Exam 4 Lesson 37 Flashcards
What are the four phases of the cell cycle?
G1, S, G2, M
What is the “start” point in yeast or “restriction point” in animal cells?
At the end of G1, before S starts.
What happens at “restriction point”?
mitogen is activated
What happens during G1?
gap and growth
What happens during G1 if cell doesn’t pass cell regulation check point?
It will go to G0.
What phases make up Interphase?
G1 + S + G2
What happens during G2 to regulate cell cycle?
Check for DNA damage
Make sure materials needed are available.
What happens at the different checkpoints of the cell cycle?
G1: Is there enough energy? Is the DNA okay? Is it damaged?
S phase end: is all DNA replicated? Is it not damaged?
M phase: are mitotic spindles attached correctly?
G2: Is DNA damaged? Are all materials ready for M phase?
What do we use (complex) to regulate the cell cycle?
We activate or repress the cdk-cyclin complex for each phase.
What cdk-cyclin complex works with each phase?
G1 = G1-cdk, G1/S-cdk
S and G2 = S-cdk
M- phase = M-cdk
How is the cdk-cyclin complex activated?
- cdk is inactive with inhibitor phosphate attached
- add cyclin
- cdk remains inactive, but has cyclin bound to it
- add cak (activating phosphate) and remove inhibitor phosphate
- cdk-cyclin complex is active
How is cdk-cyclin complex inactivated?
add phosphate inhibitor with a kinase (Wee1 kinase) or
degrade cyclin
How is cyclin degraded?
- start with active cdk-cyclin complex
- add ubiquitin ligase and UB
- Polyubiquinate cyclin
- degrade polyubiquinated cyclin in proteasome
What happens at G1? Name the steps.
- at G1, we check for DNA and material readiness
- If we decide we are damaged, we go to G0.
- If we are not damaged, we proceed as follows:
a. start with cdk 4/6 (inactive) with inhibitor phosphate
b. mitogen through Ras activated cyclin D
c. cyclin D binds to cdk 4/6
d. add cak with activating phosphate
e. lose inhibitor phosphate
f. G1-cdk complex is active
What does G1-cdk complex do?
It drives cell past “start”/”restriction point”
What are the targets of the G1-cdk complex?
Rb protein, which derepresses transcription of E2F.
What does E2F help express?
cyclin E and A
How does G1-cdk help cell get to S-phase?
It allows the assembly of G1/S-cdk and S-cdk with cyclins E and A.
When does G1/S-cdk take place?
it is activated in late G1. Cyclin E and cdk2 form G1/S-cdk. Then cyclin E levels fall in S phase.
What complex is built during S phase? How?
- start with inactive cdk 2 with Pi.
- add cyclin A
- cdk 2 still inactive but has A bound to it, and has activating phosphate
- add cak, remove Pi
- S-cdk is active
What are the targets of S-cdk?
DNA polymerase for replication.
Origin of replication complexes - prevent rereplication.
Through what phases does S-cdk work?
Through S and G2
What complex works in M phase? How is it assembled?
- start with inactive cdk1 with Pi
- add cyclin B
- still inactive but with B bound
- add Cak and activating phosphate
- use cdc25 to remove Pi
- M-cdk is active
What are the targets of M-cdk?
condensins - condense DNA for mitosis
also triggers mitotic spindle assembly and phosphorylates nuclear lamina for disassembly of nuclear lamina
What transition happens during M phase?
a metaphase-anaphase transition regulated by APC/C.
How APC/C activated?
By association with cdc20. Cdc 20 recognizes specific amino acid sequences on M-cyclin and other target proteins.
How does APC/C activation work? Name steps.
- Start with active M-cyclin complex and inactive APC/C.
- add cdc20
- bind APC/C and cdc20 to activate APC/C
- Add proteins E1, E2 and UB
- Polyubiquinate cyclin B and degrade to proteasome.
What does APC/C do after it turns M-cyclin off? How?
It initiates sister-chromatid separation by:
a. ubiquitylation and destruction of securin, which holds separate in an inactive state
b. separate can then cleave Scc1, which holds sister chromatids together
c. the pulling forces of mitotic spindle then pull sister chromatids apart
what is securin?
normally holds separate in an inactive state
what is separase?
a protease that cleaves Scc1
What is Scc1?
a submit of the cohesin complex holding the sister chromatids together
When does mitotic spindle form?
metaphase, M phase
when are sister chromatids separated?
anaphase
How does G1 phase come about after mitosis?
cdc20 and APC/C trigger M-cyclin destruction
How does M-cdk/ cdc20/APC/C cycle work?
M-cdk leads to cdc20/APCC complex activity, which destroys M-cdk. When levels of M-cdk go down, do does cdc20/APCC activity, so M-cyclin can build up again
what complex activity keeps M-cyclin levels low in G1?
