QUESTION 1:
(a)Examine the Figure below relating to the S.pombe cellcycle:
1) Describe the function of Cdc25 and Wee1and why deficit of Cdc25 or excess of Wee1 can cause G2arrest
2) b) Describe how deficit of Wee1 or excess ofCdc25 may result in abnormally small cells
(b) Theactivities of Wee1 tyrosine kinase and Cdc25 tyrosine phospatasedetermine the state of phosphorylation of tyrosine 15 in the Cdk1component of mitotic cyclin-Cdk complexes (MPF). When tyrosine 15is phosphorylated, M-Cdk is inactive; when tyrosine 15 is notphosphorylated, M-Cdk is active. Just as the activity of M-Cdkitself is controlled by phosphorylation, so too are the activitiesof Wee1 kinase and Cdc25 phosphatase. The regulation of thesevarious activities can be studied in extracts of frog oocytes. Insuch extracts Wee1 tyrosine kinase is active and Cdc25 tyrosinephosphatase is inactive. As a result M-Cdk is inactive because itsCdk1 component is phosphorylated on tyrosine 15. M-Cdk in theseextracts can be rapidly activated by addition of okadaic acid,which is a potent inhibitor of serine/threonine phosphatases. Usingantibodies specific for each component it is possible to examinetheir phosphorylation states by changes in mobility upon gelelectrophoresis.
(Phosphorylated proteins generally run slower than theirnon phosphorylated counterparts).
1) Based on theresults with okadaic acid, decide whether the active forms of Wee1and Cdc25 are phosphorylated or unphosphorylated. In the figureindicate the phosphorylated forms of Wee 1 and Cdc25 and label thearrows connecting their active and inactive forms to show whichtransitions are controlled by protein kinases and which by proteinphosphatases.
2) Are the proteinkinases and phosphatases that control Wee1 and Cdc25 specific forserine/threonine side chains or for tyrosine side chains? How doyou know?
3) How doesaddition of okadaic acid cause an increase in phosphorylation ofWee1 and Cdc25, but a decrease in phosphorylation ofCdk1?
4) If you assumethat Cdc25 and Wee1 are targets for phosphorylation by activeM-Cdk, can you explain how the appearance of a small amount ofactive M-Cdk would lead to its rapid and completeactivation?
QUESTION 1:
(a)Examine the Figure below relating to the S.pombe cellcycle:
1) Describe the function of Cdc25 and Wee1and why deficit of Cdc25 or excess of Wee1 can cause G2arrest
2) b) Describe how deficit of Wee1 or excess ofCdc25 may result in abnormally small cells
(b) Theactivities of Wee1 tyrosine kinase and Cdc25 tyrosine phospatasedetermine the state of phosphorylation of tyrosine 15 in the Cdk1component of mitotic cyclin-Cdk complexes (MPF). When tyrosine 15is phosphorylated, M-Cdk is inactive; when tyrosine 15 is notphosphorylated, M-Cdk is active. Just as the activity of M-Cdkitself is controlled by phosphorylation, so too are the activitiesof Wee1 kinase and Cdc25 phosphatase. The regulation of thesevarious activities can be studied in extracts of frog oocytes. Insuch extracts Wee1 tyrosine kinase is active and Cdc25 tyrosinephosphatase is inactive. As a result M-Cdk is inactive because itsCdk1 component is phosphorylated on tyrosine 15. M-Cdk in theseextracts can be rapidly activated by addition of okadaic acid,which is a potent inhibitor of serine/threonine phosphatases. Usingantibodies specific for each component it is possible to examinetheir phosphorylation states by changes in mobility upon gelelectrophoresis.
(Phosphorylated proteins generally run slower than theirnon phosphorylated counterparts).
1) Based on theresults with okadaic acid, decide whether the active forms of Wee1and Cdc25 are phosphorylated or unphosphorylated. In the figureindicate the phosphorylated forms of Wee 1 and Cdc25 and label thearrows connecting their active and inactive forms to show whichtransitions are controlled by protein kinases and which by proteinphosphatases.
2) Are the proteinkinases and phosphatases that control Wee1 and Cdc25 specific forserine/threonine side chains or for tyrosine side chains? How doyou know?
3) How doesaddition of okadaic acid cause an increase in phosphorylation ofWee1 and Cdc25, but a decrease in phosphorylation ofCdk1?
4) If you assumethat Cdc25 and Wee1 are targets for phosphorylation by activeM-Cdk, can you explain how the appearance of a small amount ofactive M-Cdk would lead to its rapid and completeactivation?
