You have isolated several different temperature sensitive mutant yeast strains t

Question

You have isolated several different temperature sensitive mutant yeast strains that stop proliferating when shifted from 25 degree C to 37 degree C. You analyze these mutant strains at the two different temperatures using FACS analysis in which a machine sorts cells according to the amount of DNA they contain. At 25 degree C the FACS plots for all of the strains look like this: After the mutant cell cultures have been grown at 37 degree C you again analyze these cells using FACS. The plots below represent the different plot types from mutant yeast strains: For each of the following temperature sensitive mutations predict which FACS plot (A, B or C) you would expect (Circle the letter corresponding to the FACS plot you would expect)

Explanation / Answer

An inactivating mutation in -cyclin: B As M cyclin is a marker whose concentration rises as cell enters mitosis. It helps in progression of cell cycle from G2 to M phase. So in case of inactivation mutation in M cyclin, M cyclin will not form and cell cannot enter mitotic phase but remains in G2 phase with 4N number of chromosomes. A mutation that prevented initiation of replication: A As the initiation of replication is stopped so cell cannot replicate its genetic material and cells remain in 2N state. A mutation that prevent de-phosphorylation of S-Cdk: A If the S-Cdk complex is not dephosphorylated, It cannot phosphorylate the pre replication complex and DNA replication does not happen and cell remain 2N. An inactivating mutation in anaphase promoting complex: B If anaphase promoting complex is inactivated the cells cannot proceed from metaphase to anaphase and thus remain in 4N state. A mutation stopped production of ATP: A As production of ATP is stopped cells cannot synthesise sufficient nutrients for entering the cell cycle and thus remains in 2N state.

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