Q.43 Which of the following characteristics with respect to bacterial DNA polymerase III are TRUE?
P. Initiation of chain synthesis
Q. 5′ → 3′ polymerization
R. 3′ → 5′ exonuclease activity
S. 5′ → 3′ exonuclease activity
(A) P and Q only
(B) Q and R only
(C) Q and S only
(D) P and S only
Correct Answer: (B) Q and R only
Bacterial DNA polymerase III performs 5′ → 3′ polymerization (Q) and possesses 3′ → 5′ exonuclease proofreading activity (R), but it does not initiate chain synthesis (P)—primase (DnaG) synthesizes RNA primers—and lacks intrinsic 5′ → 3′ exonuclease activity (S), which belongs to DNA polymerase I.
Option Analysis
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(A) P and Q only: Incorrect. Pol III cannot initiate de novo synthesis; requires 3′-OH primer from primase.
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(B) Q and R only: Correct. α-subunit catalyzes 5′ → 3′ polymerization (~1000 nt/s); ε-subunit provides 3′ → 5′ proofreading (error rate 10⁻⁷).
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(C) Q and S only: Incorrect. 5′ → 3′ exonuclease (S) removes RNA primers during Okazaki fragment processing (Pol I function).
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(D) P and S only: Incorrect. Neither initiation nor 5′ → 3′ exonuclease applies to Pol III holoenzyme.
Bacterial DNA polymerase III 5′→3′ polymerization and 3′→5′ exonuclease activities define core replication fidelity in Q.43, testing biochemical engineering students’ understanding of E. coli replisome dynamics.
Pol III Holoenzyme Functions
α-subunit (dnaE): Catalyzes dNTP → DNA phosphodiester bonds; palm-thumb-fingers domain; processivity >500 kb via β-clamp.
ε-subunit (dnaQ/mutD): 3′ → 5′ exonuclease removes misincorporated nucleotides (immediate editing); fidelity boost 10³-10⁴.
Non-functions: Primase (DnaG) synthesizes 10-12 nt RNA primers; Pol I (polA) performs nick translation with 5′ → 3′ exonuclease.
Replication Fork Coordination
Leading: Pol III → 1000 nt/s → β-clamp sliding
Lagging: Pol III → Okazaki fragments (1-2 kb) → SSB-coated
Maturation: Pol I → RNA removal → DNA fill-in → DNA ligase
Fidelity Cascade
Nucleotide selection (10⁻⁴-10⁻⁵) × ε-proofreading (10⁻²-10⁻³) × MMR (10⁻²) = overall 10⁻⁹-10⁻¹⁰ error rate.
GATE Biotechnology Integration
Q.43 links Q.35-42 progression: Pol III → synchronous culture timing → IPTG plasmid replication → Shine-Dalgarno expression → PPI replisome assembly → metabolic pathway localization. Essential for μ = ln(2)/G₁ doubling calculations and bioreactor nucleotide pool optimization.
Exam Strategy
Eliminate P (primase) and S (Pol I) immediately; recognize α/ε subunit specialization. Quantitative: Pol III K_m(dNTP) ~1-10 μM vs Pol I ~50 μM distinguishes replication vs repair polymerases.
