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Molecular Biology

DNA Replication Practice Questions

37 free DNA Replication practice questions for the Zoology. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.

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Question 1 of 37 Medium

Which enzyme is responsible for unwinding the DNA double helix at the replication fork by breaking hydrogen bonds between nitrogenous bases?

  1. A DNA Ligase
  2. B DNA Helicase
  3. C Topoisomerase
  4. D RNA Primase

Correct answer: DNA Helicase

DNA Helicase uses energy from ATP hydrolysis to separate the two parental strands of DNA. This creates the replication fork, allowing other enzymes access to the single-stranded templates.

Question 2 of 37 Medium

During DNA replication, the 'Leading Strand' is synthesized in which manner?

  1. A Discontinuously in the 3' to 5' direction
  2. B Continuously in the 5' to 3' direction
  3. C Discontinuously in the 5' to 3' direction
  4. D Continuously in the 3' to 5' direction

Correct answer: Continuously in the 5' to 3' direction

DNA polymerases can only add nucleotides to the 3' end of a growing strand. Since the leading strand template runs 3' to 5' toward the fork, the new strand is synthesized continuously in the 5' to 3' direction.

Question 3 of 37 Medium

What is the function of Single-Strand Binding Proteins (SSBs) during the replication process?

  1. A To catalyze the formation of phosphodiester bonds
  2. B To keep separated strands stable and prevent re-annealing
  3. C To synthesize short RNA primers
  4. D To proofread the newly synthesized DNA

Correct answer: To keep separated strands stable and prevent re-annealing

Once helicase separates the DNA strands, single-strand binding proteins (SSBs) coat the single-stranded regions to prevent re-annealing or hairpin formation. This keeps the template accessible for replication machinery.

Question 4 of 37 Medium

Which enzyme relieves the torsional strain and supercoiling created ahead of the replication fork?

  1. A DNA Polymerase I (repair enzyme)
  2. B DNA Gyrase (Topoisomerase II)
  3. C Primase (RNA primer enzyme)
  4. D Telomerase (end-repair enzyme)

Correct answer: DNA Gyrase (Topoisomerase II)

As helicase unwinds the DNA, it creates positive supercoils ahead of the replication fork. DNA Gyrase (a type II topoisomerase) cuts, passes, and rejoins DNA strands to relieve this torsional tension, preventing the fork from stalling.

Question 5 of 37 Medium

DNA Polymerase III requires a free 3'-OH group to begin synthesis. Which enzyme provides this by creating a short RNA sequence?

  1. A DNA Ligase
  2. B RNA Polymerase II
  3. C Primase (RNA primer synthase)
  4. D Helicase

Correct answer: Primase (RNA primer synthase)

Primase is a specialized RNA polymerase that synthesizes a short RNA primer (typically 5–10 nucleotides), providing the free 3'-OH group that DNA Polymerase III requires to begin extending the new DNA strand.

Question 6 of 37 Medium

The discontinuous segments of DNA synthesized on the lagging strand are known as:

  1. A Griffith fragments
  2. B Okazaki fragments
  3. C Watson-Crick sequences
  4. D Chargaff units

Correct answer: Okazaki fragments

Because the lagging strand template runs 5' to 3' toward the fork, synthesis must occur in short bursts away from the fork. These 1000-2000 nucleotide segments are named Okazaki fragments after the scientists who discovered them.

Question 7 of 37 Medium

In prokaryotes, which enzyme removes the RNA primers and replaces them with DNA nucleotides?

  1. A DNA Polymerase I
  2. B DNA Polymerase III
  3. C DNA Ligase
  4. D Exonuclease V

Correct answer: DNA Polymerase I

DNA Polymerase I has 5' to 3' exonuclease activity, allowing it to digest RNA primers. It simultaneously uses its polymerase activity to fill the resulting gaps with the appropriate DNA nucleotides.

Question 8 of 37 Medium

Which enzyme acts as the 'glue' to join the sugar-phosphate backbones of DNA fragments, such as Okazaki fragments?

  1. A Helicase
  2. B DNA Ligase
  3. C Primase
  4. D Topoisomerase

Correct answer: DNA Ligase

DNA Ligase catalyzes the formation of a phosphodiester bond between the 3'-OH of one fragment and the 5'-phosphate of the next. This step is essential for creating a continuous, unbroken DNA strand.

