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NEW TERMS
The restriction point in mammals and START in yeast are the points in late G1 when a cell becomes
committed to continuing thru the cell cycle
 Restriction point in mammals & START in yeast are the same thing = ↑ conservation over
evolution of organisms
 Once cell gets to a certain size in G1 & passes this point, it is committed to division & goes
through no matter what
 If you starve a yeast cell: it stops growing & never gets to division (thru the cell cycle)
 But if the yeast cell has already passed START, then you starve it: it will go thru with division
(thru the cell cycle) – committed to 1 more round
o Get back to G1, under those controls again that determine whether or not it will go thru
B-Type cyclins: These are the S-phase & M-phase cyclins that contain a destruction box & so are
destroyed late in mitosis
 Cyclins whose transcription are driven by G1 cyclins (mid & late G1 cyclins)
 Targets for ubiquitin ligase, so APC can break them down
 At end of mitosis: end up in kinase-free & cyclin-free period at beginning of G1
 When S-phase cyclins are first produced, they complex w/ cdk to form SPF (S-phase promoting
factor)
Sic1: The inhibitor that immediately binds & inactivates the S-phase cyclin-CDK heterodimer
 Cyclin-dependent kinase inhibitor
 Only becomes activelate in G1 when Sic1 is heavily phosphorylated
o Target of another ubiquitin ligase – gets broken down
o Releasing a large burst of SPF (S-Phase cyclin-cdk)
ORC (Origin recognition complex): A muiltmolecular complex that bind to the origins of replication
throughout the cell cycle
 Throughout cell cycle, bound to origins = docking sites for components of pre-replication
complex
Pre-replication complex: The ORC plus a variety of other proteins that bind to it & prepare the origins
for DNA replication. As long as the components of the pre-replication complex are not phosphorylated,
replication cannot begin. When the S-phase dimers become active, components of the pre-replication
are phosphorylated. Some of the components of the replication machinery are activated in this fashion
(Mcm helicase) while phosphorylated inhibitory molecules dissociate freeing the origins from inhibition.
 Some proteins are actual components of the replication machinery, some are inhibitors
o As long as they’re not phosphorylated, they’re stable
 During G1: don’t fire
 During S-phase: the origins of replication fire
o They’ve been licensed in early G1 by the pre-replication complex
o As soon as SPF activity & S-phase cyclins surge: these components get phosphorylated,
some are activated, inhibitors dissociate & replication begins
o As soon as replication begins, orgins are replicated: ORC accumulates on that origin =
proteins that form the ORC are deposited on the origin
 Origins are never free/naked
 Always have target site for condensation of pre-replication complex
 Can never build pre-replication complex in phosphorylating
environment
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Can only rebuild pre-replication complex when you dephosphorylate
the components that have dissociated – only happens in G1 after you’ve
broken down B-type cyclins
That means you can only replicate DNA once in the cell cyle
DNA DAMAGE CHECKPOINTS
ATM: A kinase that is activated by DNA damage OR a failure to complete DNA replication
 Sensor /signal for DNA damage
 Activates 2 proteins: P53 & CHK kinases
P53: The guardian of the genome. Activated by ATM when DNA is damaged. It directs the transcription
of the CDK inhibitor, p21, which then causes G1 arrest.
 Transcription factor
 If DNA damage cannot be repaired, causes cell apoptosis
CHK kinases: Activated by ATM when DNA is damaged. It phosphorylates cdc25 homologues &
inactivates them. This causes G2 & S arrests.
 Cdc25 = activating phosphatases
o In their absence, cannot activate crucial points of transition in cell cycle
o Get G2 & S phase arrests
 DNA damage shuts down cell cycle until it is repaired
 If can’t be repaired, then the cell dies