Cell Cycle II Flashcards
what are Cdks and what are they activated by?
they are cyclin dependent kinases; although they depend on the presence of cyclins, CAKs (Cdk-activating kinases) activate them by phosphorilating the cdk-cyclin complexx
what 4 steps does mitosis consist of?
1) condensation of chromosomes
2) assembly of mitotic spindle
3) breakdown of nuclear envelope
4) attachment of chromatids to spindle
what is condensin? what does it do?
it is a five subunit protein complex (2 SMC subunits and 3 non-structural maintenance of chromosomes subunits)
condensin forms a ring-like structure and uses ATP to promotor compaction and resolution of sister chromatids
what protein is condensin related to?
cohesin; the protein that holds sister chromatids together
describe the mitotic spindle
- its a bipolar array of microtubule proteins
- pulls sister chromatids apart at anaphase
- M-Cdk (M-cyclin-Cdk complex) triggers assembly of spindle
- At mitosis stage, microtubules get organized
what are the 3 types of microtubules?
kinetochore microtubules, interpolar microtubules, astral microtubules
what do the kinetochore microtubules do?
attach each chromosome to spindle pole at the kinetochore (large protein structure located at the centromere of each sister-chromatid)
what do the interpolar microtubules hold?
they hold the two halves of the spindle together; plus ends of interpolar microtubules from one pole interact with plus ends from other poles
what do the astral microtubules interact with?
they interact with cell cortex; they radiate outward from the poles and contact the cell cortex helping to position the spindle in the cell
where are microtubules nucleated from?
a specific location called microtubule-organizing center (MTOC); microtubules grow outward form the MTOC from the plus end
what do gamma-turcs do?
bind to negative ends to nucleate and elongate microtubules
what are the two major types of motor proteins on mitotic spindle?
- dyneins: tends to move to center of cell; MINUS END directed
- kinesins: move to periphery of cell; PLUS END directed; has two globular heads and elongated coil-coil tails; *plays important role in chromosome separation
- the assembly and function of the mitotic spindle depends on these proteins
what are the four major classes of motor proteins involved in spindle assembly and function?
kinesin-5
kinesin-14
kinesin-4,10
dynein
what does kinesin-5 do?
it forces centrosomes apart
two motor domains that interact with PLUS end of anti-parallel microtubule. they move these two anti-parallel microtubules past each other to force/push the spindle poles (centrosomes) apart
*walks towards plus end - forcing centrosomes apart. (like opening elevator doors)
if no kinesin-5, then spindle collapses
what does kinesin-14 do?
minus end oriented directed motor with a single motor domain
pulls poles together
what does kinesin-4,10 do?
aka chromokinesins - plus directed motors; push attached chromosomes away from the pole
dyneins
minus end directed motors
link plus ends of astral microtubules to actin skeleton at cell cortex
by moving toward minus end of microtubule, the dynein motors pull the spindle poles away from each other
describe kinetochores
- responsible for attachment of spindle to chromosomes
- spindl microtubules are attached to each sister chromatid at the kinetochore
- this is a giant multilayered protein structure built on the chromosome
- multiple microtubules attach to kinetochore (Ndc80 complex)
- there is an exposed open end for addition and removal of tubular subunits
- removal of tubular subunits leads to force pulling on kinetochore; movement of chromosomes to pole of cell
binding to kinetochore
- bipolar attachement
- formed by attaching kinetochore to microtubule on opposite side of chromosomes
- sister chromatids must attach to opposite poles of mitotic spindle (called bi-oriented)
- formation of stable attachment is detected by kinetochore by tension ( unstable connections not allowed)
what are the 3 forces of chromosome movement?
- depolymerization
- microtubule flux
- polar ejection
force one: depolymerization
a major force PULLS the kinetochore and chromosome TOWARD the spindle pole
depolymerization of the plus end of the microtubule drives the pulling of the kinetochore poleward
force two: microtubule flux
- microtubules are moved toward spindle poles while being dismantled at minus ends
- tubulin added at plus end while being removed at minus end- interpolar microtubules
- escalator
force three: polar ejection force
- kinesin-4,10 on chromosomes interact with microtubules and transport chromosomes from poles
- result in push-pull phenomenon
what are the final steps?
- spindle prepared
- everything is organized
- chromosomes are attached and ready to be separated to become daughter chromosomes
*M-Cdk activates APC/C to complete mitosis; cohesin system
what are the two types of anaphase?
anaphase A: chromosomes move apart (due to spindle microtubule depolymerization at kinetochore; shortening of kinetochore microtubules; movement of daughter chromosomes to poles; forces generated mainly at kinetochores)
anaphase B: separations of spindle poles themselves (by kinesin-5; also dynein pulls pole apart.
what is the final step of the cell cycle?
cytokinesis
its the end of anaphase
- first visible change in cell is the cleavage furrow
- underlying the cleavage furrow is the contractile ring
- composed of actin and myosin filaments
- rings contract, vesicles fuse with membrane to create new membrane
what are the 4 stages of cytokinesis?
