Cell Cycle Flashcards
3 major functional aspects of the cell cycle
- cell growth - chromosome replication
- chromosome segregation
- cell division
how is the cell cycle controlled?
by a series of biochemical switches
regulatory proteins and checkpoints
chromosomal events
s phase – chromosomes are duplicated DNA synthesis phase
m phase – chromosome segregation
phases of cell division
prophase metaphase anaphase telophase cytokinesis
GAP phases of cell cycle
cells have gap phases to allow more time for growth
includes G1 and G2 phases
G1 phase
between M and S
g2 phase
between S and M
the 4 phases of the cell cycle
g1
s
g2
m
interphase
g1
s
g2
how long is M phase?
1 hour
prophase
chromosomes are condensed into rigid sister chromatids
attach to mitotic spindle
metaphase
sister chromatids line up at equator
anaphase
sister chromatids are pulled apart to poles of spindles
telophase
spindle disassembles
chromosomes packed into separate nuclei
what are the 3 checkpoints of the cell cycle
start
g2/m
meta to anaphase
START checkpoint
if cell passes this point it is committed to the cell cycle
before start – if poor environment cell will not pass
after start – cell will continue even if in poor conditions
also called the restriction point
G2/M checkpoint
is dna replicated?
is the environment favorable?
will trigger chromosome alignment on spindle in metaphase
meta- to ana- phase checkpoint
are all chromosomes attached to spindle?
passage will trigger chromatid separation and cytokinese
the cell cycle control system uses a series of switches made of ____ that turn on various steps.
cdks
cyclin dependent kinases
they phosphorylate proteins downstream to activate them
Cdk _____ changes during the cell cycle but Cdk ____ does not change.
activity changes
level/amount of cdks does not change
Cdks are dependent on the prescence of _____ .
cyclins
must be bound to cyclin to be active
inactive Cdks
a protein called T loop blocks the cave site (active site)
cyclin binds, T loop moves, and cdk can be phosphorylated
cyclins
cyclin presence changes thru the cycle since if they present Cdk will be active - if missing cdks will be inactive
activate Cdks and can direct them to their targets
4 classes of cyclins
- g1/s cyclin
- s cyclins
- m cyclins
- g1 cyclins
g1/s cyclins
start cell cycle by activating cdks in late g1
- -trigger progression thru start
- -cyclin level drops in s phase
s cyclins
duplicate dna by activating cdks after progression thru start
- -stim chromosome duplication
- -s cyclins levels remain high until mitosis begins
m cyclins
activate cdks that stim entry into mitosis at g2/m checkpoint
are removed mid mitosis
g1 cyclins
govern activity of g1/s cyclins
aka control progression thru start
4 Cdks
g1/s - Cdk
s - Cdk
m - Cdk
g1 - cdk
cyclin levels after mitosis
fall
and must fall in order to finish cycle
CAKs
Cdk activating kinase
cause phosphorylation of cyclin-Cdk complex
when cyclin binds Cdk and displaces t loop - phosphorylation of Cdk is caused by CAK
CKIs
Cdk inhibitory proteins
inhibit cyclin-Cdk complexes
bind to both to inactivate
primary use of CKIs
for control of g1/s-Cdk and s-Cdk in early cell cycle
INK4A
CKI involved in g1 phase
- family hereditary melanoma
- -mutation in this gene = loss of activity = loss of inhibition
aka no control over cell cycle
p53
tumor suppressor that influences a lot of other genes
responsible for regulating p21
p21 is a CKI that stops cell division
Wee1 kinase
Cdk inhibitor
by phosphorylating the roof of the cave site
Cdc25
a phosphatase that dephosphorylates the roof to increase Cdk activity
to oppose/undo Wee1 kinase activity
proteolysis of _____ to turn on S-Cdks
CKIs
removal of cki = removal of inhibitory factors
how is proteolysis of CKIs achieved?
by tagging them w/ ubuiqitin and proteasomes will kill them
SCF
a ubiquitin adder
adds it to phosphorylated CDI
adds in g1
to help activate/restore S-cdks complex activity
SCF activity depends on ?
F-Box subunit
which helps scf recognize it’s targets
progression from metaphase to anaphase is triggered not by protein ______ but by _____ .
not by phosphorylation
but by protein destruction
APC/C
anaphase promoting complex/cyclosome
regulator of mvt from metaphase to anaphase
APC/C factors
- -a ubiquitin ligase enzyme
- -catalyzes addition of ubiq. to proteins
- -S and M cyclins are major targets
APC/C activation
activated by binding to cdc20
APC/C affects 2 major proteins
cohesin
securin
cohesin
- glues sister chromatids together along length
- large protein
- a structural maintenance protein
- forms rings around sisters
securin
protects cohesin protein links that hold sisters together
how? by inhibiting the separase enzyme
APC/C will ubiq. securin and it will be destroyed allowing separase to activate
separase
an enzyme that cleaves cohesin
resulting in the separation of sister chromatids
what is the role of PRE-RC in duplicating chromosomes once?
