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DNA Replication Questions
Q.
DNA replication is described as semiconservative because:
Both parental strands stay together in one daughter
Each daughter molecule has one parental and one newly synthesized strand
Only one daughter molecule is produced
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?
Sulfur-35 (35S)
Nitrogen-15 (15N)
Phosphorus-32 (32P)
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:
Origins of replication
Centromeres
Promoters
Telomeres
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:
Promoter region
Replisome
Okazaki fork
Replication 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?
DNA polymerase
Ligase
DNA helicase
Primase
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
Topoisomerase
DNA ligase
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 3' end
The 5' 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:
Coding strand
Lagging strand
Leading strand
Template 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:
Leading strand
Lagging strand
Sense strand
Non-template 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:
Okazaki fragments
Klenow fragments
Watson fragments
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.
Q.
The enzyme that joins Okazaki fragments together is:
DNA helicase
DNA polymerase I
DNA ligase
Topoisomerase
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 II
DNA polymerase IV
DNA polymerase I
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?
Primase
DNA polymerase II
DNA polymerase I
DNA polymerase III
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?
Endonuclease activity
Helicase activity
3'→5' exonuclease 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:
Cleave phosphodiester bonds
Synthesize RNA primers
Prevent re-annealing of separated DNA strands
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:
Cleave Okazaki fragments
Extend chromosome ends using an RNA template
Remove RNA primers
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.
Q.
Eukaryotic chromosomes typically have how many origins of replication per chromosome?
No fixed origins
Multiple origins
Two origins only
Exactly one origin
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:
Conservative replication
Bidirectional replication
Rolling circle replication
Unidirectional 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:
Only post-replication repair
Only base selection by DNA polymerase
Base selection, proofreading, and mismatch repair
Random chance alone
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?
RNA polymerase
DNA template
Primase
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?
Leading strand
Neither strand
Both strands equally
Lagging 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:
Protein
DNA
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:
The multiprotein complex that carries out DNA replication
A type of topoisomerase
A DNA repair enzyme
A ribosome-bound complex
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?
Mitochondria only
Retroviruses 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.