This set of Cytogenetics Multiple Choice Questions & Answers (MCQs) focuses on “Initiation – 1”.
1. The 16S rRNA binds the mRNA in its ___________ region.
a) Body
b) Head
c) Neck
d) Flap
View Answer
Explanation: The 3’-OH end of the 16S rRNA is located in the neck region of the rRNA. Near it is the Shine and Dalgarno sequence that binds the mRNA.
2. _____________ treated ribosomes will be able to carry out elongation and termination of previously initiated translation, but can’t initiate any new translation.
a) Streptomycin
b) Rifampicin
c) Tetracyclin
d) Colicin E3
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Explanation: Colicin e# effects initiation at the Shine and Dalgarno sequence, however, it has no effect on elongation of termination. Thus, it will prevent new initiation but will not halt already translating ribosomes.
3. IF1 helps in correct placement of the initiation tRNA. How does it do so?
a) By guiding it to P site
b) By binding to A site
c) By dissociating the ribosomal complex
d) By binding to the P site
View Answer
Explanation: IF1 binds to the A site of the 30S subunit so that the 1st aminoacyl-tRNA can’t sit at the A site. It then sits on the P site i.e. correct position.
4. Which initiation factor prevents association of ribosome when not bound to mRNA?
a) IF1
b) IF2
c) IF3
d) IF4
View Answer
Explanation: The IF3 binds to the 30S subunit and prevents association of 30S and 50S subunit when the ribosome is not bound to mRNA. It also helps in binding t the mRNA.
5. If you see a GTP bound IF2 it is _________
a) Before initiation
b) During initiation
c) After initiation
d) Not related to an initiation
View Answer
Explanation: IF2 guides the 1st tRNA to the P site of the ribosome. Then it is GTP bound. When properly placed the GTP hydrolyses to GDP and the IF2 is released.
6. Which proteins perform the function of preventing reassociation of ribosomal subunits, when they finish one translation, in eukaryotes?
a) eIF1 and eIF3
b) eIF3 and eIF6
c) eIF2 and eIF4
d) eIF3 and eIF5
View Answer
Explanation: eIF1 is a multi protein complex with eight subunits; this along with eIF6 prevents the reassociation of the subunits of ribosome after translation of one protein is complete.
7. What is used to regulate translation initiation in eukaryotes?
a) Turnover of mRNA
b) Ubiquitination of ribosome
c) Phosphorylation of IF2
d) Methylation of IF1a
View Answer
Explanation: Phosphorylation of IF2 prevents it from binding to f-MettRNA-f-Met, thus it prevents initiation of translation by IF2 bringing the 1st amino acid.
8. If you compare the Kozak sequence with the SD sequence of the prokaryotes which is located at the greater distance from the start site?
a) Equal distance
b) Kozak
c) Shine and Dalgarno sequence
d) Both are too far to account
View Answer
Explanation: Eukaryotes are not polycistronic and the 5’ cap corresponds to the 5’ end of the mRNA near which it must start translating. Although there is not much defined sequence but all the same there is a consensus.
9. Which of the bases are important in detecting whether the AUG should be the start site for eukaryotic initiation?
a) -2 and +3
b) -4 and +3
c) -3 and +4
d) -1 and +4
View Answer
Explanation: The consensus sequence CC(A/G)CCAUGG was discovered by Kozak, where the purine in -3 and +4 position is significant in determining which AUG should serve as initiation site.
10. If give a segment – 5’CTGACCAUGGCCTAGAUGCCTCT, which AUG will the eukaryotic ribosome choose as initiation site?
a) Any at random
b) First
c) Second
d) None
View Answer
Explanation: The ribosome scans the mRNA from 5’ end to the 3’ end searching for a Kozak consensus sequence to start at. Here, the 1st AUG indeed has a purin at -3 and +4 position so it is a good initiation site, the ribosome will not look beyond to the 2nd AUG(weaker) and start initiation.
Sanfoundry Global Education & Learning Series – Cytogenetics.
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