Part 4: DNA Replication & Enzymes (25 MCQs)
Part 4: DNA Replication & Enzymes (25 MCQs)
Q76. DNA replication is described as:
a) Conservative
b) Semi-conservative
c) Dispersive
d) None of these
Answer: b
- b) Correct: Meselson & Stahl showed DNA replication is semi-conservative.
- a) Wrong: Conservative → parental DNA stays intact.
- c) Dispersive model was rejected.
- d) Not applicable.
Q77. The Meselson–Stahl experiment was conducted on:
a) Drosophila
b) E. coli
c) Yeast
d) Bacteriophage
Answer: b
- b) Correct: They used E. coli and heavy nitrogen (¹⁵N).
Q78. Which isotope was used to label DNA in Meselson–Stahl experiment?
a) ³²P
b) ³⁵S
c) ¹⁵N
d) ¹⁴C
Answer: c
- c) Correct: ¹⁵N (heavy nitrogen) incorporated into bases.
Q79. After one round of replication in ¹⁵N medium shifted to ¹⁴N, DNA molecules are:
a) All heavy
b) All light
c) Hybrid (intermediate)
d) Half heavy, half light
Answer: c
- c) Correct: First generation = all hybrid DNA.
Q80. After two generations in Meselson–Stahl experiment, DNA showed:
a) Only heavy DNA
b) Only light DNA
c) 50% hybrid and 50% light DNA
d) All hybrid DNA
Answer: c
- c) Correct: Second generation = 50% hybrid, 50% light.
Q81. DNA replication in prokaryotes begins at:
a) Random points
b) Origin of replication (OriC)
c) Centromere
d) Ribosomes
Answer: b
- b) Correct: Prokaryotes have a single origin (OriC).
Q82. DNA replication is:
a) Bidirectional and semi-conservative
b) Unidirectional and conservative
c) Multidirectional and dispersive
d) None of these
Answer: a
- a) Correct: Replication is bidirectional & semi-conservative.
Q83. In eukaryotes, replication begins at:
a) A single origin
b) Multiple origins
c) Random ends
d) Centromere
Answer: b
- b) Correct: Eukaryotic DNA → large → multiple origins.
Q84. The enzyme responsible for unzipping DNA helix is:
a) DNA polymerase
b) Helicase
c) Ligase
d) Primase
Answer: b
- b) Correct: Helicase unwinds DNA at replication fork.
Q85. The enzyme that relieves supercoiling is:
a) Helicase
b) DNA polymerase
c) Topoisomerase (DNA gyrase)
d) Ligase
Answer: c
- c) Correct: Topoisomerase/DNA gyrase reduces tension in DNA ahead of fork.
Q86. DNA polymerase can add nucleotides only in:
a) 5′ → 3′ direction
b) 3′ → 5′ direction
c) Both directions
d) Randomly
Answer: a
- a) Correct: DNA polymerase works only in 5′ → 3′ direction.
Q87. Which strand is synthesized continuously?
a) Leading strand
b) Lagging strand
c) Template strand
d) None
Answer: a
- a) Correct: Leading strand synthesized continuously towards fork.
Q88. The lagging strand is synthesized in short fragments called:
a) Histone fragments
b) Okazaki fragments
c) Nucleosome fragments
d) Exons
Answer: b
- b) Correct: Okazaki fragments synthesized discontinuously.
Q89. Okazaki fragments are joined by:
a) DNA helicase
b) DNA ligase
c) DNA gyrase
d) RNA polymerase
Answer: b
- b) Correct: DNA ligase seals gaps between Okazaki fragments.
Q90. The enzyme that synthesizes RNA primer is:
a) DNA polymerase
b) RNA polymerase (Primase)
c) Ligase
d) Helicase
Answer: b
- b) Correct: Primase (RNA polymerase) lays RNA primer.
Q91. Which DNA polymerase in prokaryotes is mainly responsible for replication?
a) DNA Pol I
b) DNA Pol II
c) DNA Pol III
d) DNA Pol α
Answer: c
- c) Correct: DNA Pol III = main replicative enzyme in prokaryotes.
Q92. DNA polymerase I function is mainly to:
a) Unwind DNA
b) Remove RNA primers and fill gaps
c) Join Okazaki fragments
d) Proofread only
Answer: b
- b) Correct: DNA Pol I → removes RNA primers, fills with DNA.
Q93. Proofreading activity of DNA polymerase is due to:
a) 3′ → 5′ exonuclease activity
b) 5′ → 3′ exonuclease activity
c) Helicase activity
d) Primase activity
Answer: a
- a) Correct: DNA polymerase corrects errors via 3′ → 5′ exonuclease activity.
Q94. In eukaryotes, DNA polymerase α is involved in:
a) Initiation and primer synthesis
b) Proofreading
c) Repair only
d) Telomere synthesis
Answer: a
- a) Correct: Pol α = initiator + primer synthesis.
Q95. Which enzyme prevents shortening of chromosomes during replication?
a) Ligase
b) Telomerase
c) Gyrase
d) Primase
Answer: b
- b) Correct: Telomerase maintains telomeres in eukaryotes.
Q96. Telomerase is a type of:
a) RNA-dependent DNA polymerase
b) DNA-dependent RNA polymerase
c) DNA ligase
d) Helicase
Answer: a
- a) Correct: Telomerase uses RNA template to extend DNA.
Q97. Proofreading ensures replication accuracy with an error rate of:
a) 1 in 10³ bases
b) 1 in 10⁶ bases
c) 1 in 10⁹ bases
d) 1 in 10² bases
Answer: c
- c) Correct: Error rate after proofreading ≈ 1 in 10⁹ bases.
Q98. DNA replication is called semi-discontinuous because:
a) Both strands synthesized continuously
b) Both discontinuous
c) Leading continuous, lagging discontinuous
d) Random
Answer: c
- c) Correct: Leading strand continuous, lagging strand discontinuous → semi-discontinuous.
Q99. Which enzyme fills gaps after primer removal in prokaryotes?
a) DNA Pol III
b) DNA Pol I
c) Ligase
d) Helicase
Answer: b
- b) Correct: DNA Pol I fills gaps left after primer removal.
Q100. Replication bubbles are formed in eukaryotic DNA because:
a) Multiple origins of replication
b) RNA primers
c) Supercoiling
d) Histone presence
Answer: a
- a) Correct: Large eukaryotic genome initiates at multiple Ori sites → replication bubbles.
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