PRE-RC = pre replicative complex
assembles at origins of dna replication
during mitosis no new pre-rc are made thus dna is only duplicated once
assembly of PRE-RC is inhibited by ?
Cdk activity
DNA at the end of S phase
it is a tangled mess and must be organized into sisters to avoid breaks
done thru a 2 step process by condensin
condensin
a 2 step process to condense chromatids
- condense chromosomes
- resolution = sister chromatids becoming distinct separate units
what is condensin?
- -5 subunit protein complex
- -2 smc and 2 non-smc units
- -uses ATP to promote compaction and resolution
order of mitosis
- condensation of chromosomes
- assembly of mitotic spindle
- breakdown of nuclear envelope
- attachment of chromatids to spindle
3 types of microtubules
kinetochore
Interpolar
astral
kinetochore microtubules
attach plus end to kinetochores on chromatids
minus end attached to spindle pole
have associated motor proteins for pulling
Interpolar microtubules
hold the two spindle poles together
attach to Interpolar microtubules from the other pole
plus end interacts w/ other plus end
astral microtubules
connect the spindle to cell cortex
radiate out
helping to position pole in cell
astral and Interpolar motor proteins
their motor proteins work to push chromosomes towards their respective poles
kinetochores
large protein structures on chromosomes located at the centromeres
origination of microtubules
MTOC - microtubule organizing center or centrosome
minus end always stays attached to the centrosome
centrosome is also the site of the spindle poles
assembly and function of mitotic spindles depend on ?
motor proteins
dynein - mvt toward minus end
kinesin - mvt toward plus
4 major motor proteins involved in spindle function
kinesin-5
kinesin-14
kinesin-4,10
dynein
kinesin-5
interacts w/ plus end of anti-parallel microtubules
mvt forces centrosomes apart
the 2 motor domains walk towards the plus end of each mt to push centrosomes apart
if no kinesin-5?
spindles will collapse
kinesin-14
minus oriented
1 motor domain
walks towards the minus end - pulling the poles together - towards center
kinesin-5 and -14 action
oppose each other to stabilize the poles
chromokinesins
also called kinesin-4,10
kinesin-4,10
plus directed
1 motor domain
head domain attached to chromosomes
mvt towards plus end pushes chromosomes away from pole
dynein
minus end directed motor
link astral mts to actin skeleton
mvt pulls the spindle poles away from each other
3 forces of chromosomal mvt
- depolymerization
- microtubule flux
- polar ejection
kinetochore attachment
anchoring proteins - Ndc80
links kinetochore to microtubules
microtubules depolymerize at open plus end which pulls chromosomes towards the centrosome
kinetochore must be properly attached - sister chromatids must attach to opposite poles. a stable attachment is detected by ?
kinetochore tension
force 1 - depolymerization
pulls the kinetochores to centrosome
happens at the open plus end of microtubules
force 2 - microtubule flux
microtubules are moving towards poles while being dismantled at minus end
Interpolar mts add at plus, lose at minus = escalator effect
force 3 - polar ejection
kinesin-4,10 motors on chromosomes interact w/ mts and transport chromosomes
results in a push pull effect - where the chromosome ends trail behind the kinetochore that is being pulled
anaphase A vs. anaphase B
a - chromosomes move apart by mts depolymerization
b - separation of poles by kinesin-5
3 classes of extracellular signaling molecules to regulate cell size/number
mitogens
growth factors
survival factors
mitogens
binds to receptors on cells to activate the Ras-map kinase pathway
resulting in increased gene regulatory proteins - including Myc
Myc
a regulatory protein that promotes entry into cell cycle by increasing expression of g1 cyclins
cells in culture will divide on _____ dependence of ____ levels.
density
mitogens
G1-Cdk-cyclin activates a group of ?
gene regulatory factors called E2F proteins
E2F proteins
bind to promoters of g1/s cyclin and s cyclin genes
to promote DNA synthesis
growth factors
stimulate cell growth
survival factors
suppress apoptosis
E2F protein is inhibited by ?
interaction w/ Rb protein
it shuts down entry into s phase
what stops Rb?
active g1-cdk phosphorylates Rb to stop it from inhibiting E2F
what if Rb protein does not work?
no control over cell cycle
cancer
=retinoblastoma
Retinoblastoma
- -no Rb protein
- -Rb is tumor suppressor and prevents over proliferation of cells
must lose BOTH copies for retina tumors to develop
DNA damage activates ?
ATM and ATR protein kinases
ATM and ATR associate w/ sites of damage to
DNA
will phosphorylate Chk1 and Chk2
Chk1 and Chk2 targets
target p53 proteins to stimulate p21 transcription
p21 is a CKI which will stop cell cycle to allow DNA time to fix itself
AT disease
ataxia telangiectasia
mutation in AT gene - ATM protein
radiation sensitive which damages DNA
mutation in ATM protein so cell cycle is not stopped - DNA not fixed
AT symptoms
ataxia - poor coordination
telangiectasia - small dilated BVs
autosomal recessive
