Q.15 The RNA primer synthesized during the replication process in bacteria is removed by
(A) DNA gyrase (B)primase
(C) DNA polymeraseI (D) DNA polymerase II
RNA Primer Removal in Bacterial DNA Replication: Role of DNA Polymerase I
DNA polymerase I removes the RNA primer during bacterial DNA replication through its 5′ to 3′ exonuclease activity.
RNA Primer in Replication
Bacterial DNA replication requires short RNA primers synthesized by primase to initiate DNA synthesis on both leading and lagging strands. These primers provide the 3′-OH group needed for DNA polymerase III to extend new DNA strands. After synthesis, primers must be removed and replaced with DNA to complete the chromosome.
Correct Answer: DNA Polymerase I
Option (C) DNA polymerase I is correct. This enzyme possesses 5’→3′ exonuclease activity that specifically degrades RNA primers ahead of its polymerase activity, replacing them with DNA nucleotides via nick translation. DNA polymerase III performs the bulk of replication but lacks this exonuclease function.
Analysis of All Options
| Option | Enzyme | Role in Replication | Why Correct/Incorrect |
|---|---|---|---|
| A | DNA gyrase | Relieves supercoiling by introducing negative supercoils | Incorrect; manages DNA topology, not primer removal |
| B | Primase | Synthesizes RNA primers | Incorrect; creates primers, doesn’t remove them |
| C | DNA polymerase I | Removes RNA primers via 5’→3′ exonuclease, fills gaps | Correct; essential for Okazaki fragment maturation |
| D | DNA polymerase II | DNA repair, restarts stalled replication forks | Incorrect; no significant role in primer removal |
DNA gyrase prevents torsional stress during unwinding. Primase (DnaG) initiates but never removes primers. DNA polymerase II handles damage repair but lacks primer processing capability.
Mechanism of Primer Removal
On the lagging strand, DNA polymerase I binds at the RNA-DNA junction of each Okazaki fragment. Its exonuclease domain removes ribonucleotides while polymerase adds deoxyribonucleotides, maintaining a nick. DNA ligase then seals the phosphate backbone. This process ensures accurate genome duplication without RNA remnants.
