Cell Cycle/Control of Cell Cycle Flashcards
what occurs during G1 phase
cells become committed to continue division OR exit from the cell cycle (G0 = quiescence)
what occurs if cells exit from the cell cycle?
G0, they can become non-cycling, terminally differentiated cell OR re-enter the cell cycle in response to external cues (GF, nutrients)
how is the cell transitioned from G1 to S
a mitogenic stimulus is bound to activated mitogen receptor, and a signal transduction cascade initiates to express genes that are needed to proceed to S phase
what is S phase, how long does it take
the cells duplicate its genenomic material, replicated semi-conservatively, takes 8 hours
what are sister chromatids joined by?
cohesin rings around the two molecules
what are cohesin rings made of?
2 core subunits SMC1 and SMC3
what structures are duplicated during S phase?
chromosomes (1 chromatid –> 2 chromatid) centrosome
what is the centrosome
microtubule organizing center of the cell that forms the poles of the bipolar mitotic spindle during mitosis
what is G2?
interval between S phase and mitosis period of rapid cell growth and protein synthesis AN IMPORTANT CHECKPOINT for entering M phase
5 phases of mitosis (M phase)
prophase prometaphase metaphase anaphase telophase
how long does mitosis last?
90 min
what occurs during prophase
- chromatin condenses (become visible) 2.centrosomes separate and move to opposite sides of the nucleus 3. centrosomes start the mitotic spindle
what protein complex condeses chromosomes during prophase?
condensins consist of two core proteins SMC2 and SMC4 ring like complexes
how are condensins activated?
mitotic cyclinB-cdk1 complexes phosphorylate condensin subunits early in mitosis, resulting in their assembly on chromosomes
what occurs during prometaphase
- nuclear envelope breaks down 2. chromosomes become more condensed 3. kinetochore formed at centromere and microtubules attach to it
what causes the nuclear envelope to break down?
phosphorylation of the nuclear pore proteins and lamins by mitotic cyclinB-Cdk1
what are kinetochores
proteins on chromatids where the microtubules of the mitotic spindle attach, that are at the centromere (center) of the chromatids (where the two sister chromatids are linked)
what occurs during metaphase
- chromosomes align on an axis called metaphase plate ANOTHER REGULATION POINT, called the mitotic spindle checkpoint
what is the mitotic spindle checkpoint?
a regulation checkpoint during metaphase where the cell makes sure that every kinetochore is attached by a spindle microtubule (this makes sure that each daughter cell will have one copy of each chromosome)
what makes up the mitotic spindle?
- aster microtubules 2. kinetochore microtubules 3. interpolar microtubules
what are aster microtubules
hold centrosomes in place at the poles of the mitotic spindle
what are kinetochore microtubules
attach to the kinetochores on each side of the sister chromatids and links the chromosomes to the spindle pole
what are interpolar microtubules
two microtubules from opposite poles interact with each other via various microtubule-associated proteins and motor proteins in the middle of the spindle pole THESE ARE NOT ASSOCIATED WITH KINETOCHORES
-formation of this is due to cyclinB-cdk1 phosphorylating microtubule associated proteins
when do the sister chromatids actually separate
in the transition from metaphase to anaphase
how do the sister chromatids separate?
the cohesin keeping them attached is cleaved by separase
what is separase and how is it regulated?
separase cleaves the cohesin complex that keeps sister chromatids together, and it is kept in an inhibited state by securin
how is separase activated
at the metaphase to anaphase transition, APC (anaphase promoting complex) is activated, which mediates the degradation of securin.
once securin is degraded, separase is freed from inhibition
how are sister chromatids freed from each other?
- the enzyme that cleaves the cohesin complex, separase, is inhibited by securin.
- at the transition of metaphse to anaphase, the anaphase promoting complex (APC) is activated and degrades securin.
