Selected MCQ

Current Question
What is the sedimentation coefficient (S value) of a complete eukaryotic ribosome?
  • A. 70S
  • B. 80S
  • C. 90S
  • D. 100S
Correct Answer: B
Explanation:
A eukaryotic ribosome is 80S, composed of a 60S large subunit and a 40S small subunit (70S in bacteria).
Related Question 1
The 5′ cap added to eukaryotic mRNAs is:
  • A. A string of adenine residues at the 5′ end
  • B. A modified guanosine (7-methylguanosine) linked 5′→5′
  • C. A poly-uridine sequence at the 3′ end
  • D. A phosphate group at the 5′ end
Correct Answer: B
Explanation:
The 5′ cap is a 7-methylguanosine joined via a 5′-to-5′ triphosphate linkage to the first nucleotide of the mRNA, protecting it from degradation and aiding translation.
Related Question 2
Which mechanism allows a single eukaryotic gene to encode multiple protein isoforms?
  • A. Alternative splicing
  • B. Alternative sigma factors
  • C. Gene duplication
  • D. Horizontal gene transfer
Correct Answer: A
Explanation:
Alternative splicing of a pre-mRNA can include or exclude exons, yielding different mRNAs and thus different protein isoforms from one gene.
Related Question 3
In eukaryotic pre-mRNA splicing, which nucleotides are found at the 5′ and 3′ ends of most introns?
  • A. GU at 5′ and AG at 3′ ends
  • B. GA at both ends
  • C. AA at 5′ and TT at 3′ ends
  • D. CT at 5′ and TC at 3′ ends
Correct Answer: A
Explanation:
The consensus splice donor site at the 5′ end of introns is “GU,” and the splice acceptor site at the 3′ end is “AG” in most eukaryotic genes.
Related Question 4
Which eukaryotic RNA polymerase transcribes tRNA genes?
  • A. RNA polymerase I
  • B. RNA polymerase II
  • C. RNA polymerase III
  • D. RNA polymerase IV
Correct Answer: C
Explanation:
RNA Pol III transcribes tRNAs and 5S rRNA. Pol I makes most rRNAs, and Pol II makes mRNA and some snRNAs.
Related Question 5
Where in the nucleus are ribosomal RNAs transcribed in eukaryotic cells?
  • A. Nucleolus
  • B. Euchromatin
  • C. Chromatin loops
  • D. Nuclear lamina
Correct Answer: A
Explanation:
The nucleolus is the site of rRNA gene transcription (by RNA Pol I) and ribosome assembly.