Question

Which of the following temperature-sensitve mutations could not be responsible for the behavior of this strain of yeast?

a) inactivation of a protein kinase that acts on the mitotic Cdk kinase
b) inactivation of an enzymy that ubiquitylates M cycline
c) inactivation of a phosphatase that acts on the mitotic Cdk kinase

Answer 1

The mutation that could not be responsible is the inactivation of a phosphatase that acts on the mitotic Cdk kinase since its role typically deactivates the kinase rather than activating it, which is necessary for the cell to progress through the cell cycle.

Step-by-step explanation:

The question relates to the behavior of a strain of yeast with a temperature-sensitive mutation and which mutation would not account for this behavior. The strain's unusual behavior could result from a mutation in the cyclin-dependent kinases (Cdks), which are regulating proteins in the cell cycle. Cdks must bind to a cyclin protein and be phosphorylated to fully activate and advance the cell cycle past a checkpoint.

The mutations described in the options include:

• Inactivation of a protein kinase that acts on the mitotic Cdk kinase.
• Inactivation of an enzyme that ubiquitylates M cyclin.
• Inactivation of a phosphatase that acts on the mitotic Cdk kinase.

Considering that Cdks need to be phosphorylated to become active, a mutation that inactivates a protein kinase (which would phosphorylate the Cdk) could explain sensitivity to temperature, as would a mutation that affects ubiquitylation of M cyclin, regulating its degradation. However, the inactivation of a phosphatase, which generally removes phosphates and thus would typically act to deactivate Cdk kinase, would not likely cause issues at restrictive temperatures since dephosphorylation of active Cdk kinase would potentially inhibit cell cycle progression at permissive conditions instead.

Answer 2

The question pertains to temperature-sensitive mutations in yeast that affect cell cycle regulation. Mutations in kinase, phosphatase, or ubiquitination enzymes can all impact Cdk activity, but without specific information on the yeast's behavior, it's challenging to pinpoint which option could not be responsible. Option c is the correct answer.

Step-by-step explanation:

The question relates to the behavior of a strain of yeast and temperature-sensitive mutations affecting the cell cycle, particularly regarding cyclin-dependent kinases (Cdks), protein kinases, and associated regulatory proteins like protein phosphatases and enzymes involved in ubiquitination. In the context of the cell cycle, Cdk activity is crucial for cell cycle progression, and these kinases must be properly regulated by phosphorylation and association with cyclins. Given this information, we can analyze the options provided for the mutations listed:

• (a) Inactivation of a protein kinase that acts on the mitotic Cdk kinase
• (b) Inactivation of an enzyme that ubiquitylates M cyclin
• (c) Inactivation of a phosphatase that acts on the mitotic Cdk kinase

Based on our understanding that both kinases and phosphatases are critical for regulating the activity of Cdk (kinases adding and phosphatases removing phosphate groups), and that ubiquitination typically marks proteins for degradation (like cyclins after they have fulfilled their role), it appears that all three mutations could potentially disrupt the normal function of the Cdk complex. Each mutation would affect the cell cycle, but not all in the same way. Option (a) would prevent the necessary activation of Cdk by phosphorylation, option (b) would interfere with the degradation of cyclins, potentially leading to prolonged Cdk activation, and option (c) would prevent the deactivation of Cdk. However, without further detail about the 'behavior' of the yeast strain mentioned, it is difficult to definitively say which mutation could not be responsible. A clarification of the specific behavior in question would be necessary to identify the unrelated mutation.