You have isolated a strain of temperature-sensitive mutant yeast that divides normally at 25°C. However, at 37°C, the mutant pauses in the cell cycle right before M phase. These mutant yeast cells are NOT defective in M- Cdk. For each one of the following conditions, state whether or not it could be responsible for the behavior of this strain of yeast. Explain your reasoning for each choice.
A. Inactivation of an enzyme that degrades M-Cyclin.
B. Inactivation of the Wee-1 kinase.
C. Inactivation of the Cdc25 phosphatase

Explanation:

A. Inactivation of an enzyme that degrades M-Cyclin.

         M-phase cyclins form M-CDK complexes and drive the cell's entry into mitosis from G2 phase as non-degradation of M-Cyclin resulting from inactivated enzyme can only result in constitutive entry of the cell in to M phase instead cell cycle being arrested at G2 phase,

B. Inactivation of the Wee-1 kinase.

         Wee1 is a nuclear kinase that plays a key role in regulating cell cycle progression. Wee1 inhibits the entry of the cell into M phase by inhibiting Cdk1. Wee1 phosphorylates Cdk1 there inhibiting the kinase activity of Cdk1 which is essential for progression from G2 to M phase. As such any mutation of wee1, and the resulting loss of activity will result in premature entry of cells into M phase instead of getting arrested at the G2 phase resulting in smaller yeast cell.

C. Inactivation of the Cdc25 phosphatase

               CDC25 phosphatases removes inhibitory phosphate residues from cyclin dependent kinases resulting in the activation of CDKs which causes the cells in G2 phase to entry to M phase. Mutation in CDC25 phosphates removes the ability of CDC25 to remove phosphate residues from CDKs thus preventing the entry into M-phase.

Therefore, mutant phenotype of cell cycle arrest prior to M phase can only result form mutation in

Cdc25 phosphatase(option 3)

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