Cell Cycle Flashcards
1
Q
Cell Cycle Stages
A
G1, S, G2, M
2
Q
G-phase
A
- ‘gap’ or ‘growth’ phases
- cell regulates conditions to decide whether or not to divide
3
Q
S phase
A
- DNA replication
- many origins of replication
- sister chromatids are held together by cohesion complexes
- requires replication of both DNA and chromatin proteins associated with it
- S CDK activation leads to DNA unwinding protein activation
- this initiates replication at replication origins and inhibits proteins needed to allow that origin to initiate DNA replication again
4
Q
Prophase
A
- chromosome condensation
- spindle forms, coordinated by centrosomes
- DNA wound around histones in a bead on a string formation
- histones wrap around each other and attach to a scaffold protein
- these loops associate with each other
- centrosomes are made of 2 centrioles (9x microtubule triplets)
- microtubules nucleate from centrosomes
5
Q
Spindle Formation/Structure
A
- bipolar microtubule array with minus ends of the poles and plus ends radiating out
- microtubule motor proteins help the spindle function
- in animal cells, centrosomes provide prefabricated spindle poles to facilitate spindle assembly
6
Q
Prometaphase
A
- nuclear envelope breakdown
- gives more space in cell with which to divide
- M CDK phosphorylation of nuclear pore complexes in nuclear envelope initiates disassembly of npc / dissociation from envelope
- this phosphorylation of nuclear lamina = vesicular disassembly of nuclear membranes
7
Q
Kinetochores
A
- protein complex bridging between the nucleic acid and microtubules
- also associate with motor proteins that use ATP to move chromosomes
8
Q
Chromosome Separation
A
- chromatids are held together by a cohesin complex
- APC (anaphase promoting complex) activates separase (protease cleaving cohesins)
- APC destroys securin (separase inhibitor)
9
Q
Chromosome Migration
A
- uses the dynamic instability of microtubules (grow with GTPase and shrink when it is removed)
- kinetochore microtubules shorten so chromosomes move to poles (ie. force is generated at the kinetochores)
- overlapping microtubules generate the sliding force from opposite poles to push poles apart
- astral microtubules pull centrosomes on poles to move them apart
10
Q
Telophase
A
- daughter chromosomes arrive at poles/decondense
- reformation of the nuclear envelope
11
Q
Cytokinesis
A
- uses myosin and actin to form a contractile ring at equator
- actin ring contracts to separate two cells
12
Q
Cytokinesis
A
- uses myosin and actin to form a contractile ring at equator
- actin ring contracts to separate two cells
- cleavage furrow narrows to form the midbody, which persists as a tether between the cells before complete separation
- mitotic spindle ensures the correct timing and place for cytokinesis & releases signals initiating furrow midbody
- after cytokinesis the cells enter stable G1 state of low CDK activity (regulatory mechanisms prevent CDK activity)
13
Q
Hartwell & Nurse
A
- worked with budding yeast cells and fission yeast and mutated genes important in cell cycle regulation
- used complementation = method to identify a mutation in the genome / where it is located
- ## found that CDC2 was a key gene regulator and that it makes protein kinases
14
Q
Hartwell & Nurse
A
- worked with budding yeast cells and fission yeast and mutated genes important in cell cycle regulation
- used complementation = method to identify a mutation in the genome / where it is located
- found that CDC2 was a key gene regulator and that it makes protein kinases
- CDC2 activity also correlated with cell cycle stages (cyclic and periodical function)
15
Q
Tim Hunt
A
- examined sea urchin oocytes that are all stuck at the same cell phase + are large enough to visualise
- injected them with radioactive methionine and saw how it was incorporated into the cell cycle activity
- cyclin proteins appear just before mitosis
- cyclin is the activating partner of CDC2 - ie. is the on switch for protein kinase activity
16
Q
Restriction Point
A
- ‘commitment’ point of the cell
- retinoblastoma protein serves as the link between the G1 to S phases and is normally phosphorylasted in the G2/S/M transition
- G1 cyclins bind and activate CDK proteins that hyperphosphorylate it
