This set of Cytogenetics Multiple Choice Questions & Answers (MCQs) focuses on ” Ribosome – 1″.
1. Eukaryotic ribosomal subunits are _____________
a) 30S and 50S
b) 30S and 50S
c) 40S and 60S
d) 30S and 60S
Explanation: The eukaryotic ribosomal subunits are 40S and 60S and together they make 80S. On the other hand the prokaryotic ribosomal subunits are 30S and 50S making 70S.
2. Which of these is transcribed by RNA pol III?
Explanation: While most of the eukaryotic rRNAs are transcribed by the RNA pol I, the 5S rRNA is transcribed by RNA pol III. 16S rRNA is however absent in eukaryotes.
3. If you gradually decrease the Mg2+ concentration of the solution of ribosomes, what will you expect?
a) The subunits will associate faster
b) The subunits will dissociate
c) rRNA will separate from its protein
d) The two subunits will dissociate into its rRNA
Explanation: Low Mg2+ concentration facilitates dissociation of the subunits of RNA polymerases. On the contrary increasing Mg2+ concentration will promote association in vitro.
4. Which of this is not a ribosomal protein?
Explanation: While naming the ribosomal proteins L and S stand for larger and smaller subunits respectively. The larger subunit has 34 proteins while the smaller one has 21, so there could not be S34 protein in the ribosome.
5. Which ribosome recognized the transcription initiation site of the mRNA?
Explanation: The 18S rRNA of a smaller subunit for eukaryotes and the 16S rRNA of the smaller subunit for prokaryotes recognizes the mRNA initiation site.
6. The ribosomal peptidyl transferase transfers ___________
a) An isolated amino acid to the peptide chain
b) A peptide chain to an amino acid
c) A peptide chain to another peptide chain
d) An amino acid to a tRNA
Explanation: Ribosomal peptidyl transferase transfers a peptide chain over the tRNA in P site of the ribosome to the tRNA’s amino acid in the A site of the same ribosome.
7. The tRNA f-Met-tRNA binds to __________
a) Formyl methinone
b) Formic acid
Explanation: f-Met tRNA actually binds to normal methionine (there is no readily available f-Met), then N10-formyl-THF converts that methionine to f-Met.
8. Which metal acts as a cofactor for Methionyl aminopeptidase?
Explanation: The Methionyl aminopeptidase which removes the terminal methionine residue in the peptide chain uses Co2+ as a cofactor. While on the other hand many polymerase use Mg2+ and DNA binding domain uses Zn2+.
9. Methionine amino peptidase will remove the N terminal formyl methionine if the N+1 amino acid is_________
b) Phenyl alanine
Explanation: The N terminal amino acid can be removed only if the N+1 amino acid is small, i.e. Gly, Ala, Ser, Thr, Pro, Val or Cys. Here all other than Glycine are larger amino acid. So, only glycine in the N+1 position will suffice the result.
10. The f-Met tRNA binds to ribosomal _______ site
Explanation: fMettRNA-fMet binds to the P site of the ribosome in translational initiation and then the site is blocked by initiation factor.
11. The 50S ribosomal subunit is folded in ______ major domains.
Explanation: While the smaller 30S subunit of the ribosome is folded into 4 major domains the larger 50S subunit had 6 major domains with its 23S and 5S rRNA.
12. State whether the enzyme PDF is a novel drug target for antibiotics.
Explanation: PDF co-Translationally removes the formyl group from the formyl-methionine amino acid. In absence of this, the N terminal methionine would pose a problem in the structure and function of the protein.
13. The Shine and Dalgarno is located in ____________
a) 5.8S rRNA ~ 20 nucleotide upstream to start site
b) mRNA ~ 10 nucleotide upstream to start site
c) 16S rRNA ~ 10 nucleotide upstream to start site
d) 23S rRNA ~ 10 nucleotide upstream to start site
Explanation: The 16S rRNA forms the smaller subunit which recognizes the mRNA via its Shine and Dalgarno sequence. This is a purine rich sequence about 10 amino acids upstream to translation start site.
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