1. REGULATION OF CELL CYCLE

3. REGULATION CHECKPOINTS
 Thesestagesrepresentpointswhereacellbecomescommitted
tobeginningacrucialevent

4.  Entry of a cell into M phase is initiated by a protein called Maturation Promoting Factor
(MPF)
 MPF -like enzymes are called Cyclin Dependent Kinases (Cdks)
MPF
A sub-unit
with Kinase
activity
A
regulatory
subunit -
Cyclin

6.  In yeast, a same Cdk (cdc 2) is active at both G1- S and G2-M transition points, but it is
stimulated by different cyclins
 Passage through these points requires transient activation of the Cdk by a specific
cyclin
 In addition to regulation by cyclins, Cdk activity is also controlled by two kinases- CAK
(Cdk activating kinase) and Wee 1 & a phosphatase Cdc 25
 Passage through G1-S (START) requires activation of cdc 2 by G1 cyclins
 Passage from G2- M requires activation of cdc 2 by the Mitotic cyclins

8.  CAK phosphorylates a Threonine residue (Thr 161 ) of cdc 2
 Wee 1 phosphorylates a Tyrosine residue (Tyr 15) of cdc 2
 The effect of Wee 1 keeps the Cdk inactive until the end of G2
 At the end of G2- phosphate at Tyr 15 is removed by cdc 25
 This activates the Cdk molecule and drives the cell into M phase