Question 51:
In E.coli, the enzyme that fills gaps in duplexes by stepwise addition of nucleotides to the 3’ end is called:
The correct answer is (B) DNA polymerase I.
In E. coli, DNA polymerase I fills gaps in DNA duplexes during Okazaki fragment processing by adding nucleotides to the 3′ end after its 5’→3′ exonuclease removes RNA primers.
Option Breakdown
(A) Type II restriction endonucleases
These enzymes cleave DNA at specific recognition sequences, creating gaps but not filling them; they defend against foreign DNA, not replication.
(B) DNA polymerase I
Pol I (polA gene product) has 5’→3′ polymerase activity to fill gaps stepwise at the 3′ OH end, plus 5’→3′ exonuclease for primer removal and 3’→5′ proofreading in E. coli replication.
(C) DNA ligase
This seals nicks in phosphodiester backbones after gaps are filled, forming covalent bonds but not adding nucleotides.
(D) Terminal transferase
A template-independent enzyme adding nucleotides to 3′ ends, used in lab labeling, not gap-filling in duplexes during replication.
Introduction to E coli Enzyme Fills Gaps Duplexes
The E coli enzyme fills gaps duplexes by stepwise 3′ addition during lagging strand synthesis, identifying DNA polymerase I as key in replication. Essential for GATE Life Sciences molecular biology sections.
Role in DNA Replication
After primase lays RNA primers, Pol III elongates; Pol I then excises primers via 5’→3′ exonuclease and fills gaps with dNTPs at the 3′ end before ligase seals. This matsutake ensures continuous DNA strands.
Pol I’s Klenow fragment exemplifies this multifunctional repair role.
Enzyme Functions Comparison Table
| Option | Primary Function | Fills 3′ Gaps? | E. coli Replication Role |
|---|---|---|---|
| Type II Restriction | Cleaves specific sequences | No | Defense |
| DNA Polymerase I | Gap filling, primer removal | Yes | Okazaki processing |
| DNA Ligase | Seals nicks | No | Final joining |
| Terminal Transferase | Adds to blunt 3′ ends | Template-free | Lab use |
This clarifies the E coli enzyme fills gaps duplexes for exams.
GATE Exam Relevance
Mastering Pol I’s role links to prior liver queries via replication fidelity in prokaryotic models, boosting genetics scores.
