Cell Division Flashcards
Identify the stages of cell division, and the function of each.
INTERPHASE-
G1 phase (longest phase): checking if enough nutrients, etc.
to move into cell division
S phase: DNA replication
G2 phase: check that all genetic info accurately replicated and ready to move into mitosis. The cell continues to grow and prepares for mitosis. Protein synthesis and organelle replication occur.
MITOSIS-
Mitosis (shortest phase): Consists of division of nucleus + division of cytoplasm
What is the main class of enzymes controlling the cell cycle ?
Cyclin dependent kinases
How are CDKs activated/inactivated ?
Initially inactive: active site of CDK sterically inhibited by T loop
Activated by binding of cyclins (co-factor. Upon cyclin binding to CDK, T loop pulled away from active site and expose bound ATP, allowing access to target proteins) + phosphorylation (CDK activating kinase adds phosphate to threonine in the T loop and inhibitory kinase adds inhibitory phosphate to threonine/tyrosine) and dephosphorylation (acivating protein phosphatase removed inhibitory phosphate).
Describe the concentration of cyclin and activity of CDKs in the different cell division stages.
INTERPHASE:
- cyclin concentrations at lowest during G1, progressively increases through S and G2.
- CDK activity mainly non-existant (because not until cyclin concentrations rise to certain levels that Cdk become active)
- active S-Cdk in G1/S
- acitve M-Cdk in G2/M
MITOSIS:
- cyclin concentrations reach maximum
- CDK activity high
Describe how phosphorylation and dephosphorylation regulate CDK activity.
- Cyclin-cdk complex initatlly inactive
- Then phosphorylated on two sites (one is inhibitory- Wee1, one is activating- CaK) by protein kinases
- Cell makes sure all conditions are satisfied for mitosis to take place
- An activating Phosphatase (Cdc25) comes and removes inhibitory phosphate, resulting in active complex
Describe the normal role of CDC25 and wee1 proteins, and hence the result of CDC25 and wee1 mutations.
CDC25: Activating phosphatase which removes inhibitory phosphate (hence, promotes activation of CDKs)
Wee1: Inhibitory kinase which puts inhibitory phosphate on Cdk (inducing interphase and preventing entry into mitosis, hence preventing cell from dividing when very very small)
CDC25 mutant: lacks CDC25 so no signal to move into mitosis so arrive at size for division (since main overriding factor determining cell division in yeast is size) but continue growing. Results in division at bigger size than normal.
Wee1 mutant: lacks wee1 so nothing preventing early entry into mitosis. Results in cell able to divide at very small size.
What is the main overriding factor determining when cell division begins, in fission yeast ?
Size of the cell
What mutations are responsible for division of abnormally big and abnormally small cells in fission yeast ?
CDC25 mutations: division of abnormally big cells
Wee1 mutations: division of abnormally small cells
Which specific proteins are responsible for the following in fission yeast:
- Putting inhibitory phosphate on CDK
- Putting activating phosphate on CDK
- Removing inhibitory phosphate on CDK
- Putting inhibitory phosphate on CDK: Wee1 (inhibitory kinase)
- Putting activating phosphate on CDK: CaK (CDK activating kinase)
- Removing inhibitory phosphate on CDK: CDC25 (activating phosphatase)
Describe the concentrations of M cyclins and S cyclins during the different stages of cell division, and identify when exactly S-CDK and M-CDK become activated as a result.
The concentrations of S cyclins begins to rise halfway through G1, reaching a maximum at G1/S boundary (which is when S-CDK becomes activated). The concentrations of S cyclins then remains constant at said maximum until the beginning of M phase at which point it progressively decreases (reaches 0 halfway through mitosis)
The concentration of M cyclins is null during G1 and S phases. It begins rising during G2 and reaches a maximum at the G2/M boundary (which is when M-CDK becomes activated). Soon after, still during mitosis, the concentration of M cyclins begins to decreases (reaches 0 halfway through mitosis, a little after S cyclins after reached 0)
Identify the specific components (specific cyclin and specific enzyme) of the main cyclin-CDK complexes, and state the stages of cell division which each complex controls.
D cyclin and CDK4/6 (most of G1, until R point)
E cyclin and CDK2 (G1 after R point and some of S)
A cyclin and CDK2 (half of S)
A cyclin and CDC2 (=CDK1) (other half of S and G2)
B cyclin and CDC2 (=CDK1) (M phase)
How are Cyclin-CDK complexes inactivated after fulfilling their role ?
Cyclin part is ubiquitilated
This tells the cell machinery to destroy the cyclin
CDK becomes inactive
At which stage of cell division is Cyclin B destroyed ?
