15.1-15.7 Flashcards
most cells have what between their S and M phases
gap/growth phases
what happens during G1/G2 phases
growth and ensuring major cell cycle events completed
true or false: CDCs occur continuously
false
how do surroundings influence CDC cycle?
- resource availability for unicellular organisms
- neighbor constraints for multicellular organisms
purpose of checkpoints
- ensure cell cycle event doesn’t start before previous one completed
- dependence of initiation of cycle on cell’s surroundings
what type of organisms are inherently synchronous?
sea urchin, frog, and clam oocytes
how can sea urchin, frog, and clam oocytes be induced to undergo meiotic maturation synchronously
by treatment with appropriate hormones
- go from interphase-arrested state to metaphase-arrested state
are the embryonic divisions after fertilization also synchronous?
the early divisions are, not once you get to blastula
why is size of oocyte amenable for studyign
can be micro-injected with biomolecules
budding yeast
Saccharomyces cerevisiae
fission yeast
Schizosaccharomyces pombe
conditional loss-of-function mutants
grow normally at permissive growth condition and arrest when switched to restrictive growth condition
temperature-sensitive lethal mutations
ex. growth occurs at 25 C and stops at 37 C
what is indicator of CDC stage in S. cerevisiae
bud size
what is indicator of CDC stage in S. pombe
cell length
other experimental organisms besides yeast
- Aspergillus nidulans
- Drosophila melanogaster
- mammalian tissue culture cells
normal primary cells
directly organism, stop dividing after a little bit
cancerous cells
immortalized, can divide indefinitely with the right supplements
hybrid cells (hybridomas)
between normal and cancerous cells - immortalized
fusion of S-phase cell with G1
accelerated G1 phase nucleus into DNA replication
fusion of mitotic cells with interphase cells
chromosomes to prematurely condense
fusion of G2 cell with S-phase cell
S-phase continue to replicate, G2 cell didn’t begin to re-replicate
cyclin-dependent protein kinases
small family of protein kinases which are key components of central cell cycle control system
cyclin-dependent kinase structure
2 polypeptides, CDK subunit binds ATP and contains active site
CDK is also active when bound to what?
second polypeptide cyclin
do CDK or cyclin concentrations rise and fall with onset of mitosis
cyclin
each CDK-cyclin complex is only active when/
specific stage of CDC
3 requirements for CDK to be active:
- needs cyclin
- need inhibitory phosphates by Wee1 removed by Cdc25
- needs activating phosphate from CAK
function of Cdk1
promotes mitosis (acts in late G2) by phosphorylating laminate proteins so nuclear envelope can break down
- also phosphorylates other proteins to regulate mitotic spindle assembly
true or false: all cells have only a single cycling and a single CDK
false
true or false: the transition between cycle phases are sharp and irreversible
true
Wee1 kinase
adds inhibitory phosphates to specific tyrosine and threonine residues of Cdk subunit
what are inhibitory phosphates removed by
Cdc25 family of phosphatases
CDK-activating kinase
phosphorylates threonine residue to activate Cdk
2 mains families of cyclin kinase inhibitors
- p16
- Cip/Kip (like p27)
p16
CKI interacts with CDK subunit to prevent cyclin association
Cip/Kip
binds/inhibits CDK-cycling complexes
how is regulation of CDK accomplished
- CKIs
- inhibitory phosphates
- segregating into different regions
- periodic availability of cyclins
what does cyclin B1 have
NES, cytoplasmic retention signal
after use, what happens to cyclins
become highly. unstable and are irreversibly destroyed
what is responsible for abrupt instability of cyclins
activation of specific ligase complexes
ubiquitin ligase E3
covalently attaches multi-ubiquitin chain, marking cyclin for degradation by proteasome
Ub ligase E3 composition
- Ub-activating E1
- Ub-conjugated E2
how do Cdks program own inactivation?
activate cyclin uniquing ligases that lead to cyclin destruction - feed-forward control
if cells don’t receive signal to divide, what happens
enter quiescence (G0) state
can cells get out of quiescence
yes
can cells get out of senescence
no
when do cells become irreversibly committed to cell cycle
late in G1
point when yeast become committed to cell division
START
point when multicellular eukaryotes become committed to cell division
restriction point
extracellular factor that stimulates apoptosis
tumor necrosis factor a
(TNF-a)
reentry into cell cycle is primarily by what?
ubiquitin-mediated destruction of CKIs to relieve inhibition of G1 CDK-cyclin complexes
growth factors
class of peptides and hormones that stimulate proliferation
platelet-derived growth factor
growth factor released by platelets upon blood clotting, contributes to rapid cell proliferation required for wound healing
early response genes
expressed very quickly after serum addition
- activate delayed early response genes
what is cyclin D an example of
delayed early response gene
how are CDKs with their cyclins able to get through the restriction point
by phosphorylating tumor suppressor Rb (retinoblastoma)
what happens when Rb is unphosphorylated
it is bound and inhibiting the transcription factor E2F
what initially phosphorylates Rb
cyclin D in complex with either Cdk 4 or 6
what happens when Rb is phosphorylated
dissociates from E2F, E2F stimulates its own expression and expression of other genes such as cyclin E
what also phosphorylates Rb
Cdk2-cyclin E
autonomously replicating sequences
DNA sequences in yeast that are able to replicate independently of the context of a chromosome
- part of origins of replication in yeast
characteristics of origin region in some organisms
- small consensus sequences
- A/T-rich regions
organisms with more than one chromosome usually have how many OR per chromosome?
multiple
issues with have multiple origins of replication
- firing origins only during S phase
- making sure certain replication complete before mitosis
- firing each origin only once
which origins are activated in early S phase
actively transcribed genes, euchromatin
which origins are activated in late S phase
non-tcr genes, constitutive heterochromatin
prior to replication initiation, what must be assembled on origin?
pre-replication complex
steps of pre-replication complex formation
- origin recognition complex binds
- Cdc6 and Cdt1 bind
- MCM binds
- ORC and Cdc6 not needed
when does assembly of pre-RC take place
end of M to beginning of S (G1)
why is the pre-RC only in G1
- Cdc6 only available during G1
- Cdt1 negatively regulated by geminin, no activity outside of G1 window
what is Cdt1 negatively regulated by
geminin
which parts of the pre-RC are AAA+ ATPases
- Cdc 6
- MCM
what does MCM stand for
minichromosome maintenance complex
how is pre-RC assembly restricted by mitotic CDK-cyclin activity
- CDK phosphorylation of Cdc6 inactivates it
- CDK phosphorylation of MCM removes it from the DNA (S phase)
what 2 kinases help cells transition from pre-replicative to replicative states
- CDK-cyclin - prevents pre-RC assembly
- Dbf4-dependent kinase - initiates DNA synthesis, phosphorylates MCM to change structure and allow initiation
rate-limiting step for replication
binding of Cdc45
what does Cdc45 depend on
CDK-cyclin and DDK activity
what results in the unwinding of DNA
binding of Cdc45 + GINS
what role does Cdk2-cyclin A have
works with DDK to phosphorylate and activate Cdc45+GINS
what does Cdc45+GINS do
activates MCM to convert it from an assembly factor to a helicase (remains part of elongation complex)
when ssDNA is exposed, what happens?
ssDNA bp and RPA bind, then loading of primase/DNA pol a complex
what is DNA pol a replaced with
DNA pol S
what happens to MCM as S phase proceeds
dislodged from chromatin (likely by Cdk-cyclins)
when is cohesin added
S phase