Question 9 of 37 Medium

DNA replication is described as 'Semi-conservative.' This means that:

  1. A Both strands of the daughter DNA are entirely new
  2. B Both strands of the daughter DNA are from the original parent
  3. C Each daughter molecule contains one parental strand and one new strand
  4. D Only half of the genes are replicated in each cycle

Correct answer: Each daughter molecule contains one parental strand and one new strand

The Meselson-Stahl experiment confirmed semi-conservative replication: each parental strand serves as a template, producing two daughter helices each containing one original strand and one newly synthesized strand.

Question 10 of 37 Medium

The 3' to 5' exonuclease activity of DNA Polymerase III is primarily used for:

  1. A Removing RNA primers from the lagging strand
  2. B Proofreading and correcting mismatched bases
  3. C Degrading the parent strand ahead of synthesis
  4. D Synthesizing the leader sequence at the start site

Correct answer: Proofreading and correcting mismatched bases

The 3' to 5' exonuclease activity of DNA Polymerase III allows it to detect and excise a mismatched nucleotide immediately after it is added, then resume correct synthesis — an essential proofreading mechanism that maintains replication fidelity.

Question 11 of 37 Medium

Which specific region of the DNA serves as the starting point for replication?

  1. A Promoter (transcription start site)
  2. B Origin of Replication (OriC)
  3. C Terminator (replication stop site)
  4. D Operator (repressor binding site)

Correct answer: Origin of Replication (OriC)

Replication begins at a defined DNA sequence called the origin of replication. Prokaryotes like E. coli have a single OriC, while eukaryotic chromosomes have thousands of origins to enable rapid, parallel replication.

Question 12 of 37 Medium

Eukaryotic DNA replication faces the 'end-replication problem' at the telomeres. Which enzyme solves this by extending the ends of chromosomes?

  1. A DNA Polymerase Alpha
  2. B Telomerase
  3. C Ribonuclease
  4. D Ligase

Correct answer: Telomerase

Telomerase is a ribonucleoprotein that carries its own RNA template. It adds repetitive DNA sequences to the 3' ends of chromosomes, preventing the loss of vital genetic information during successive rounds of division.

Question 13 of 37 Medium

What is the role of the 'Sliding Clamp' (beta-subunit in E. coli) in DNA replication?

  1. A To unwind the DNA at the replication fork
  2. B To keep DNA polymerase attached to the template strand
  3. C To identify and bind the origin of replication
  4. D To hydrolyze ATP for powering helicase movement

Correct answer: To keep DNA polymerase attached to the template strand

The sliding clamp (beta-subunit in E. coli; PCNA in eukaryotes) encircles the DNA and tethers DNA polymerase to the template, dramatically increasing its processivity so it can synthesize thousands of nucleotides without falling off.

Question 14 of 37 Medium

During replication, which of the following provides the energy required for the formation of the phosphodiester bond?

  1. A ATP hydrolysis by helicase at the replication fork
  2. B Cleavage of pyrophosphate from incoming dNTPs
  3. C The sodium ion concentration gradient across the membrane
  4. D Heat energy released from the surrounding cytoplasm

Correct answer: Cleavage of pyrophosphate from incoming dNTPs

Each incoming deoxynucleoside triphosphate (dNTP) donates its alpha-phosphate to form the phosphodiester bond. The subsequent hydrolysis of the released pyrophosphate (PPi) by pyrophosphatase drives the reaction forward thermodynamically.

Question 15 of 37 Medium

Which eukaryotic DNA polymerase is primarily responsible for the synthesis of the lagging strand?

  1. A Polymerase Alpha
  2. B Polymerase Gamma
  3. C Polymerase Delta
  4. D Polymerase Epsilon

Correct answer: Polymerase Delta

In eukaryotes, DNA Polymerase Epsilon is typically associated with leading strand synthesis, while DNA Polymerase Delta handles the lagging strand. Polymerase Gamma is reserved for mitochondrial DNA replication.

