Lecture 7

Question 1

G2/M Phase Progression

Answer 1

During G2 CDK1 is kept in an inactive state by the kinases
WEE1 and MYT1. As M phase approaches, the phosphatase
CDC25 is activated activating the CDK1/CyclinB complex
* The activation of CDK1 at the G2/M boundary includes two
independent events: one is CAK-mediated phosphorylation
(indicated in blue) which is necessary for cyclin B-CDK1
complex formation, and the other involves Cdc25-mediated
dephosphorylation.

Question 2

Targets of Active CDK1/Cyclin B
include..

Answer 2

• Directly acting on cell architecture
  – phosphorylates serine residues on lamin
  (breakdown of nuclear membrane)
  – Phosphorylates histones (packaging of DNA into
  nucleosomes, chromosome condensation)
  – Phosphorylates microtubule associated proteins
  (altered behaviour during mitosis)

Question 3

CDC25 Phosphatases

Answer 3

Multiple Key Phosphorylation Events Controls the activity of CDC25
phosphatases
* CDC25 phosphatases (A, B, C) dephosphorylate specific tyrosine and
threonine residues on CDKs (which inhibit CDK function)
* CDC25s are involved in G1-S and G2-M transition
* dephosphorylates inhibitory phosphorylation residues on CDK2 in G1
(CDC25A may be main player here?)
* G2-M transition dephosphorylation of CDK1-CDC25 (A,B,C)
* CDC25 phosphatases are targets of the checkpoint machinery during
DNA damage. They are inactivated or degraded to stop cell cycle
progression
* CDC25A and B overexpression-frequent in cancer-high grade tumours
and poor prognosis

Question 4

G2/M DNA damage Checkpoint

Answer 4

In normal cells Cell Cycle progression is
blocked if DNA damage detected
CDC25 are targets of Checkpoint kinases (CHK)
that are activated in response to DNA damage

Question 5

CDC25 Phosphatases are Overexpressed

Answer 5

in many primary cancers

Question 6

Spindle Assembly Checkpoint

Answer 6

Spindle assembly checkpoint ensures proper segregation of sister chromatids by
inhibiting metaphase to anaphase transition until all chromosomes are attached
correctly to the mitotic spindle. Sister chromatids are held together by cohesins.
In the presence of unaligned chromosomes, separase (a protease which cleaves
cohesins) is kept inactive by securin and Cdk1-cyclinB. After proper alignment of
the chromosomes to the mitotic spindle, cyclinB and securin are ubiquitinylated by
the Anaphase Promoting Complex/cyclosome (APC)-CDC20 complex and degraded.
This leads to the activation of separase, a protease which cleaves a protein link
between sister chromatids called cohesins, releasing sister chromatids, allowing for
metaphase to anaphase transition.