Selected MCQ

Current Question
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 1
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 2
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 3
Which enzyme untangles DNA during replication by cutting and re-ligating both strands?
  • A. DNA ligase
  • B. Topoisomerase I
  • C. Topoisomerase II
  • D. DNA helicase
Correct Answer: C
Explanation:
Topoisomerase II (a type II enzyme) makes double-strand breaks to relieve supercoils or decatenate DNA, then re-ligates the strands.