MCB 252 FINAL EXAM
QUESTIONS AND ANSWERS 2026
- 2027
What are the four key drivers of the cell cycle?
G1/S CDKs, S CDKs, M CDKs, and APC/C
Why is the cell cycle considered irreversible?
Once one switch is turned off, it cannot turn back on until the next
cycle
What is the overall logic of cell cycle regulation?
Each CDK activates events for its stage, activates the next stage, and
helps inactivate itself
What triggers START in budding yeast?
Peak G1/S CDK activity activates S-CDKs
What are the three major outcomes of S-CDK activation at
START?
DNA replication, transcription of M cyclins, and degradation of G1/S
cyclins
How is Sic1 removed to activate S-CDKs?
G1/S CDKs phosphorylate Sic1, allowing SCF ubiquitin ligase to
degrade it
Why can't G1/S cyclins accumulate again immediately
S and M CDKs target them for degradation
,Why are M cyclins produced during S phase
To accumulate inactive M CDKs ahead of mitosis
Why are M CDKs initially inactive?
Wee1 phosphorylates and inhibits them
What creates rapid switch-like activation of M CDKs
Active M CDKs activate Cdc25 and inhibit Wee1, creating positive
feedback
Why are S and M CDKs synthesized ahead of time?
So a large inactive pool can be rapidly activated all at once
What type of enzyme is APC/C?
An E3 ubiquitin ligase
What does APC/C-Cdc20 target?
S cyclins and a large fraction of M cyclins for degradation
What is APC/C-Cdc20 required for
Sister chromatid separation and onset of anaphase
What does APC/C-Cdh1 do after anaphase?
Degrades the remaining M cyclins
Why is cyclin degradation important?
It allows exit from mitosis
What happens if cycle cannot be degraded?
cells remain stuck in mitosis
What does Cdc14 activates
APC/C-Cdh1
How does Cdc14 help reset the cell cycle?
, It dephospahorylates Sic1 and Rb, returning them to their G1 state
Why is APC/C activity maintained into early G1?
To prevent premature accumulation of S and M cyclins
What is the role Rb?
Rb represses transcription of genes required for progression
What happens when Rb is phosphorylated?
It releases transcription factors, allowing cyclin gene expression
Why is Rb important in cancer?
Loss-of-function Rb mutations allow uncontrolled cell cycle
progression
what are the three microtubule based structures in eukaryotes?
cillia (moves fluid across cells)
flagella (propel cells)
mitotic spindle (separates chromosomes during mitosis)
where are minus ends anchored in cells
at centrosomes, spindle poles, or Basal bodies (MTOCs)
what are microtubules made of and how does GTP relate to
tubular?
Microtubules are made of alpha/beta tubular heterodimers. Both bind
GTP, but only beta tubular hydrolyzes GTP to GDP
why is the exchangeable GTP on beta tubulin important?
GTP exchange and hydrolysis regulate microtubule growth,
shrinkage, and dynamic instability
QUESTIONS AND ANSWERS 2026
- 2027
What are the four key drivers of the cell cycle?
G1/S CDKs, S CDKs, M CDKs, and APC/C
Why is the cell cycle considered irreversible?
Once one switch is turned off, it cannot turn back on until the next
cycle
What is the overall logic of cell cycle regulation?
Each CDK activates events for its stage, activates the next stage, and
helps inactivate itself
What triggers START in budding yeast?
Peak G1/S CDK activity activates S-CDKs
What are the three major outcomes of S-CDK activation at
START?
DNA replication, transcription of M cyclins, and degradation of G1/S
cyclins
How is Sic1 removed to activate S-CDKs?
G1/S CDKs phosphorylate Sic1, allowing SCF ubiquitin ligase to
degrade it
Why can't G1/S cyclins accumulate again immediately
S and M CDKs target them for degradation
,Why are M cyclins produced during S phase
To accumulate inactive M CDKs ahead of mitosis
Why are M CDKs initially inactive?
Wee1 phosphorylates and inhibits them
What creates rapid switch-like activation of M CDKs
Active M CDKs activate Cdc25 and inhibit Wee1, creating positive
feedback
Why are S and M CDKs synthesized ahead of time?
So a large inactive pool can be rapidly activated all at once
What type of enzyme is APC/C?
An E3 ubiquitin ligase
What does APC/C-Cdc20 target?
S cyclins and a large fraction of M cyclins for degradation
What is APC/C-Cdc20 required for
Sister chromatid separation and onset of anaphase
What does APC/C-Cdh1 do after anaphase?
Degrades the remaining M cyclins
Why is cyclin degradation important?
It allows exit from mitosis
What happens if cycle cannot be degraded?
cells remain stuck in mitosis
What does Cdc14 activates
APC/C-Cdh1
How does Cdc14 help reset the cell cycle?
, It dephospahorylates Sic1 and Rb, returning them to their G1 state
Why is APC/C activity maintained into early G1?
To prevent premature accumulation of S and M cyclins
What is the role Rb?
Rb represses transcription of genes required for progression
What happens when Rb is phosphorylated?
It releases transcription factors, allowing cyclin gene expression
Why is Rb important in cancer?
Loss-of-function Rb mutations allow uncontrolled cell cycle
progression
what are the three microtubule based structures in eukaryotes?
cillia (moves fluid across cells)
flagella (propel cells)
mitotic spindle (separates chromosomes during mitosis)
where are minus ends anchored in cells
at centrosomes, spindle poles, or Basal bodies (MTOCs)
what are microtubules made of and how does GTP relate to
tubular?
Microtubules are made of alpha/beta tubular heterodimers. Both bind
GTP, but only beta tubular hydrolyzes GTP to GDP
why is the exchangeable GTP on beta tubulin important?
GTP exchange and hydrolysis regulate microtubule growth,
shrinkage, and dynamic instability
