What is proofreading in DNA replication?

What is proofreading in DNA replication?

• A. RNA polymerase's proofreading activity.
• B. Ligase seals mispaired bases.
• C. Topoisomerase proofreads mismatches.
• D. DNA polymerase's 3' to 5' exonuclease activity corrects mispaired bases during replication.

Answer: (D)

Proofreading during DNA replication is the built-in quality check of the DNA polymerase. As the enzyme adds nucleotides in the 5' to 3' direction, it constantly tests the recently formed base pair. If a wrong nucleotide is incorporated, the polymerase pauses and uses its 3' to 5' exonuclease activity to remove that incorrect nucleotide from the end of the growing strand. After removal, the polymerase resumes synthesis, inserting the correct nucleotide opposite the template. This backtracking and removal step happens right at the moment of replication, dramatically increasing fidelity by catching errors before the new strand is completed. Other enzymes mentioned operate in different roles. RNA polymerase is used for transcribing DNA into RNA, not proofreading DNA during replication. DNA ligase seals nicks after fragments are synthesized, not mismatches. Topoisomerase relieves supercoiling ahead of the replication fork, also not proofreading. Remember that cells also rely on separate mismatch repair after replication to fix any errors that slip past proofreading.

Proofreading during DNA replication is the built-in quality check of the DNA polymerase. As the enzyme adds nucleotides in the 5' to 3' direction, it constantly tests the recently formed base pair. If a wrong nucleotide is incorporated, the polymerase pauses and uses its 3' to 5' exonuclease activity to remove that incorrect nucleotide from the end of the growing strand. After removal, the polymerase resumes synthesis, inserting the correct nucleotide opposite the template. This backtracking and removal step happens right at the moment of replication, dramatically increasing fidelity by catching errors before the new strand is completed.

Other enzymes mentioned operate in different roles. RNA polymerase is used for transcribing DNA into RNA, not proofreading DNA during replication. DNA ligase seals nicks after fragments are synthesized, not mismatches. Topoisomerase relieves supercoiling ahead of the replication fork, also not proofreading. Remember that cells also rely on separate mismatch repair after replication to fix any errors that slip past proofreading.