Question

The RNA primer synthesized during bacterial DNA replication is removed by .

• A. DNA gyrase
• B. DNA Polymerase I
• C. Primase
• D. DNA Polymerase III

Hint:

Recall that removing an RNA primer needs a 5' to 3' exonuclease activity paired with polymerase activity; only one bacterial DNA polymerase has both.

Answer 1

The Correct Option is B

Step 1: List the enzymes at a bacterial replication fork.
Bacterial DNA replication uses several enzymes together. DNA gyrase relieves the supercoiling ahead of the fork. Primase lays down short RNA primers so synthesis can start. DNA Polymerase III extends these primers and builds most of the new strand. DNA Polymerase I acts later, after Polymerase III has finished a stretch of DNA.

Step 2: Recall why an RNA primer must be removed.
Every Okazaki fragment on the lagging strand, and the very start of the leading strand, begins with a short RNA primer made by primase, since DNA polymerases cannot start a new chain on their own; they can only add to an existing 3' end. Once the DNA polymerase has copied past this stretch, the RNA piece has to be taken out and replaced with DNA, otherwise the genome would carry RNA nucleotides mixed into it.

Step 3: Identify the enzyme that removes the primer.
DNA Polymerase I is the enzyme that does this job in bacteria. It carries a 5' to 3' exonuclease activity that chews away the RNA primer ahead of it, while its 5' to 3' polymerase activity fills the gap with DNA at the same time. This combined action is called nick translation. DNA ligase then seals the remaining nick between the old and new DNA.

Step 4: Rule out the other options.
DNA gyrase only removes supercoils; it does not touch RNA primers. Primase makes the RNA primer, it does not remove it. DNA Polymerase III is the main replicating enzyme with high speed and processivity, but it lacks a 5' to 3' exonuclease activity, so it cannot excise primers.

Final Answer:
The RNA primer is removed by DNA Polymerase I, using its 5' to 3' exonuclease activity coupled to its polymerase activity (nick translation). \[ \boxed{\text{DNA Polymerase I}} \]