Question

1. Which of the following enzymes involved in cell cycle progression is a phosphatase?

   		a)Cdc2
   		b)Cdk 4/6
   		c)Cdc25
   		d)CAK
   		e)Wee 1

1 points   

QUESTION 2

1. Which of the following steps activates MPF for G2 to M phase transition ?

   		a)Dephosphorylation of Cdc2
   		b)Phosphorylation of Cdk 4/6
   		c)Phosphorylation of Cdc25
   		d)Phosphorylation of CAK
   		e)Dephosphorylation of Wee 1

1 points   

QUESTION 3

1. Which of the following enzymes involved in G2 to M phase transition inactivates MPF?

   		a)Cdc2
   		b)Cdk4/6
   		c)Cdc25
   		d)CAK
   		e)Wee 1

1 points   

QUESTION 4

1. Which of the components of MPF is responsible for the actual phase transition of G2 to M?

   		a)Cdc2
   		b)Cyclin E
   		c)Cdk4/6
   		d)Cdc25
   		e)Cyclin B

1 points   

QUESTION 5

1. Which of the following morphologies would be correct for a cell in which CAK were inactivated?

   		A
   		B
   		C
   		D

Answer 1

1) The correct answer is - Cdc25 .
2) The correct answer is - phosphorylation of Cdc25.
The Cdc2 kinase in its active form exists as a complex with cyclin B and together they form MPF (mitosis promoting factor). The activity of MPF in the cell cycle act as a triggering factor for entry of the cell into M-phase. It catalyzes the phosphorylation of lamins and histone1 and is involved in the regulation of events receding cell division like spindle formation, chromatin condensation and fragmentation of the nuclear envelope and organelles like golgi apparatus and endoplasmic reticulum. DNA damage activates the DNA PK or ATM kinases that lead to the activation ofp53, chk 1 and chk2. The chk thus phosphorylates the mitosis-promoting phosphatase, Cdc25c. Phosphorylation of Cdc25c creates a binding site for 14-3-3 proteins and Cdc25c is either catalytically inhibited or sequestered in the cytoplasm. In any case, the phosphorylated 14-3-3 bound form of Cdc25c is prohibited from dephosphorylating and activating the mitotic Cyclin B-Cdc2 kinase and the damaged cells are effectively blocked from entering mitosis.
3) The correct answer is - Wee1
Wee1 phosphorylates the amino acids Tyr15 and Thr14 of Cdk1 and thus lowers the kinase activity of Cdk1 and prevents entry into mitosis.
4) The correct answer is - Cyclin B
A crucial regulatory element of M-phase is the concentration of Cyclin B and its concentration increases with the entry into S-phase which in turn triggers mitosis. The extent of phosphorylation and the activity of the Cdc25 phosphatase are subject to both positive and negative control. Therefore, this regulation system ensures a rapid increase in activity of MPF at the G2/M-phase transition.