Backgroundinformation:

1.      Temperature-sensitive cell cycle (cdc) mutants are important toolsfor studying cell cycle. These mutants grow and divide normally atlow temperature but express the mutant phenotype at highertemperature.

2.      Hydroxyurea inhibits the enzyme ribonucleotide reductase, and thusblocks DNA synthesis. Hydroxyurea inhibition of cell cycle can beovercome by changing the incubation media to remove the drug.

Experimental design andresults:

A.      Aculture of cdcx mutations is incubated at 37°C for two hours (theapproximately length of the cell cycle in yeast), and thentransferred to a medium containing hydroxyurea at 20°C.

a.      None of the cdcx mutants divide.

B.      Youfirst incubate cdcx cells at 20°C for 2 hours with hydroxyurea, andthen transfer them to a medium without the drug at 37°C.

a.      The cdcx mutants divide once.

C.      Yourepeat these two experiments with the cdcy mutant cells.

a.      The cdcy mutant cells do not divide in either experiment.

·        Identify what phase of the cell cycle the cdcx mutantcells are blocked at for the restrictive temperature. Providereasoning.

·        Identify what phase of the cell cycle the cdcx mutantcells are blocked at for the restrictive temperature. Providereasoning.