Question 16 of 37 Medium

The direction of DNA synthesis is always:

  1. A 3' to 5' because that is the direction the template is read
  2. B 5' to 3' because nucleotides add to the free 3'-OH group
  3. C C-terminus to N-terminus along the polypeptide chain
  4. D Random, depending on local dNTP availability

Correct answer: 5' to 3' because nucleotides add to the free 3'-OH group

DNA synthesis always occurs in the 5' to 3' direction: the incoming nucleotide's 5'-phosphate attacks the 3'-OH of the last nucleotide in the growing chain, forming a phosphodiester bond.

Question 17 of 37 Medium

What is a 'Replisome'?

  1. A The multi-protein machine that carries out DNA synthesis at the replication fork
  2. B The specific DNA sequence where replication terminates
  3. C A specialized organelle where DNA is packaged and stored
  4. D A class of mutation induced by ultraviolet radiation

Correct answer: The multi-protein machine that carries out DNA synthesis at the replication fork

The replisome is the large, coordinated protein complex at the replication fork, comprising helicase, primase, DNA polymerases, sliding clamps, and accessory proteins that work together to replicate DNA efficiently and accurately.

Question 18 of 37 Medium

In eukaryotes, the 'Pre-Replication Complex' is licensed during which phase of the cell cycle?

  1. A S phase, when replication begins
  2. B G1 phase
  3. C G2 phase, after replication is complete
  4. D M phase, during chromosome segregation

Correct answer: G1 phase

To ensure DNA is replicated only once per cycle, the Origin Recognition Complex (ORC) and licensing factors (Mcm2-7) assemble at origins during G1. This 'licensing' step is inactivated once the cell enters S phase, preventing re-replication.

Question 19 of 37 Medium

Which enzyme is unique for its ability to synthesize a DNA strand using an RNA template?

  1. A DNA Polymerase I
  2. B Reverse transcriptase
  3. C Primase, which makes RNA primers
  4. D RNA Polymerase III

Correct answer: Reverse transcriptase

Reverse transcriptase synthesizes a DNA strand using an RNA template — the hallmark of retroviruses like HIV. DNA Polymerase I and RNA Polymerase III work only on DNA or RNA templates in the conventional direction; primase lays down short RNA primers but cannot use RNA as a template for sustained DNA synthesis.

Question 20 of 37 Medium

What happens to the histones in eukaryotic cells during DNA replication?

  1. A They are permanently destroyed to allow access to DNA
  2. B They remain bound to the leading strand only
  3. C They are displaced and then reassembled on both daughter strands
  4. D They migrate to the nucleolus until replication is complete

Correct answer: They are displaced and then reassembled on both daughter strands

As the replication fork progresses, nucleosomes are transiently disrupted. Old histones are distributed to both daughter strands, and newly synthesized histones are deposited by chaperone proteins such as CAF-1, restoring chromatin structure on each daughter duplex.

Question 21 of 37 Medium

The direction of DNA synthesis during replication is:

  1. A 5' to 3'
  2. B 3' to 5'
  3. C Both directions simultaneously
  4. D Random direction

Correct answer: 5' to 3'

DNA polymerases synthesize new DNA strands only in the 5' to 3' direction. This is due to the requirement of a free 3'-OH group.

Question 22 of 37 Medium

Which enzyme unwinds the DNA double helix at the replication fork?

  1. A DNA ligase
  2. B Helicase
  3. C Primase
  4. D Topoisomerase

Correct answer: Helicase

Helicase breaks hydrogen bonds between DNA strands, unwinding the double helix during replication.

Question 23 of 37 Medium

Which enzyme joins Okazaki fragments on the lagging strand?

  1. A Helicase
  2. B DNA polymerase
  3. C DNA ligase
  4. D Primase

Correct answer: DNA ligase

DNA ligase seals the gaps between Okazaki fragments by forming phosphodiester bonds.

Question 24 of 37 Medium

The discontinuous DNA segments formed on the lagging strand are called:

  1. A Coding exons
  2. B Non-coding introns
  3. C Okazaki fragments
  4. D Bacterial plasmids

Correct answer: Okazaki fragments

Okazaki fragments are short DNA segments synthesized discontinuously on the lagging strand.

Question 25 of 37 Medium

Which enzyme replaces RNA primers with DNA nucleotides in prokaryotes?

