Selected MCQ

Current Question
Why does the lagging strand of DNA require more DNA ligase activity during replication than the leading strand?
  • A. It contains more errors to fix
  • B. It is synthesized more continuously
  • C. It is synthesized discontinuously in Okazaki fragments
  • D. It is shorter than the leading strand
Correct Answer: C
Explanation:
The lagging strand is made in short Okazaki fragments, each of which must be joined by DNA ligase. The leading strand is continuous and requires minimal ligation.
Related Question 1
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.
Related Question 2
DNA polymerases proofread the newly synthesized DNA strand using which activity?
  • A. 5′→3′ exonuclease
  • B. 3′→5′ exonuclease
  • C. Primase activity
  • D. Helicase activity
Correct Answer: B
Explanation:
Many DNA polymerases have 3′→5′ exonuclease activity that excises incorrectly paired nucleotides immediately after incorporation (proofreading).
Related Question 3
In eukaryotes, removal of RNA primers during DNA replication is primarily carried out by:
  • A. DNA polymerase I
  • B. RNase H
  • C. DNA ligase
  • D. DNA gyrase
Correct Answer: B
Explanation:
RNase H degrades the RNA in RNA-DNA hybrids (RNA primers). DNA Pol I is bacterial; eukaryotes use RNase H (and FEN1) before ligation.
Related Question 4
Which pH environment is required for optimal activity of lysosomal enzymes?
  • A. ~7.4
  • B. ~6.8
  • C. ~5.0
  • D. ~8.0
Correct Answer: C
Explanation:
Lysosomal hydrolases function best at an acidic pH around 5.0, maintained by proton pumps in the lysosomal membrane.
Related Question 5
Alternative splicing occurs during gene expression at the level of:
  • A. DNA replication
  • B. Transcription initiation
  • C. Pre-mRNA processing (RNA splicing)
  • D. Protein folding
Correct Answer: C
Explanation:
Alternative splicing of pre-mRNA allows different exons to be joined or skipped, producing multiple mRNA variants from one gene during RNA processing.