Question: The first step in characterizing mutants in the cell cycle (cdc) mutants is defining the cell cyc…

The first step in characterizing
mutants in the cell cycle (cdc) mutants is defining the cell cycle
phase at which the mutational block stops the cell’s progress
through the cell cycle. Temperature-sensitive cdc mutants are
useful because they grow and divide normally at a permissive, low
temperature, but express the mutant phenotype at a restrictive,
higher temperature. One can use the drug hydroxyurea to
characterize temperature-sensitive cdc mutants. Hydroxyurea
inhibits the enzyme ribonucleotide reductase, and thus blocks DNA
synthesis by limiting nucleotide substrate availability.
Hydroxyurea inhibition of cell cycle can be overcome by changing
the incubation media to remove the drug. Consider the following
results with two hypothetical yeast cdc mutants (cdcx and
cdcy).

A culture of cdcx mutations is
incubated at the restrictive temperature of 37°C for two hours (the
approximately length of the cell cycle in yeast), so that the
mutant phenotype is expressed. You then transfer the yeast to a
medium containing hydroxyurea at the permissive temperature of
20°C. None of the cdcx mutants divide.

You now reverse the order of the
treatment. You first incubate cdcx cells at 20°C for 2 hours with
hydroxyurea, and then transfer them to a medium without the drug at
37°C. The cdcx mutants divide once.

You repeat these two experiments
with the cdcy mutant cells. The cdcy mutant cells do not divide in
either experiment.

a). Explain the results of the
reciprocal temperature-shift experiments for the cdcx mutants and
identify what phase of the cell cycle the cdcx mutant cells are
blocked at for the restrictive temperature. Use only complete
sentences.

b). Explain the results of the
reciprocal temperature-shift experiments for the cdcy mutants and
identify what phase of the cell cycle the cdcy mutant cells are
blocked at for the restrictive temperature. Use only complete
sentences.