1) initiation
2) contraction
3) membrane insertion
4) completion
what does the assembly of the ring result in?
formation of new actin filaments depend on formin; formin is the protein that nucleates assembly of linear, unbranched actin filaments
actin-myosin exerts enough force to split cell into two
what are the 3 classes of extracellular signaling molecules that regulate cell size and number?
class 1: mitogens - stimulate cell division by triggering G1/S-Cdk activity
class 2: growth factors - stimulate cell growth
class 3: survival factors - suppress form of programmed cell death known as apoptosis
mitogens: stats
cells of body only divide when they need to
for this to happen, extracellular signals must be detected from mitogens
examples: PDGF, EGF, EPO (erythropoietin)
how do mitogens act? describe cell-cycle entry into S-Phase via mitogenic activation of the Ras-MAPK pahtway
- mitogen binds to receptor
- Ras causes activation of MAP kinase cascade (mitogen activated protein kinase)
- leads to increase of gene regulatory proteins including Myc
- Myc promotes entry into cell cycle by increasing expression of G1 cyclins
- this activates G1 Cdk-cyclins
- G1-Cdk-Cyclin activates group of gene regulatory factors called E2F proteins
- E2F binds to promoters of G1/S cyclin and S cyclin genes (leads to DNA transcription)
- Enter into S phase of cell cycle
- DNA synthesis begins duplication of chromosomes
how does Rb protein fit into this?
Rb is a regulator
E2F protein is inhibited by interaction with Rb protein
shuts down entry int S phase of cell cycle
**active G1-Cdk phosphorylates Rb to reduce binding to E2F
what happens if Rb protein is not working?
then you would have no control in going into cell cycle and cancer can occur (retinoblastoma)
stats about retinoblastoma protein family
- Rb identified through studies of inherited eye cancer in children called retinoblastoma
- rare childhood tumor: 4 per million
- occurs before age 2
- 95% of cases diagnosed before age 5
- Rb is tumor suppressor (inhibits cell division)
- Rb prevents over-proliferation of cells
- Loss of both copies of Rb genes (no Rb protein made) leads to cell and tumor proliferation of retina (no inhibition means retinoblastoma)
speak on cancer and the cell cycle and ATM/ATR
- DNA damage blocks cell division
- Cell cycle cannot proceed with DNA damage
- DNA damage activates ATM and ATR protein kinases
- these kinases associate with site of damage and phosphorylate (activate) Chk1 and Chk2 proteins (checkpoint kinase 1 and 2)
what is the major target of these Chk1/Chk2?
p53 protein, which stimulates transcription of p21
what does p21 do?
p21 ( a CKI) binds to G1/S-Cdk and S-Cdk to INHIBIT ACTIVITY
- no cell division because there’s DNA damaged, so it must be repaired
this is why if the ATM/ATR systems are not working, they can cause cancer
talk about AT
Ataxia telangiectasia
it is a syndrome, radiation-sensitive or cancer prone in which DNA damage checkpoint is disrupted; in this case, ATM protein
AT patients exhibit higher incidence of lymphoma and leukemia
Ataxia - poor coordination
Telangiectasia - name means small dilated blood vessels
both seen in AT
ATM PROTEIN IS DEFECTIVE IN AT; AUTOSOMAL RECESSIVE; NO DNA REPAIR
AT case study
ATM sense DNA damage
major substrates are Chk1 and Chk2
these activate p53
p53 cases cell cycle arrest and DNA repair occurs
*If DNA damage is real bad then p53 causes apoptosis
*whenever diagnose AT - no X-rays at all. ever
cancer gene stats
Ras - mutated in 30% of cancers
p53 mutated in 50% of human cancers
what is the most important growth signaling pathway?
PI-3 kinase pathway
PI-3 kinase adds ATP to inositol phospholipids (PIP2); activates TOR (target of rapamycin) which activates many factors for cell growth
what are the three mechanisms to coordinate cell growth with Division?
1) rate of cell division determined by extracellular factor leading to cell growth
2) cell growth and division controlled separately by growth factors and mitogens
3) cell growth and division both stimulated by extracellular factor
what is myostatin?
it is a factor that inhibits muscle cell growth
if you knockout myostatin, mouse has a very large muscle
these are being used to treat DMD