  1. A DNA polymerase I
  2. B DNA polymerase III
  3. C Ligase
  4. D Helicase

Correct answer: DNA polymerase I

DNA polymerase I removes RNA primers and replaces them with DNA nucleotides in prokaryotes.

Question 26 of 37 Medium

Which enzyme synthesizes the main DNA strand during replication in prokaryotes?

  1. A DNA polymerase I
  2. B DNA polymerase II
  3. C DNA polymerase III
  4. D Reverse transcriptase

Correct answer: DNA polymerase III

DNA polymerase III is the primary enzyme responsible for DNA synthesis in prokaryotes.

Question 27 of 37 Medium

Which structure serves as the starting point for DNA replication?

  1. A Replication fork
  2. B Origin of replication
  3. C Transcriptional promoter region
  4. D Gene operator sequence

Correct answer: Origin of replication

The origin of replication is a specific DNA sequence where replication begins.

Question 28 of 37 Medium

What is the role of single-strand binding proteins (SSBs)?

  1. A Join adjacent DNA fragments
  2. B Stabilize unwound DNA strands
  3. C Synthesize short RNA primers
  4. D Break double-stranded DNA molecules

Correct answer: Stabilize unwound DNA strands

SSBs bind to single-stranded DNA to prevent re-annealing and protect it during replication.

Question 29 of 37 Medium

The leading strand is synthesized:

  1. A Discontinuously
  2. B Continuously
  3. C In fragments
  4. D Randomly

Correct answer: Continuously

The leading strand is synthesized continuously in the direction of the replication fork.

Question 30 of 37 Medium

Which strand is synthesized discontinuously?

  1. A Leading strand
  2. B Lagging strand
  3. C Template strand
  4. D Coding strand

Correct answer: Lagging strand

The lagging strand is synthesized discontinuously as Okazaki fragments due to opposite orientation.

Question 31 of 37 Medium

What provides energy for DNA polymerization?

  1. A ATP
  2. B NADH
  3. C dNTPs
  4. D Proteins

Correct answer: dNTPs

The hydrolysis of deoxynucleotide triphosphates (dNTPs) provides energy for forming phosphodiester bonds.

Question 32 of 37 Medium

Which enzyme has 3' to 5' exonuclease activity for proofreading?

  1. A DNA helicase enzyme
  2. B RNA primase enzyme
  3. C DNA polymerase III
  4. D DNA ligase enzyme

Correct answer: DNA polymerase III

DNA polymerase III has proofreading ability, correcting errors during replication.

Question 33 of 37 Medium

What is a replisome?

  1. A A single enzyme catalyzing DNA synthesis
  2. B A structural motif within chromatin
  3. C A large complex of replication enzymes
  4. D A regulatory DNA-binding protein

Correct answer: A large complex of replication enzymes

A replisome is a complex of multiple proteins involved in DNA replication, including polymerases and helicases.

Question 34 of 37 Medium

Which enzyme extends telomeres in eukaryotic cells?

  1. A DNA polymerase
  2. B Telomerase
  3. C Ligase
  4. D Helicase

Correct answer: Telomerase

Telomerase adds repetitive DNA sequences to chromosome ends, solving the end-replication problem.

Question 35 of 37 Medium

Which enzyme is unique for synthesizing DNA using an RNA template?

  1. A DNA-dependent DNA polymerase
  2. B DNA-dependent RNA polymerase
  3. C DNA ligase enzyme
  4. D Reverse transcriptase

Correct answer: Reverse transcriptase

Reverse transcriptase synthesizes DNA from an RNA template. It is used by retroviruses.

Question 36 of 37 Medium

During DNA replication, histones:

  1. A Are permanently degraded and removed
  2. B Remain completely unchanged throughout
  3. C Completely block the replication process
  4. D Are disassembled and then reassembled

Correct answer: Are disassembled and then reassembled

Histones are temporarily displaced during replication and then reassembled onto newly formed DNA.

Question 37 of 37 Medium

Which enzyme forms phosphodiester bonds between nucleotides?

  1. A DNA polymerase
  2. B Helicase
  3. C DNA ligase
  4. D Topoisomerase enzyme

Correct answer: DNA polymerase

DNA polymerase catalyzes the formation of phosphodiester bonds between nucleotides during DNA synthesis.

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