24 – Cell Cycle I Flashcards
Cell cycle
G0: cells terminally differentiated
-Cells no longer need dividing
G1: Growth-phase
-Cells grow
-Cell sense growth enough = ready for division
S phase:
-DNA synthesis
G2: 2nd phase of growth
-Not present in all organisms
M pahse:
cell division
Central requirements for cell division
2
- Increase your size
-To prevent daughter cells being smaller & smaller - Duplicate your DNA
How do cells know they have doubled in size? what can cell measure?
Cells can measure concentration
(Enzymatic activity)
–Concentration comes from accumulation
—Accumulation comes with time
more kinase = more phosphorylation
-higher concentration=forward rates of reactions increase.
Timer for a cell
Cyclin concentration = timer
-Completion of mitosis = correlate with cyclin concentration going down
Cyclin = made continuously/Destroyed periodically
Cyclin build up over time = cell know to of under cell division
Mutant yeasts
Mutant yeast – used to:
identify genes that control entry to mitosis
identify a kinase that controls entry to mitosis
The cdc2- mutant fission yeast = failed to go under fission
Cyclin-dependent kinase (CDK)
CDK binds to cyclin = active molecule
-Thresholds of CDK activity activate DNA synthesis (S) & mitosis (M)
End of degradation - binds CDK = synthesize cyclin = synthesis
What if something goes WRONG?!
what could be wrong?
- Bad environment
- DNA damage
…keeps Cyclin-CDK under control
Wee1 Kinase
o Until cells have grown sufficiently
it phosphorylates a critical tyrosine on cyclin dependent kinase, slowing down its activity.
defiencient of wee 1 =
small cells
-progress too rapidly through the cell cycle and cells become too small.
…activates Cyclin-CDK
CDC25 – activates Cyclin-CDK = initiate cell division
-phosphatase- removes the inhibitory phosphorylation on tyrosine 15
It activates it to make sure that you actually continue through cell division.
absence of Cdc25
forget to go through cell division and and your cells grow very large.
Wee1 completes with Cdc25 to regulate activity of CDK complexes
Excess of Wee1 = deletion of excess Cdc25=large cell phenotype
Excess of Cdc25 = deletion of Wee 1=small cell phenotype
cell Measuring length
Protein gradient measures cell length in S. pombe (fission yeast cell)
-Small cells
Cdr2 = wants to inhibit Wee1 but Pom1 inhibits Cdr2
-Large cells
Pom1 stays at poles & Cdr2 inhibits Wee1
POM 1
accumulates only on their poles, only on the distal ends of the cell.
inhibiting proteins that are localized to the middle of the cell.
Cyclin-CDK
Cyclin-CDK phosphorylates DNA replication machinery
Cyclin-CDK phosphorylation = breaks down nuclear envelope