- separase is no longer inhibited by securin, and can cleave cohesin, freeing the sister chromatids
what occurs during anaphase
- each centromere splits, making 2 free chromatids (freed from cohesin by separase) and centromeres
- each chromatid moves toward a pole
- cell begins to elongate, caused by motor proteins along interpolar microtubules
what happens during anaphase A
chromosomes move toward opposite poles as the kinetochore associated microtubules DEPOLYMERIZE at their plus ends (bc they are microtubules)
what occurs during anaphase B
- elongation and sliding of interpolar microtubules past one aother pushes the two poles apart
- forces exerted by outward facing astral microtubules at each spindle pole pull the poles away from each other
what occurs during telophase
- 2 sets of chromosomes arrive at the poles of the spindle
- nucleus and nucleolus begin to reform
- short and thick chromosomes begin to elongate again to form long and thin chromatin
- formation of the cleavage furrow begins (shallow groove where metaphase plate was)
- division of the clyoplasm begins with the assembly of contractile ring
what allows for the nuclear envelope to reform during telophase?
the nuclear pore proteins and lamins that were phosphorylated to cause breakdown of the envelope during prometaphase are DE-PHOSPHORYLATED by protein phosphatases and reform around chromosomes
what is cytokinesis
division of the cytoplasm during telophase
what makes up the contractile ring during cytokinesis during telophase
myosin and actin filaments, act to pinch in the cell to create 2 daughter cells
how does the cell ensure that the daughter cells get an equal amount of membrane bound organelles?
during G2, the membrane bound organelles either grow in size or number
which membrane bound organelles grow in size to ensure equal amounts are dispersed?
ER
golgi
which membrane bound organelles grow in number to ensure equal dispersion
mitochondria
lysosomes
ribosomes
etc
how goes the golgi partition to both daughter cells, and why is it different than mitochondrial and ER?
in prophase, golgi begins to break up into small vesicles
small golgi vesicles then are partitioned equally between the two daughter cells
this occurs becuase the golgi is only on one side of the nucleus AND single copy, unlike the ER and mitochondria which are multiple copy, in multiple locations and can be easily evenly distributed
what proteins control cell cycling?
Cdk and Cyclins
what are cdk’s?
cyclin dependent protein kinases
are serine/threonine kinases and are only active when bound to cyclin
there are 3 major: cdk1, cdk 2, cdk 4/6
these concentrations are constant through the cell cycle
what are cyclins
family of proteins that control progression through cell cycle when interacting with cdks, and are expressed in cycles (cyclin concentration oscillates during cell cycle because there are different cyclins for each stage)
how are specific target proteins phosphorylated during different stages?
different combinations of cyclin/cdk phosphorylate different proteins
how are cyclin-cdk complexes formed?
they are formed in an inactive state due to inhibitory phosphorylation
they may contain activating phosphorylation, but this is not enough to override the inhibitory
how are cyclin/cdk complexes activated
cell cycle cues activate phosphatase that removes the inhibitory phosphorylations and make it active
how are cyclin signals terminated?
cell cycle cues direct ubiquitination of specific cyclins = destruction
what is ubiquitination
covalent linkage of ubiquitin to lysines of a protein which tags it to multi-complex subunit called proteasome, which degrades it
what cyclin/cdk complexes are involved with driving a cell out of G1 and into proliferative state (S)
cyclinD-cdk4/6
and cyclin E-cdk2
what is Rb? What does it do in relation to initiation of replication for mitosis?
a tumor suppressor protein that binds to E2F (a transcription factor) so that it is unable to induce transcription
in mitosis, Rb is phosphorylated when cyclinD-cdk4/6 and cyclinE-cdk2 are activated. this phosphorylation ensures that Rb CANNOT bind to E2F, therefore, is inactive. this inactivity allows E2F to induce transcription of genes required for DNA synthesis in the S phase
how does E2F induce transcription?
PCNA
DNA polymerases
how is Sphase initated in the cell starting from G1?
- In G1, the ORC binds to origins on chromosomes, and it is joined by MCM. Throughout G1, cdc6 is bound to ORC/MCM which keeps it inactive.
- during transition to S phase, cyclinE-cdk2 complexes phosphorylate cdc6, marking it for degradation so the ORC/MCM is free to initiate assembly of replication fork
what cyclin/cdk is associated with inducing transition into S phase from G1)
cyclinE-cdk2
what cyclin/cdk complexes are associated with inducing proliferation (during G1)
cyclinD-cdk4/6 and cyclinE-cdk2
when does accumulation of cyclin B start and end?
starts at S phase, rises gradually up to mitosis
what does an increase in cyclin B indicate?
an increase in cyclinB-cdk1 complexes
what are cyclinB-cdk1 complexes also called?
