This set of Cytogenetics Assessment Questions and Answers focuses on “Transcription in Eukaryotes : Gene Silencing at Telomere”.
1. RAP1 has a ___________________
a) Positive effect on transcription
b) No effect on transcription
c) Negative effect on transcription
d) Immensely positive effect on transcription
Explanation: RAP1 is necessary for silencing at the telomere. As silenced gene can’t be expressed so it has a negative effect. The effect can be even called very negative as the silencing is very strong.
2. We know that loss of DNA bases is highly mutagenic. But a loss of DNA bases from which region will not have much affect at all?
b) Satellite region
c) Hot spots for recombination
Explanation: Telomere genes are mostly silenced so in case of any deletion mutation the deletion will not be detected. However, loss of bases from anywhere else in the chromosome which transcribe RNA could be disastrous for the cell.
3. A eukaryotic cell can divide only a limited number of times.
Explanation: As the cell goes on dividing in each division love genes at the telomere is lost for the primer RNA at the telomere. Thus the DNA keeps shortening. If it shortens beyond a limit the cell undergoes apoptosis.
4. Yeast mating factor alpha is located in _________________
Explanation: The mating factor alpha is located in the left telomere at HML. This gets transferred to MAT locus for expression. HMR has the ‘a’ mating factor.
5. If a parent yeast is of mating type a, what will be the mating type of its bud?
Explanation: In yeast mating type alternates with every generation, so if a parent is ‘a’ then bud will be ‘alpha’ and vice versa. Yeast with a-alpha mating type is sterile while there is no rho mating type of yeast.
6. Under which condition will the yeast express a-alpha mating type?
a) HAT activation
b) TBP activation
c) HDAC inactivation
d) HAT inactivation
Explanation: HAT acetylates the histone H3 and H4 so it activates the gene while HDAC deacetylates them and inactivates the genes. Under condition when HAT is active but HDAC is inactive we will get an expression of both the telomere gene which is normally repressed. This will lead to a-alpha mating type.
7. RAP1 binds to ____________
a) At the loop between enhancer and promoter
b) Linear repeated sequences
c) At the nuclosomes in telomere region
d) Specific sequences at telomere
Explanation: RAP1 has no sequence specification. It binds to the repeated sequences at the telomeric region in the linear DNA lacking nucleosomes. Then, it loops over and spreads the repression to the entire telomere.
8. Which of these SIR proteins are having HDAC activity?
Explanation: The SIR or silent information regulator proteins are responsible for repression at the telomere by deacetylating the histones. While SIR3and SIR4 bind to the N-termini the SIR2 protein which is attracted by SIR 3-4 is having the HDAC activity.
9. You transfer the DAM gene to the telomere of yeast. What will be your expected result?
a) Methylation at the A sites in specific sequence
b) Methylation at the C sites in specific sequence
c) No methylation at the A sites in specific sequence
d) No methylation at the C sites in specific sequence
Explanation: DAM is a methylase that methylates the A of GATC sequence. When expressed in Yeast it methylated the yeast DNA, however, in telomere the expression is blocked by the telomere silencing effect. Thus, we don’t get methylated DNA.
10. What is the range of telomere suppression?
Explanation: The telomere suppression spreads up to 4kb from the telomere. This is possible due to the looping action of RPA1 and SIR2-4 proteins by protein-protein interactions. The history at this regions are hemi-acetylated.
Sanfoundry Global Education & Learning Series – Cytogenetics.
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