This set of Cytogenetics Multiple Choice Questions on “Cell Cycle Stages – 1”.
1. Check points in the cell cycle are dependent on __________________
A. Time dependent regulation
B. Volume dependent regulation
C. Positive feedback regulation
D. Negative feedback regulation
Answer: D
Clarification: If the dependence was on positive feedback then it would be hard to differentiate between different levels of partially and fully positive cases. But in case of negative feedback, even one negative signal can be easily detected and its removal will trigger the process.
2. Which is the irreversible stage for progression of cell division?
A. G1
B. S
C. G2
D. M
Answer: B
Clarification: The S phase is marked by DNA replication. Once this phase is done the cell has to undergo division else its DNA content will remain double that of normal cell.
3. Which of the following ubiquitin ligase concentration is regulated for proper cell cycle progression?
A. E1
B. E2
C. E3
D. E4
Answer: C
Clarification: E3 ubiquitin ligase is finally responsible for adding the poly ubiquitin tail to the substrate to mark it for destruction. Thus regulating the E3 level will help to regulate the concentration of the substrate.
4. Which one of the following acts as CKI?
A. P20
B. p27
C. p35
D. p53
Answer: B
Clarification: p27 can bind to the CDK-cyclin complex in a way that the catalytic site is no longer able to interact with the substrate. This leads to inactivation; thus, it’s a CDK inhibitory protein or CKI.
5. Which of the following acts as a licensing factor for replication?
A. ORC
B. Cdt
C. MCM
D. CDC42
Answer: C
Clarification: MCM acts as the licensing factor to set on the M phase of cell cycle. In its absence, the ORC although present in the nucleus can’t reinitiate replication. Cdc6 participates in replication and not cdc42, cdt is bound by geminin and is not the licensing factor.
6. Which CDK triggers origin firing?
A. G1
B. G1-S
C. S
D. M
Answer: C
Clarification: S CDK phosphorylates the residues on MCM and on ORC thus triggering ORC firing leading to DNA replication in the cell. ORC is fired once and only once during cell cycle.
7. What is the fate of cdc6 after origin firing?
A. It is sequestered by geminin
B. It is phosphorylated and thus can no longer interact
C. It is irreversibly bound to ORC
D. It is phosphorylated, ubiquitin is added and destroyed
Answer: D
Clarification: The continued presence of CDC6 and MCM could lead to re-initiation of replication at the ORC. Thus, to prevent this, the CDC6 is phosphorylated and marked for destruction.
8. Which CDK is responsible for nuclear membrane disintegration?
A. G1
B. S
C. M
D. A
Answer: C
Clarification: The M-cdk on activation leads to the M phase. It breaks down the nuclear membrane by specific phosphorylation activity allowing the licensing factor to re-enter the nucleus.
9. Which Cdc is responsible for the destruction of M cyclin – Cdk complex?
A. Cdc6
B. Cdc20
C. Cdc30
D. Cdc42
Answer: B
Clarification: The Cdc20 – APC complex is responsible for the breakdown of the M cyclin cdk complex which leads to escape from the anaphase.
10. M cyclin CDK _________________ cdc20 and ______________ Hct1?
A. Activates, activates
B. Activates, inactivates
C. Inactivates, activates
D. Inactivates, activates
Answer: B
Clarification: Both Cdc20 and Hct1 are APC ligases which help in escape from mitosis by destruction of M CDK, but cdc20 is activated by M cyclin – CDK by while Hct1 is inactivated by the same through phsphorylation.
11. In the G1 phase which of the following has a high level?
A. Hct1
B. CDC20
C. M cyclin CDk
D. S cyclin
Answer: A
Clarification: Hct1 that is triggered by cdc20 at the end of the anaphase remains high in concentration even in the early G1 phase preventing the CDk activity.
12. Sic1 can bind and inhibit ___________ Cdk and it is inhibited by ________ cdk.
A. G1, M
B. M, M
C. S, S
D. M, S
Answer: B
Clarification: M Cdk inactivated sic1 and Hct1 activity. It is however inhibited by the same. With decrease in M Cdk activity in the early G1 stage sic1 binds and inactivated it.
13. M cyclin cdk is activated by positive feedback loop.
A. True
B. False
Answer: A
Clarification: The M-cyclin Cdk when bound to sic1 in turn also causes slight transcription of M cyclin gene. Increasing concentration of M cyclin further increases its activity. This works as a positive feedback loop which ultimately leads to M cdk activation.
14. Which of the following Cdk is resistant to Hct1?
A. G1
B. G1-S
C. Both G1 and G1-S
D. G1, G1 S and S