Which enzyme catalyzes the 5'→3' elongation of a new DNA strand during replication?
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ADNA polymerase
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BHelicase
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CDNA ligase
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DPrimase
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ETopoisomerase I
Answer
Correct Answer: DNA polymerase
Explanation
Introduction / Context:Multiple enzymes coordinate at the replication fork. Identifying which one directly adds deoxyribonucleotides to a growing DNA chain is foundational knowledge for molecular biology.
Given Data / Assumptions:
- Question targets the catalytic polymerization step.
- Standard fork proteins act with distinct roles: unwinding, priming, elongating, ligating, and topology control.
Concept / Approach:DNA polymerases catalyze nucleotidyl transfer: the 3'-OH of the primer strand attacks the alpha-phosphate of an incoming dNTP, releasing pyrophosphate and forming a phosphodiester bond, extending DNA 5'→3'.
Step-by-Step Solution:
1) Determine which function is “elongation of DNA”.2) Match to enzyme class: DNA polymerase performs template-directed addition of dNTPs.3) Conclude DNA polymerase is the elongating enzyme.Verification / Alternative check:In vitro, purified DNA polymerases synthesize DNA on primed templates; other fork proteins cannot catalyze chain elongation alone.
Why Other Options Are Wrong:
- Helicase unwinds duplex DNA.
- DNA ligase seals nicks after synthesis.
- Primase synthesizes short RNA primers.
- Topoisomerase I relaxes supercoils; no chain elongation.
Common Pitfalls:Confusing primase (primer synthesis) with polymerase (DNA extension).
Final Answer:DNA polymerase