Selected MCQ

Current Question
In the *E. coli* trp operon, attenuation causes premature termination of transcription when tryptophan levels are:
  • A. High, because the ribosome does not stall on the leader peptide
  • B. Low, because the ribosome stalls on the leader peptide
  • C. High, because tryptophan activates the terminator hairpin
  • D. Low, because tryptophan activates the attenuator sequence
Correct Answer: A
Explanation:
High tryptophan allows the ribosome to quickly translate the leader peptide, enabling formation of a terminator hairpin and early transcription termination (attenuation).
Related Question 1
In the lac operon of *E. coli*, if both lactose and glucose are abundant in the medium, what is the expected effect on lac operon transcription?
  • A. High transcription of lac genes
  • B. Low transcription due to catabolite repression
  • C. No transcription at all
  • D. On only if allolactose is present
Correct Answer: B
Explanation:
High glucose lowers cAMP levels, so CAP is inactive. Even with lactose present (inactivating the repressor), transcription is reduced (catabolite repression).
Related Question 2
Which plasmid-based technique is used to enrich and detect low-abundance mRNAs by reverse transcription into cDNA?
  • A. RFLP analysis
  • B. Northern blotting
  • C. RT-PCR (reverse-transcription PCR)
  • D. Affinity chromatography
Correct Answer: C
Explanation:
RT-PCR involves reverse-transcribing RNA to cDNA and then amplifying specific sequences by PCR. It is very sensitive for detecting low-abundance transcripts.
Related Question 3
What distinguishes reverse transcription PCR (RT-PCR) from standard PCR?
  • A. Use of DNA ligase
  • B. Amplification of RNA by first creating complementary DNA
  • C. Higher temperature cycles
  • D. Use of radioactive nucleotides
Correct Answer: B
Explanation:
RT-PCR involves reverse transcription of RNA into cDNA before PCR, enabling amplification of RNA targets.
Related Question 4
A single gene mutation causes both phenylketonuria (affecting skin pigment) and neurological impairment. This phenomenon is called:
  • A. Epistasis
  • B. Pleiotropy
  • C. Codominance
  • D. Penetrance
Correct Answer: B
Explanation:
One gene affecting multiple, seemingly unrelated traits (e.g., PKU’s effects on pigment and brain) is pleiotropy.
Related Question 5
In bacterial transcription, the σ (sigma) factor of RNA polymerase is required for:
  • A. RNA chain elongation
  • B. Recognizing promoter sequences
  • C. DNA replication
  • D. Termination of transcription
Correct Answer: B
Explanation:
The sigma subunit binds the promoter sequences (e.g., –10 and –35 regions) and directs RNA polymerase to initiate transcription at the correct site.