Topic 9: Cell Cycle Flashcards
List and describe the stages of the cell cycle and the time it takes
G knot: cell is stationary and not in the process of anything
G1: growing (9 hours)
S: DNA replication and repair (10 hours)
G2: growing (4.5 hours)
M: division of cells (30 minutes)
what factors drives each stage to each stage
G0 to G1:
G1 to S: growth factors, nutrients, cell size, DNA damage
S to G2: no factors, moves there when done synthesizing
G2 to M: Cell size, DNA damage and replication
M to cytokenisis (chromosome attachment to spindle)
what is the order of the Cdk complexes?
G1-Cdk complex
G1-S Cdk complex
S-Cdk complex
M-Cdk complex
what is the role of Cdk?
(cyclin dependant kinase) they are expressed consistently throughout the cell cycle and only become activate when their cyclin binds totem allowing them to control the cell activity
what is the role of Cyclins?
they increase in expression when they need activate the Cdks of that stage so they can control the activity of the cell
what are the cyclins and Cdk involved in the mid G1-Cdk complex?
Cyclin D and Cdk4&6
what are the cyclins and Cdk involved in the G1-S Cdk complex?
Cyclin E & Cdk2
what are the cyclins and Cdk involved in the S-Cdk complex?
Cyclin A and Cdk2
what are the cyclins and Cdk involved in the M-Cdk complex?
Cyclin A & B and Cdk 1&2
how does a cyclin activate a CDK? (steps)
1) The cyclin binds to the Cdk and the T loop of the Cdk goes into the cyclin
2) the Cak (Cdk activating kinase) adds a phosphate on the Thr residue to activate the complex
3) at the same time the Wee1 (Cdk-inhibiting kinase) adds an extra phosphate on the Tyr residue to inhibit the complex
4) then the activated Cdc25 comes and removes the phosphate again so the complex becomes active
explain the positive feedbacks of the Cdk Activation
1) the active Cdk phosphoylates the Cdc25 phosphatase
2) the active Cdk stops the Wee1 from adding its inhibitory phosphate because it wants to stay activate
how can cyclin and cdk activity be inhibited and what are some examples?
an inhibitor called p27 (usually in the s phase) pulls part the active complex
p21 will do the dame in s phase if it detects DNA damage
how are cyclins and CKI regulated?
It is when the polyubiquitylation chain gets added to the complex tagging to be degraded by a proteasome
- called proteolysis
how does M-cdk trigger mitosis and what events does it regulate?
the accumulation of M-Cdk after G2 triggers the
- chromsome condensation: pack so its easier to divide
- phorsphorlaytion of nuclear lamins: breaks the nuclear envelope
- spindle formation
- chromosome separation
how does chromosome condensation occur?
protein called condensin (5 subunit with 2 SMC subunits) uses ATP to loop the DNA strands and tighten them
how does the mitotic Cdk promote spindle formation?
it phosphorylates kinesin 5 motor proteins to activate kinetochore microtubules and stimulates centrosome separation
- the kinetochores are the ones that attach to the centrosome of the sister chromatids
- kinetochore have 16 proteins at the centrosome
why is it important for stable chromosome attachment to the kinetochores
so there is an even divide of the DNA in each daughter cell
what is the key regulator of the transition form metaphase to anaphase?
all the chromosomes being attached to their kinetochores, then cdc20 activates APC/C
when does Cdc20 get synthesized and how does it get activated?
it gets synthesized as the cell reaches mitosis and it gets activated by M-Cdk
why is APC/C and what does it allow?
APC/C complex is an ubiquitin ligase family of enzymes that catalyzes the ubiquitiination and degradation of
- serurin
- Mitotic cyclin
What happens when securin is destroyed?
it activates separase and that destroys the cohesion that keep the chromatins together so anaphase can occur
how is mitotic cylcin destroyed and what controls it?
- Cdc20 and a factor called Cdh1 makes APC kill the m cyclin
- cdh1 gets activated in late anaphase and is regulated by G1cylin/cdk complex
what is the key factor that allows exit of mitosis
Cdc14 must dephosphorlyate Cdh1
what is the mechanism for regulation of the G1/S transition
the restriction point: the point of no return, mitogens are no longer needed to reach S phase
What allows a cell in G1 to go into S phase
mitogens and they enter through cell surface receptor like Ras to initiate intrasignalling pathways
describe the mechanism that drives expression of early response gene
1) mitogen ligand binds to mitogen receptor which interacts with Ras
2) MAP kinase is phosphorylated
3) activatatin of transcription regulatory protien
4) Early response genes are expressed
what does early response genes do?
they encode transition factors like FOS, c-Jun and Mac that induce expression of delayed response genes
what does the Delay response genes do?
they are genes that code for Cdk and Mid G1 cyclins (cyclins D) followed by late G1 cyclins (cyclin E)
what is the process of inducing DNA synthesis?
G1 cdk inactivates protien Rb so it can release E2F which encodes genes for cyclins E (G1/S) and A (S) to activate S-cdk
what is Rb
it is a tumor suppressor and it getting inactivated is one of the main events responsible for passage through the restriction point
how is S phase cyclin inhibited and how it is released?
Sic is inhbititing it and it gets free by its protoelysis (polyubiquitinated and degradation) activated by the G1/S cyclin so the s phase cyclin complex is free
why do the G1/S page Cdks phosphorylate Cdh1
it inactivates APC/C so the S phase and the Mitotic cyclins can accumulate
how does DNA get replicated only once?
in early G1and late mitosis, a prereplication complex (preRC) is assembles that the origins of replication. This occurs when APC/C activity is high
how does S cdk trigger S phase?
- phosphorylates several initiator proteins on helicases so that a kinase called DDK can phosphorylate and activate the helicase
- phosphorylates some preRc components to cause it to dismantle
these can not get undone til early G1 due to APC/C activity
how is DNA damage detected?
by the DNA damage response
- ATM: serine kinase
- ATR: theronine kinase
what is checkpoint 1 and 2?
1: G2 to M phase
2: Metaphase to Anaphase
what happens if cell gets halted at check point?
- it will phosphorylate and inhibit Cdc25 on a site with inactive cyclin and Cdk phosphorylation
- activation of p53 to express the CDK inhibitor p21
how does p21 get activated?
by p53 that is not longer being degraded by Mdm2 after seeing DNA damage
what happen when p53 is mutated?
it causes cancer since it
- allows cells to go through the cell cycle even if damaged
- doesn’t induce apoptosis in dire situation
- makes cancer resistant to drugs
what is the extreme reaction of p53
it will cause apoptosis
what damage does ATM fine and what checkpoint does it get checked?
double stranded breaks checked at checkpoint 2
what damage does ATR find and what checkpoint does it get checked?
single strand focus
- mix match
- nucleotides
- stalled replication forks
all checked at both check points
understand the spindle assembly checkpoint and its relationship with the cell cycle machinery
kinetochore with attached chromosomes will send a wait signal of Mad proteins to deliver Cdc20 to the APC/C so cohesion degradation can form
