Q.53 Proteins involved in DNA replication are written below.
Arrange them chronologically in terms of function:
A. DNA polymerase I
B. DNA polymerase III
C. Helicase
D. Ligase
E. Primase
Choose the correct answer from the options given below.
- C, E, B, D, A
- C, E, B, A, D
- A, B, C, D, E
- B, A, C, E, D
The correct sequence of proteins in DNA replication is C, E, B, A, D. This chronological order reflects the prokaryotic replication process: unwinding, priming, elongation, gap filling, and sealing.
DNA Replication Sequence
DNA replication begins at the origin, forming a replication fork with leading (continuous) and lagging (discontinuous) strands. Prokaryotic enzymes act in a precise order to ensure semi-conservative duplication.
Correct Chronology: C → E → B → A → D
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C. Helicase: Unwinds DNA double helix at origin/replication fork by breaking H-bonds (ATP-dependent).
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E. Primase: Synthesizes short RNA primers (10-12 nt) on both strands for polymerase initiation.
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B. DNA polymerase III: Main replicative polymerase; extends primers (5’→3′ direction, leading strand continuous; lagging via Okazaki fragments).
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A. DNA polymerase I: Replaces RNA primers with DNA (5’→3′ polymerase + 5’→3′ exonuclease removes RNA); fills gaps.
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D. Ligase: Seals nicks between Okazaki fragments/DNA segments (forms phosphodiester bonds).
Option Analysis
Option Sequence Explanation C, E, B, D, A Wrong Ligase (D) before Pol I (A); gaps unsealed prematurely . C, E, B, A, D Correct Matches fork progression: unwind-prime-elongate-fill-seal . A, B, C, D, E Wrong Starts with Pol I (gap filler); no unwinding/priming first . B, A, C, E, D Wrong Pol III/Pol I before helicase/primase; impossible without fork/primer . Proteins involved in DNA replication chronological order is essential for NEET/GATE Life Sciences, outlining the five-step prokaryotic mechanism from unwinding to sealing.
Step-by-Step Functions
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Helicase (C): Initiates fork by separating strands.
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Primase (E): Provides RNA primers for synthesis start.
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DNA Pol III (B): Elongates strands (1000 nt/sec, processive).
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DNA Pol I (A): Removes primers, fills DNA gaps.
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Ligase (D): Joins fragments into continuous strand.
Exam Tips
Focus on prokaryotic (E. coli) order; eukaryotes use Pol α/δ/ε but sequence similar. Mnemonics: “Happy Elephants Bring Apples Daily” (Helicase, Primase, PolIII, PolI, Ligase).
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