Cdh1-APC/C complex
Let’s look at entire cell cycle control system. What will stimulate G1-cdk activity? What will G1-cdk lead to?
favorable extracellular environment
G1/S-cyclin synthesis + S-cyclin synthesis
What inhibits both G1/S-cyclin and S-cyclin?
DNA damage
What does S-cdk lead to?
Synthesis and G2
What inhibits M-cdk?
unreplicated DNA (from synthesis) and damaged DNA
What does M-cdk inhibit? Stimulate?
DNA re-replication
Phase M
What inhibits APC/C?
chromosomes unattached to spindle
What does APC/C lead to?
anaphase
What happens if cell can’t replicate DNA or divide?
CDK inhibitors expressed/Activated (CKIs)
Example of a CKI
p27
What happens if there is DNA damage due to X-rays?
a. DNA is broken
b. ATM/ATR kinase activation
c. Chk1/chk2 kinase activation
d. p53 is phosphorylated
e. p53 enables CKI p21 expression
f. p21 inhibits G1/S-cdk and S-cdk
what is p53?
Normally held inactive by Mdm2, it is a transcription factor that, when stabilized, can lead to expression of CKI p21
How do we control CDK inhibitors?
a. activate SCF complex with F-box protein
b. phosphorylate active CDK
c. add active SCF complex, proteins E1 and E2 (ubiquitylation enzymes) and UB
d. add polyubiquitin chain to CKI and degrade in proteasome
What is SCF?
ubiquitin ligase
what is the spindle assembly checkpoint?
Mad2 present if unattached spindle
Summary of cell cycle checkpoints?
a. what monitors pathways?
b. what initiates response?
c. what halts cell cycle progression and initiates repair?
a. molecular sensors
b. signaling pathways initiate response
c. effector proteins (CKI)
Let’s look at G1 phase overall. How does it start? How is it inactivated? What checkpoint is there?
G1
a. starts with cyclin D and cdk 4,6
b. inactivated when ATM/R activates p53 activates CKI p21cip
c. DNA damage checkpoint before replication
Let’s look at G1/S and S phases overall. How do they start? how are they inactivated? Are there checkpoints?
G1/S is made from cdk2 and cyclin E
S-cdk is made from cdk2 and cyclin A
both are inactivated by ATM/R, p53, p21cip
ATM/R leads to chk1/2, which inhibits cdc25A. cdc25a usually stimulated cdk complex. At G1/S and S, there is a DNA damage checkpoint
S-cdk goes through which cell phases?
S and G2
Let’s look at M-cdk. How does it start? How is it inactivated? what checkpoint is there?
M-cdk built with cyclin B and cdk 1
Inhibited by two things:
a. ATR stimulates chk1, which inhibits cdc25c, which stimulates cyclin B
b. ATM/R stimulates p53, which stimulates p53, which stimulates p21cip, which inhibits cyclin B
The checkpoint after M-cdk checks for DNA damage
during M-cdk and M-phase, how does cell go from metaphase to anaphase?
First, it checks to make sure spindles are attached correctly. Mad2 does this. Mad2 inhibits APC/C-cdc20 complex if not correct. Then, if all is correct, APC/C binds cdc20 to make polyubiguitylation of securin to start anaphase and separate sister chromatids.
What happens regulation wise after anaphase?
Telophase
APC/C-Cdh1 polyubiquinate cyclin B. This only happens if spindle position checkpoint is cleared. If it is cleared, cdc14 is stimulated, which leads to APC/C-Cdh1 complex and to Sic1
what is inhibited by ATM/R –> p53–>p21 cip
M-cyclin, G1/S-cdk, S-cdk, G1-cdk
what is cdc25?
a phosphatase. cdc25A dephosphorylates G1/S-cdk, S-cdk to activate
cdc25c dephosphorylates M-cdk to activate it
what does chk1/2 do?
it stimulates cdc25
role of cdks
phosphorylate many targets; drive cells through different stages of cell cycle
CAK (cdk activating kinase)
activate cdks
Wee1 kinase
inhibits cdks
cdc25 phosphatase
activates cdks
cdc14 phosphatase
activates cdh1 to degrade mitotic spindle
What controls cell cycle checkpoints (2)?
accelerators “Proto-oncogens”
brakes “tumor suppressors”
Where do accelerators and brakes work in the cell cycle?
At the start/restriction point between G1 and S. Accelerators will promote movement to S, whereas brakes will stop the cell cycle.
what does gain of function mean with proto-oncogene?
Ras turns proto-oncogene into oncogene, which will drive cell past checkpoint and continue replication and division.
what happens when a tumor suppressor experiences “loss of function”?
a checkpoint is lost (p53 or p21, which can disable a cdk-complex
What defect do 30% of all cancers have?
a defect in the RAS pathway. RAS mutant is constitutively active. Can’t hydrolyze GTP.
