How are RNA primers removed and replaced with DNA on the lagging strand?

• A. RNA primers are removed by RNase and replaced with DNA by DNA polymerase, then ligase joins gaps.
• B. DNA polymerase removes primers and replaces with RNA.
• C. Primers are not removed.
• D. Helicase removes primers.

Answer: (A)

On the lagging strand, RNA primers laid down to start each Okazaki fragment must be removed and replaced with DNA so the strand can become continuous. The RNA primer is removed by a nuclease (an RNase in this context), and then DNA polymerase fills the resulting gap with DNA. After that, DNA ligase seals the remaining nicks between fragments, producing a continuous sugar-phosphate backbone.

In bacteria, RNase H (or related nucleases) removes the RNA primer, and DNA polymerase I fills in the DNA in place of the RNA primer. In eukaryotes, RNase H and other nucleases remove the primer, DNA polymerase delta fills in the DNA, and DNA ligase I seals the final nick. The key idea is that primers are removed and replaced with DNA, not RNA, and then the fragments are ligated together.

Primers aren’t removed by helicase, which unwinds DNA, and DNA polymerase doesn’t replace primers with RNA, so those options aren’t correct.