DNA Replication
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All Questions for: DNA Replication
Question 1: DNA replication is described as semiconservative because:
- A Each daughter molecule has one parental and one newly synthesized strand
- B Both parental strands stay together in one daughter
- C Both daughter strands are entirely new
- D Only one daughter molecule is produced
Explanation: Each daughter DNA molecule contains one parental (conserved) strand and one newly synthesized strand.
Question 2: The Meselson-Stahl experiment used which isotope to demonstrate semiconservative replication?
- A Nitrogen-15 (15N)
- B Phosphorus-32 (32P)
- C Sulfur-35 (35S)
- D Carbon-14 (14C)
Explanation: Meselson and Stahl (1958) labeled DNA with heavy nitrogen (15N) and tracked its distribution in successive generations using density gradient centrifugation.
Question 3: Sites on DNA where replication initiates are called:
- A Origins of replication
- B Telomeres
- C Centromeres
- D Promoters
Explanation: Origins of replication are specific DNA sequences recognized by initiator proteins that recruit the replication machinery.
Question 4: The Y-shaped structure where DNA is being unwound and copied is called the:
- A Replication fork
- B Okazaki fork
- C Promoter region
- D Replisome
Explanation: The replication fork is the site where parental DNA is separated into two template strands and new DNA is synthesized.
Question 5: Which enzyme unwinds the DNA double helix at the replication fork?
- A DNA helicase
- B DNA polymerase
- C Primase
- D Ligase
Explanation: DNA helicase uses ATP hydrolysis to break hydrogen bonds between base pairs and separate the two strands.
Question 6: Which enzyme synthesizes RNA primers during DNA replication?
- A Primase
- B DNA polymerase III
- C DNA ligase
- D Topoisomerase
Explanation: Primase is an RNA polymerase that creates short RNA primers, providing a 3'-OH for DNA polymerase to extend.
Question 7: DNA polymerase can only add nucleotides to which end of a growing strand?
- A The 3' end
- B The 5' end
- C Both ends simultaneously
- D Only the middle of the strand
Explanation: DNA polymerases synthesize DNA strictly in the 5'→3' direction, adding to the free 3'-OH end of the growing chain.
Question 8: The strand synthesized continuously toward the replication fork is called the:
- A Leading strand
- B Lagging strand
- C Template strand
- D Coding strand
Explanation: The leading strand is synthesized in the same direction as fork movement (5'→3'), so it grows continuously with a single primer.
Question 9: The strand synthesized discontinuously in short fragments is called the:
- A Lagging strand
- B Leading strand
- C Non-template strand
- D Sense strand
Explanation: The lagging strand is synthesized away from the fork direction in short Okazaki fragments, each requiring its own primer.
Question 10: The short DNA fragments synthesized on the lagging strand are called:
- A Okazaki fragments
- B Klenow fragments
- C Watson fragments
- D Kornberg fragments
Explanation: Okazaki fragments are typically 100–200 nt in eukaryotes and 1000–2000 nt in prokaryotes; they are later joined by DNA ligase.
Question 11: The enzyme that joins Okazaki fragments together is:
- A DNA ligase
- B DNA helicase
- C DNA polymerase I
- D Topoisomerase
Explanation: DNA ligase seals the nick between adjacent fragments by forming a phosphodiester bond between the 3'-OH and 5'-phosphate.
Question 12: In E. coli, which DNA polymerase is primarily responsible for chromosomal DNA replication?
- A DNA polymerase III
- B DNA polymerase I
- C DNA polymerase II
- D DNA polymerase IV
Explanation: DNA polymerase III is the main replicative enzyme in E. coli, with high processivity and 3'→5' proofreading exonuclease activity.
Question 13: In E. coli, which DNA polymerase removes RNA primers and fills the resulting gaps?
- A DNA polymerase I
- B DNA polymerase III
- C DNA polymerase II
- D Primase
Explanation: DNA polymerase I possesses a unique 5'→3' exonuclease activity that removes RNA primers while simultaneously extending the DNA chain.