MPF (maturation promoting factor)
what inhibitory phosphorylations keep cyclinB-cdk1 complexes inactive?
Wee1
what does it mean that the cyclinB-cdk1 complex is “poised to become active”
cyclinB-cdk1 has been phosphorylated by cdk-activating kinase (cak) but this phosphorylation cannot override the inhibitory phosphorylation done by Wee1
how is cyclinB-cdk1 activated near the end of G2?
cell cycle cues activate Cdc25 phosphatase, which removes the inhibitory phosphorylation done by Wee1, which activates cyclinB-cdk1
what is unique about cyclinB-cdk1?
it can phosphorylate/activate cdc25 phosphatase, which increases the amount of active cyclinB-cdk1
aka it forms a positive feedback loop
what is an important function of cyclinB-cdk1 in prophase?
activation of condensin subunits onto the chromosomes that are being condensed early in prophase
what is an imporant function of cyclinB-cdk1 in prometaphase?
cyclinB-cdk1 phosphorylates the nuclear pore proteins and lamins to degrade the nuclear envelope
what is an important function of cyclinB-cdk1 in prometaphase/metaphase
phosphorylates the microtubule-associated proteins, resulting in formation of interpolar microtubule connections and formation of the mitotic spindle
what are the purpose of the checkpoints of the cell cycle?
to inhibit activity of cyclin-cdk complexes if something is awry or has not been completed
what are the checkpoints during G1?
- damaged DNA
- unfavorable cellular environment - growth factors, nutrients, intercellular signals
is a restriction point where cell decides to enter S phase and complete the division cycle
what is the checkpoint during S?
damaged or incompletely replicated DNA
what is the checkpoint during G2?
damaged or incompletely replicated DNA
what is the checkpoint for M?
chromosome improperly attached to mitotic spindle
what occurs if there is DNA damage detected in G1?
DNA damage in G1 or incomplete DNA replication in S phases activate kinases that phosphorylate p53.
- activated p53 stabilized, accumulates in nucleus, TF for p21
- p21 binds to and inhibits cyclinD-cdk4/6 and cyclinE/cdk2, causing cell arrest in G1 or S
what occurs when p53 is phosphorylated?
p53 is stabilized and accumulates in the nucleus, and acts as a transcription factor to induce expression of cdk inhibitor p21
what happens when cdk inhibitor p21 is expressed
p21 binds to and inhibits both cyclinD-cdk4/6 and cyclinE-cdk2 causing cell cycle arrest in either G1 or S, depending where damage occurred
what happens if there is no damage detected in DNA, and kinases DO NOT phosphorylate p53?
p53 is degraded in proteasomes (doesn’t accumulate in the cell)
what occurs if damage/incomplete DNA synthesis is detected in G2 heading to M phase?
- kinases that phosphorylate p53 activated
- p53 phosphorylated, stabilized, accumulates in nucleus
- p53 acts as transcription factor to induce expression of cdk inhibitor p21
- p21 binds to and inhibits cyclinB-cdk1, causing cell cycle arrest in G2
what is the checkpoint for mitosis itself?
check if spindle microtubules attached to the kinetochores, if not, cannot proceed to anaphase (4th step mitosis)
what occurs if there are defects in any of the checkpoints?
cancer! unregulated cell division, so the cell stays in a high mitotic index
what do chemotherapeutics that are mitotic inhibitors do?
target microtubules, freezes cells in mitosis by inhibiting microtubule dynamics
ex. paclitaxel
vinblastine
vincristine
what are chemo drugs that antimetabolites?
target DNA replication, prevents replicating DNA by inhibiting synthesis of thymidine
ex. methotrexate
5-fluorouracil
what do chemo agents that are DNA damaging agents do?
by artificially damaging DNA, cancer cells can be overwhelmed, since many do not have G2/M checkpoint that monitors DNA damage
too much damage/fragmentation prevents them from completing mitosis
ex. doxorubicin
