Question

Background information:

1.       Temperature-sensitive cell cycle (cdc) mutants are important tools for studying cell cycle. These mutants grow and divide normally at low temperature but express the mutant phenotype at higher temperature.

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

Experimental design and results:

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

a.       None of the cdcx mutants divide.

B.      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.

a.       The cdcx mutants divide once.

C.      You repeat 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 mutant cells are blocked at for the restrictive temperature. Provide reasoning.

·         Identify what phase of the cell cycle the cdcx mutant cells are blocked at for the restrictive temperature. Provide reasoning.

Answer 1

Answer 1: According to the information, the temperature fluctuations prevents division of the cell culcture and changing the media with fresh one with appropriate growth factors resumes the bacterial growth. This holds true for both type of experiments, in which the strain experiences first low then high or first high then low ambient temperature. However, in both experiments, the cells cease to divide after the temperature fluctuates. This clearly suggests that the cell cycle progression halted at either the interphase or immediately after the first division cycle. This is because during interphase, the cell remains in quinscent stage but examines all the necessary conditions in it which favour its division. The ambient temperature acts as a stimulus to accomplish this process because change in temperature activates heat shock proteins which prevent not only cell division (by regulating expression and activity of cell cycle control proteins) but also forces the cell to remain in inert condition.

Answer 2: After the temperature has been restricted, it is very likely that the cells remaind in the G1 phase of cell cycle. This is because the S phase is longer in duration and once the DNA replication takes place in the S phase, it is very unlikely that division will stop. Once completed S phase, a cell is bound to cross the G2 and M phase.