At the end of mitosis
Describe the process of ubiquitination of cyclins.
- Ubiquitin covalently bound to E1 activating enzyme (there are 2 E1 enzymes in mammalian cells). Energy consuming process generating high energy bond.
- Ubiquitin eventually transferred to E2 conjugating enzyme (35 E2 enzymes in cells).
- Once transferred, it can work in junction with E3 ubiquitin ligase to specifically ubiquinate its favourite substrate (E3 enzyme provides substrate specificity). (100s of these, specific for different substrates in human cells)
Ubiquitin usually transferred to lysine on substrate to generate this kind of bond. Ubiquitins themselves may be polymerised to generate a chain.
Where does destruction of cyclins take place ?
Proteasome
Describe how the structure of proteasomes helps in its function of cyclin destruction.
Core particle in middle forming channel
Regulatory part in top and bottom
Particular proteins in the regulatory particle which recognise ubiquitin
They start to cleave off ubiquitin (which can be re-used) and unfold protein (so can move in energy dependant manner through core aprticle)
In core particle, protein is chopped up into constituents AAs, which are lost out of the bottom of proteosome
How many Sverdberg units does a proteasome have ?
26S
Describe what happens in the R-point.
R-point = G1/S transition
-Essentially, chooses either growth (move into synthesis) or quiescence
1) In non-proliferating cell:
-CDK is kept in inactive form because CDK inhibitor present
-Hence specific protein (Rb) is unphosphorylated, so active and bound to transcription factor adjacent to certain genes.
-Tells cell not to transcribe genes, we do not want to move
into S phase, so do not want S phase specific genes expressed, like for DNA polymerase
2) In proliferating cell (ready to divide)
- Cyclin dependant kinase inhibitor inactivated or not present at all
- CDK activated, and ready to phosphorylate its first substrate (target protein)
- Key target protein in G1 to S phase transition is Rb protein
- Rb becomes phosphorylated, inactive, and falls off DNA
- Transcription factors become active, so get synthesis of genes required for entry into S phase
What is a possible treatment option for cancer, targeting the R point ?
Specific inhibitors of CDK4/6
How do cells ensure that phosphorylated CDC6 cannot somehow rebind to replicative complex ?
CDC6 is degraded by ubiquitin specific pathway
Describe the main mechanisms by which transcription of cell cycle can stop (following DNA damage).
2 main ones:
- p53: directs transcription of cdk-inhibitors (directs transcription of cdk-inhibitors, which structurally interfere with active CDK and convert it into inactive form)
- CHK2 (inhibits cdc25 which is the phosphatase responsible for final step of activating CDK)
How does DNA condensation occur ?
Condensins form handcuffs around DNA and help it become more compact
How does nuclear envelope breakdown and reformation take place ?
- CDK target nuclear lamines (proteins found on inside of nuclear envelope) and pore proteins
- When proteins become phosphorylated by CDK, that contributes to falling apart of nuclear envelope in prometaphase
- Dephosphorylation of Nuclear Pore proteins and nuclear lamines contributes to the later formation of the nuclear envelope in telophase by allowing fusion of nuclear envelope vesicles (which were bound to phosphorylated lamins)
How do chromosomes attach to spindle ?
-Through constricted region present in centromere of chromosomes (kinetochore)
-These kinetochores bind directly to microtubules
-Need to bind to microtubules at both ends of the
spindle
How are sister chromatids held together ?
Through cohesin rings (hold them together until the right stage of the cell cycle)
Describe Roberts Syndrome.
Due to defect in cohesion of sister chromatids (further apart)
Mutation on 8th chromosome
Very rare
Limb and facial abnormalities
How are sister chromatids separated in anaphase ? How is this regulated ?
Separase (proteolytic enzyme) chopps up cohesins into AAs such that chromosomes under tension attached to opposite ends of spindle can now move apart (results in cleaved and dissociatied cohesins)
- Separase normally kept in inactive form (dimer with inhibitory protein called securin).
- At right stage of cell cycle, Anaphase Promoting Complex is E3 ubiquitin ligase and targets securin for ubiquitylation and subsequent degradation
- Without inhibitor protein (securin), separase becomes active and can chop up cohesins to release chromosomes
What are possible consequences of defects in separation of sister chromatids ?
Paired chromosome moving to same end
What happens if cyclins and securins are not degraded by ubiquitination pathway ?
Cells unable to exit mitosis
What are possible consequences of defects at spindle checkpoint (between metaphase and anaphase) ?
Aneuploidy (“presence of an abnormal number of chromosomes in a cell”), which “is consistently observed in virtually all cancers”
