Unit 11

Question 1

Cyclically activated protein kinases

Answer 1

• Phosphorylate & activate multiple proteins involved in DNA replication, mitosis & cytokinesis
• Activated only at appropriate times in cycle, after which they quickly deactivate
• Phosphorylation can be quickly reversed by protein phosphatases
• Concentration does not change during cell cycle
• Activity of each kinase rises & falls cyclically

Question 2

Cyclins

Answer 2

• Bind to cell-cycle kinases before they become enzymatically active
• Cell cycle kinases are known as Cyclin-dependent protein kinases (Cdks)
• Unlike Cdks, cyclins’ concentrations vary in a cyclical fashion during cell cycle

Question 3

M-phase Cdk

Answer 3

• Activated by Cyclin B
  1.) Synthesis of cyclin B begins in S phase
  2.) Accumulates & forms complexes with M phase Cdk throughout S & G2 phases. M phase Cdk still enzymatically inactive
  3.) As complex forms, M-phase Cdk phosphorylated by:
  a.) An activating kinase at site required for its activity
  b.) Inhibitory kinase that overrides activation kinase & inhibits its activity
  4.) M-phase Cdk activated by Cdc25
  5.) M-phase Cdk phosphorylates & activates a variety of target proteins required in M-phase. One target is Cdc25, leading to positive feedback
  6.) Near end of mitosis, M-phase Cdk inactivated by proteolytic degradation of Cyclin B
  7.) Leads cell to exit mitosis & undergo cytokinesis, returning to interphase
  8.) In G1 phase inactivated M-phase Cdk dephosphorylated by specific protein phosphatase. Now ready to form complex with cyclin B

Question 4

Cdc 25

Answer 4

• Activates M-phase Cdk by removing inhibitory phosphate group, leaving activating phosphates
• Is then phosphorylated by M-phase Cdk, which further activates its phosphatase activity, creating positive feedback loop

Question 5

Condensin

Answer 5

• Activated by Cyclin B-M-Cdk-mediated kinase activity
• Participates in chromatin condensation
• Leading to visible M-phase chromosomes

Question 6

Microtubule-associated proteins

Answer 6

• Activated by Cyclin B-M-Cdk-mediated Complex in prophase
• Activation promotes their ability to cross-link microtubules growing from opposite spindle poles; leading to mitotic spindle formation

Question 7

Lamins

Answer 7

• Activated by Cyclin B-M-Cdk-mediated complex in prometaphase
• Causes nuclear envelope to break up into vesicles