 if you were able to treat them with an inhibitor that specifically inactivated its only Cdk a Cdc25 phosphatase would be activated earlier than usual b The cells would remain in G1 and would not progress toward mitosis c Wee1 kinase activity would be turned off as well d The cells would be driven from G1 to mitosis e The cells would explode 12 You isolate a new species of budding yeast and sequencing the genome you find that it only has one cyclin gene one CDK gene a START checkpoint and a mitotic checkpoint Your advisor suggests that you do a temperature sensitive screen for cell cycle mutants to show that cyclin and CDK work like they do in other organisms In designing the experiment you are thinking about the phenotype that you would look for in the mutants What would you expect to see in a cyclin mutant at the restrictive temperature a A mixture of cells with various bud sizes b Cells with large buds c Cells with small buds d Cells with no buds e A mixture of cells with no buds and cells with large buds 13 The catalytic subunit of maturation promoting factor a produces ATP b transfers a phosphate group from ATP to certain amino acids of specific protein substrates c transfers a phosphate group from ATP to all serine residues of specific protein substrates d converts ATP to ADP e b and d 3 For the following questions 14 21 refer to the diagram below Wee1 CAK and Cdc25 affect entry into mitosis Assume that Wee1 kinase acts first on cyclin CDK Then as a result of Wee1 phosphorylation CAK adds the second phosphate After that Cdc25 phosphatase acts when the cell decides to pass start and proceed Note that there must be another enzyme in the mix a phosphatase that takes the activating phosphate off of CDK during mitosis to reset the system to the inactive state 14 If you made a