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Week Number: 11
Seminar Date: Wednesday 18th October
Time: 10am-11am
Module: Cell Biology BI503
Continued from Wednesday 18th October: Cell division and the cell cycle.
Cdc2 lives in an inactive and an active state, once it enters miosis it enters the active state,
wee1 keeps cdc2 inactive.
Maturation promoting factor (MPF) – this is a cell cycle checkpoint that regulates the
passage of a cell from the G2 growth phase to the M phase.
James Maller & Paul Nurse worked for Cancer research UK
Their study used a Western Blott on yeast and frogs and for the primary antibody they used
one that recognised cdc2. This was a key study in cell biology.
MPF coincides beside cyclin B. Cyclin B and cdc create cdc2 through phosphorylation. As
soon as a cell is ready to enter mitosis, during the metaphase and anaphase, you have the
spindle formulation that helps create MPF. It is essential mitosis only happens once during
the cell cycle, this is made sure during the metaphase and anaphase cycle.
Key points in time:
1) Paul nurse identifies cdc2 in yeast up to Man: temp sensitive cdc2 mutant in fission
yeast.
2) James Maller purified MPF, isolated it and found that it is made up of cdc2 and cyclin
B.
This led to research on the G2/M transition: there is a universal mechanism of cell cycle
control: cdc2/cyclin B. Cdc2 is a protein kinase; cyclin B is its regulatory Subunit that
interacts with the Cdk.
There are several protein kinases that exist in cells, these are called Cyclin Dependent
protein Kinases or Cdk’s. Cdc2 is now known as cdk1.
How does this work? (image)
1) Cyclin B comes together with Cdc2 (tyrosine and thialdine residue).
2) Wee1 phosphorylates/primes the inactive MPF (kinase).
3) CAK1 phosphorylates the rest of the residue.
4) Cdc25 then dephosphorylates tyrosine15 which opens the substrate binding surface
of MPF to bind to it.
1
, What are substrates of Cdk1/CyclinB?
Consider a cell in interphase:
When a cell goes into mitosis it looks very different.
-Microtubules get integrated into the mitotic spindle.
-Golgi/Er fragment and segregate into the daughter cells.
-Nucleolus gets dispersed.
Cdk1/cyclin B is the master switch that controls the G2/m transition.
G1/S transition
In yeast, Cdk1 is the protein kinase that controls both G1/S and G2/M. At G1/S cdk1 is
associated with G1 cyclins: cln1,2 and 3. In mammalian cells G1/S is regulated by cdk2 in
association of cyclin D and cyclin E. These regulate the early stage of the transition.
Image of cdk1/cyclin B
What are the substrates of cdk2/cyclin D?
1) Transcription factors
o The cell cycle consists of 4 phases.
o Two major control points: initiation of DNA synthesis G1/S entry to mitosis G2/M
o Control points are guarded by Cdks. Cdks are regulated by phosphorylation &
dephosphorylation
Synthesis & destruction of the cell
Another level of regulation:
o Cyclin dependent kinase inhibitors: Cki’s i.e. Protein21 (p21), P27, P15, P16 & P19
In an exam you need to demonstrate you understand the concept, include examples and
more details of them.
2
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