Selected MCQ

Current Question
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 1
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 2
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 3
Which organelle contains hydrolytic enzymes and is involved in the degradation of cellular components during autophagy?
  • A. Lysosome
  • B. Proteasome
  • C. Golgi apparatus
  • D. Smooth endoplasmic reticulum
Correct Answer: A
Explanation:
Lysosomes contain acid hydrolases that fuse with autophagosomes to degrade cellular organelles and proteins.
Related Question 4
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.
Related Question 5
Which of the following is NOT typically controlled during industrial fermentation in a bioreactor?
  • A. Temperature
  • B. pH
  • C. Oxygen concentration
  • D. DNA sequence of the microorganism
Correct Answer: D
Explanation:
A, B, and C (temperature, pH, oxygen) are critical parameters controlled in fermentation. The chosen strain fixes the DNA sequence and is not a controlled parameter during the run.