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DNA Replication Questions
Q.
DNA replication is described as semiconservative because:
Only one daughter molecule is produced
Both parental strands stay together in one daughter
Each daughter molecule has one parental and one newly synthesized strand
Both daughter strands are entirely new
Explanation:
Each daughter DNA molecule contains one parental (conserved) strand and one newly synthesized strand.
Q.
The Meselson-Stahl experiment used which isotope to demonstrate semiconservative replication?
Phosphorus-32 (32P)
Nitrogen-15 (15N)
Sulfur-35 (35S)
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.
Q.
Sites on DNA where replication initiates are called:
Telomeres
Centromeres
Promoters
Origins of replication
Explanation:
Origins of replication are specific DNA sequences recognized by initiator proteins that recruit the replication machinery.
Q.
The Y-shaped structure where DNA is being unwound and copied is called the:
Replisome
Replication fork
Promoter region
Okazaki fork
Explanation:
The replication fork is the site where parental DNA is separated into two template strands and new DNA is synthesized.
Q.
Which enzyme unwinds the DNA double helix at the replication fork?
Primase
DNA helicase
DNA polymerase
Ligase
Explanation:
DNA helicase uses ATP hydrolysis to break hydrogen bonds between base pairs and separate the two strands.
Q.
Which enzyme synthesizes RNA primers during DNA replication?
DNA polymerase III
Primase
DNA ligase
Topoisomerase
Explanation:
Primase is an RNA polymerase that creates short RNA primers, providing a 3'-OH for DNA polymerase to extend.
Q.
DNA polymerase can only add nucleotides to which end of a growing strand?
Only the middle of the strand
Both ends simultaneously
The 5' end
The 3' end
Explanation:
DNA polymerases synthesize DNA strictly in the 5'→3' direction, adding to the free 3'-OH end of the growing chain.
Q.
The strand synthesized continuously toward the replication fork is called the:
Template strand
Lagging strand
Coding strand
Leading strand
Explanation:
The leading strand is synthesized in the same direction as fork movement (5'→3'), so it grows continuously with a single primer.
Q.
The strand synthesized discontinuously in short fragments is called the:
Sense strand
Non-template strand
Lagging strand
Leading strand
Explanation:
The lagging strand is synthesized away from the fork direction in short Okazaki fragments, each requiring its own primer.
Q.
The short DNA fragments synthesized on the lagging strand are called:
Watson fragments
Klenow fragments
Kornberg fragments
Okazaki fragments
Explanation:
Okazaki fragments are typically 100–200 nt in eukaryotes and 1000–2000 nt in prokaryotes; they are later joined by DNA ligase.
Q.
The enzyme that joins Okazaki fragments together is:
DNA ligase
Topoisomerase
DNA polymerase I
DNA helicase
Explanation:
DNA ligase seals the nick between adjacent fragments by forming a phosphodiester bond between the 3'-OH and 5'-phosphate.
Q.
In E. coli, which DNA polymerase is primarily responsible for chromosomal DNA replication?
DNA polymerase III
DNA polymerase I
DNA polymerase IV
DNA polymerase II
Explanation:
DNA polymerase III is the main replicative enzyme in E. coli, with high processivity and 3'→5' proofreading exonuclease activity.
Q.
In E. coli, which DNA polymerase removes RNA primers and fills the resulting gaps?
DNA polymerase II
DNA polymerase III
Primase
DNA polymerase I
Explanation:
DNA polymerase I possesses a unique 5'→3' exonuclease activity that removes RNA primers while simultaneously extending the DNA chain.
Q.
Which exonuclease activity enables DNA polymerase to remove mismatched nucleotides during replication?
3'→5' exonuclease activity
Helicase activity
Endonuclease activity
5'→3' exonuclease activity
Explanation:
The 3'→5' exonuclease activity provides proofreading, removing incorrectly incorporated nucleotides and increasing replication fidelity.
Q.
Single-strand binding proteins (SSBs) function during replication to:
Prevent re-annealing of separated DNA strands
Cleave phosphodiester bonds
Synthesize RNA primers
Add supercoils to DNA
Explanation:
SSBs coat single-stranded DNA to prevent premature re-annealing and to protect the strands from nucleases.
Q.
In bacteria, which enzyme introduces negative supercoils ahead of the replication fork?
DNA gyrase
DNA helicase
DNA ligase
DNA polymerase I
Explanation:
DNA gyrase (a type II topoisomerase) uses ATP to introduce negative supercoils, relieving positive torsional stress generated by fork progression.
Q.
Telomerase functions to:
Remove RNA primers
Cleave Okazaki fragments
Synthesize mitochondrial DNA
Extend chromosome ends using an RNA template
Explanation:
Telomerase is a reverse transcriptase that uses its own RNA template to extend telomeric repeats at chromosome ends, compensating for end-replication loss.
Q.
Eukaryotic chromosomes typically have how many origins of replication per chromosome?
Multiple origins
Exactly one origin
Two origins only
No fixed origins
Explanation:
Eukaryotic chromosomes are long and replicate from multiple origins, allowing genome duplication within a single S phase.
Q.
Replication that proceeds in both directions away from a single origin is described as:
Unidirectional replication
Rolling circle replication
Bidirectional replication
Conservative replication
Explanation:
Bidirectional replication creates two replication forks moving in opposite directions from each origin, doubling the rate of genome duplication.
Q.
The high fidelity of DNA replication (error rate ~1 in 10^9) is achieved by:
Random chance alone
Only post-replication repair
Base selection, proofreading, and mismatch repair
Only base selection by DNA polymerase
Explanation:
Base selection, 3'→5' proofreading exonuclease activity, and post-replication mismatch repair together produce the extremely low error rate observed.
Q.
Which of the following is NOT required for DNA replication?
DNA template
Primase
RNA polymerase
dNTPs
Explanation:
RNA polymerase is the enzyme for transcription. DNA replication requires a template, primers, dNTPs, and the replisome proteins, not RNA polymerase.
Q.
Okazaki fragments are found on which strand?
Both strands equally
Lagging strand
Leading strand
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.
Q.
Primers in DNA replication are made of:
DNA
Protein
Polysaccharide
RNA
Explanation:
Primase synthesizes short RNA primers (not DNA) because DNA polymerases can only extend an existing 3'-OH group, which RNA primers provide.
Q.
The replisome is best described as:
A type of topoisomerase
A ribosome-bound complex
A DNA repair enzyme
The multiprotein complex that carries out DNA replication
Explanation:
The replisome is the multiprotein machine at the replication fork composed of helicase, primase, DNA polymerases, and accessory factors.
Q.
Which of the following organisms uses a theta (θ) structure during DNA replication?
Retroviruses only
Mitochondria only
Bacteria with circular chromosomes
Eukaryotic linear chromosomes
Explanation:
Theta replication occurs in circular DNA molecules such as bacterial chromosomes and plasmids, producing a characteristic θ-shaped intermediate.