In Escherichia coli DNA replication, which enzyme synthesizes the short RNA primers required to initiate Okazaki fragment synthesis on the lagging strand?

Introduction / Context:
DNA polymerases cannot start synthesis de novo; they extend an existing 3'-OH. Cells therefore use an RNA primase to lay down short RNA primers that provide the first 3'-OH for polymerase extension, especially important for discontinuous lagging-strand synthesis (Okazaki fragments).

Given Data / Assumptions:

• Organism: E. coli.
• Task: create RNA primers on the lagging strand.
• Protein naming convention: DnaA, DnaB, DnaC, DnaG are replication factors with distinct roles.

Concept / Approach:
DnaG is the primase in E. coli. It binds the helicase (DnaB) at the replication fork to intermittently synthesize short RNA primers (about 10–12 nucleotides), allowing DNA polymerase III to extend these primers into Okazaki fragments.

Step-by-Step Solution:
Identify the needed activity: RNA primer synthesis.Map E. coli proteins to functions: DnaG = primase.Select DnaG as the correct enzyme.

Verification / Alternative check:
Mutations in dnaG abolish primer synthesis and block lagging-strand replication, confirming its essential role.

Why Other Options Are Wrong:

• DnaA: initiator that melts oriC; does not synthesize RNA.
• DnaC: loads DnaB helicase; no primase activity.
• “All of these”: only DnaG has primase activity.
• DNA polymerase I: removes RNA primers (5'→3' exonuclease) and fills gaps; it does not create primers.

Common Pitfalls:
Conflating priming (RNA synthesis) with primer removal and gap filling (Pol I); these are separate steps carried out by different enzymes.

Final Answer:
DnaG (primase)