Question 14: Which exonuclease activity enables DNA polymerase to remove mismatched nucleotides during replication?
- A 3'→5' exonuclease activity
- B 5'→3' exonuclease activity
- C Endonuclease activity
- D Helicase activity
Explanation: The 3'→5' exonuclease activity provides proofreading, removing incorrectly incorporated nucleotides and increasing replication fidelity.
Question 15: Single-strand binding proteins (SSBs) function during replication to:
- A Prevent re-annealing of separated DNA strands
- B Cleave phosphodiester bonds
- C Synthesize RNA primers
- D Add supercoils to DNA
Explanation: SSBs coat single-stranded DNA to prevent premature re-annealing and to protect the strands from nucleases.
Question 16: In bacteria, which enzyme introduces negative supercoils ahead of the replication fork?
- A DNA gyrase
- B DNA helicase
- C DNA ligase
- D DNA polymerase I
Explanation: DNA gyrase (a type II topoisomerase) uses ATP to introduce negative supercoils, relieving positive torsional stress generated by fork progression.
Question 17: Telomerase functions to:
- A Extend chromosome ends using an RNA template
- B Cleave Okazaki fragments
- C Remove RNA primers
- D Synthesize mitochondrial DNA
Explanation: Telomerase is a reverse transcriptase that uses its own RNA template to extend telomeric repeats at chromosome ends, compensating for end-replication loss.
Question 18: Eukaryotic chromosomes typically have how many origins of replication per chromosome?
- A Multiple origins
- B Exactly one origin
- C Two origins only
- D No fixed origins
Explanation: Eukaryotic chromosomes are long and replicate from multiple origins, allowing genome duplication within a single S phase.
Question 19: Replication that proceeds in both directions away from a single origin is described as:
- A Bidirectional replication
- B Unidirectional replication
- C Rolling circle replication
- D Conservative replication
Explanation: Bidirectional replication creates two replication forks moving in opposite directions from each origin, doubling the rate of genome duplication.
Question 20: The high fidelity of DNA replication (error rate ~1 in 10^9) is achieved by:
- A Base selection, proofreading, and mismatch repair
- B Only base selection by DNA polymerase
- C Only post-replication repair
- D Random chance alone
Explanation: Base selection, 3'→5' proofreading exonuclease activity, and post-replication mismatch repair together produce the extremely low error rate observed.
Question 21: Which of the following is NOT required for DNA replication?
- A RNA polymerase
- B Primase
- C dNTPs
- D DNA template
Explanation: RNA polymerase is the enzyme for transcription. DNA replication requires a template, primers, dNTPs, and the replisome proteins, not RNA polymerase.
Question 22: Okazaki fragments are found on which strand?
- A Lagging strand
- B Leading strand
- C Both strands equally
- D Neither strand
Explanation: Okazaki fragments are exclusively synthesized on the lagging strand because this strand must be synthesized in short segments away from fork movement.
Question 23: Primers in DNA replication are made of:
- A RNA
- B DNA
- C Protein
- D Polysaccharide
Explanation: Primase synthesizes short RNA primers (not DNA) because DNA polymerases can only extend an existing 3'-OH group, which RNA primers provide.
Question 24: The replisome is best described as:
- A The multiprotein complex that carries out DNA replication
- B A type of topoisomerase
- C A DNA repair enzyme
- D A ribosome-bound complex
Explanation: The replisome is the multiprotein machine at the replication fork composed of helicase, primase, DNA polymerases, and accessory factors.
Question 25: Which of the following organisms uses a theta (θ) structure during DNA replication?
- A Bacteria with circular chromosomes
- B Eukaryotic linear chromosomes
- C Retroviruses only
- D Mitochondria only
Explanation: Theta replication occurs in circular DNA molecules such as bacterial chromosomes and plasmids, producing a characteristic θ-shaped intermediate.