- Rb interacting proteins like E2F are found in the G1 phase but not S/G2/M
- E2F is a transcription factor that is released by a conformational change and that transactivates genes for S phase entry (transcription specific genes)
17
Q
Quiesence
A
- reversible withdrawal from cell cycle
- referred to as G0
18
Q
S phase entry
A
- extracellular signal molecule triggers G1 cyclin production
- signal cascades trigger transcription factor to produce G1 cyclin
19
Q
Directionality
A
- cyclins give cell cycle directionality
- S cyclin binds to CDK that phosphorylates the G cyclins that activated them
- phosphorylated G cyclin becomes unstable and undergoes proteolysis (ubiquitin binding also leads to this)
- this pattern repeats in a negative feedback cycle
20
Q
Cyclin and CDK functions
A
- G1 = pass through restriction point
- S = DNA replication
- G2 = verify DNA replication
- M = activates APC & stimulates mitotic entry
21
Q
Summary of Cyclin Activity in the Cycle
A
- when cell proliferation conditions are right, internal and external signals activate G1 CDK
- gene expression of genes encoding G1/S cyclins
- G1/S CDK activation drives the cell past the restriction point
- G1/S CDK unleashes S CDK activity to initiate chromosome duplication
- M CDK activation triggers progression through the G2/M transition/alignment of chromatids at spindle equator
- APC/C and its activator (CDC 2) triggers securin and cyclin destruction causing mitosis completion
22
Q
CDK Activation
A
- inactive form has the active site partly obscured by a protein loop
- cyclin binding causes the loop to move away from the active site leading to partial CDK activation
- CAK (cyclin activation kinase) phosphorylates an amino acid near the active site, ie. a conformational change leads to an increase in CDK activity
- CKI (cyclin kinase inhibitors) are inhibitor proteins for the cyclin CDK complexes
23
Q
Metaphase Progression
A
- triggered by protein destruction rather than by cyclin signals
- ubiquitin ligase enzymes (APC) catalyse the ubiquitylation and destruction of securins
- securins inhibit separases that cleave chromatid pairs together and the S/M cyclins so their proteins are dephosphorylated
24
Q
Microtubule Instability
A
- during mitosis you have more microtubules with a shorter half life and are more dynamic and short
- increased ability to be nucleated by centrosomes
- result = dense and dynamic array of microtubules suited for sister chromatid capture
25
Q
Chromosomal Environment
A
- create a local environment promoting spindle formation
- chromatin bound GEF activates RAN GTP binding
- RAN GTP releases microtubule stabilising proteins
- kinetochores attach spindle and sister chromatids
- end is attached to the ND8 complex which attaches the kinetochore
- tension regulates incorrect kinetochore attachment, ie. low tension leads to loosened grip of tubules and high tension shuts off inhibitory signals
26
Q
Spindle Forces
A
- pulls kinetochore + chromatid along the microtubule to the spindle ( + end depolymerization)
- microtubule flux (microtubules are pulled to the poles and dismantled on - ends)
- polar ejection force
27
Q
Cell Division Control
A
- mitogens = trigger G1/S CDK activity and stimulates cell division
- growth factors = stimulates cell growth by promoting macromolecule synthesis
- survival factors = promotes cell survival by suppressing apoptosis
28
Q
Mitogens
A
- stop CDK suppressing activity, ie. ‘releasing brakes’
- mitogens - Myc transcription protein produced - G1 cyclin - G1 CDK activity
- G1 CDK complexes activate E2F proteins that promote a wide range of cell proliferation activities
- without mitogens, E2F dependent gene expression is inhibited by E2F - Rb interaction
- mitogen stimulation causes G1 CDK to phosphorylated Rb, reducing their E2F binding
- Positive feedback = complex and irreversible progression
29
Q
DNA Damage
A
- cell control system can detect and arrest cell cycle entry at S phase or G2/M transition
- p53 protein stimulates gene transcription of p21 gene that binds to CDKs and inhibits them
- replicative cell senescence = permanent nondividing state
- every time a cell divides the telomeres lose some parts and eventually leave DNA exposed leading to cell death
