Lecture 6

Question 1

THE ACTIVATION OF CYCLIN E/CDK2

Answer 1

As cyclinD-CDK4/6 complexes accumulate in mid-G1, cyclin Ddependent kinases sequester CKIs of the CIP/KIP/CDKN1
family facilitating cyclinE-CDK2 activation. At same time
expression of Cyclin E is occuring-E2F regulated.

Question 2

As cells progress into late G1, active cyclin E-CDK2 phosphorylates

Answer 2

p27
(which inhibits Cyclin E-CDK2) which marks it for degradation by the
proteosome (SCFSkp2 Ubiquitin protein ligase involved).Allows for more
active cyclin E-CDK2 complexes to be generated-leads to
hyperphosphorylation of RB

Question 3

SUMMARY: RB IS HYPERPHOSPHORYLATED WHICH

Answer 3

INACTIVATES RB
WHICH IS CRITICAL FOR CELL CYCLE PROGRESSION
“R-FACTOR”=CYCLIN D/CDK4 AND CYCLIN E/CDK2
CRITICAL DECISION MAKING POINT

Question 4

RB-E2F
* Nuclear RB protein is a member of the “pocket proteins

Answer 4

The binding of HDAC (histone deacetylase) and the E2F transcription factor to the pocket
region of RB is important for its function.
* RB in hypophosphorylated state binds to HDAC and E2F (which blocks its transactivation
domain)-genes required for S phase gene expression (cyclin E/A are not expressed).
* Growth signal-RB is phosphorylated by cyclin/CDKs-leads to conformational change in RB
protein leading to sequential release of HDAC and E2F.

Question 5

After the cell has progressed through the Restriction point or R-Point
then

Answer 5

removal of growth factors from the medium does not influence
whether it will go forward through S, G2 M.

Question 6

The sequential phosphorylation of RB by Cyclin D/CDK4/6 and then Cyclin E-CDK2
complexes leads to the release of

Answer 6

E2F From RB. E2F is a key transcriptional regulator of
S-phase genes-Positive regulator of cell cycle progression. Active E2Fs1-3 induce
expression of late G1 genes, required for S phase. As the cell traverses the G1/S
transition into S phase cyclin A becomes activated and together with CDK2
phosphorylates both E2F and DP subunits; leads to their dissociation, and loss of
transcriptional activities. E2F1 is degraded.

Question 7

Hsp (heat shock protein)-chaperone protein; E2Fs1, 2 and 3a are classified as

Answer 7

‘activating E2Fs’ positively
regulating S phase gene expression. CAK=CDK activating kinase leads to fully active cyclin/CDK

Question 8

S-Phase gene expression leads to…

Answer 8

Synthesis of S-phase Machinery
* DNA polymerase, topoisomerase, ligase etc
* Initiation Factors
* Structural proteins (e.g. Histones)
*Proteins involved in synthesizing DNA precursor
nucleotides -thymidine kinase
*Spindle assembly checkpoint protein MAD2