3' → 5' exonuclease function allows polymerases to perform what action?

3' → 5' exonuclease function allows polymerases to perform what action?

• A. Break the phosphodiester backbone on a single strand of DNA and remove a nucleotide
• B. Extend the growing DNA strand by adding nucleotides
• C. Ligate DNA fragments
• D. Initiate primer synthesis

Answer: (A)

3' to 5' exonuclease activity provides proofreading during DNA synthesis. When a wrong nucleotide is incorporated, the polymerase uses this exonuclease function to back up from the 3' end of the growing strand and remove the mispaired nucleotide. This action breaks the phosphodiester bond at the end of the nascent strand and excises the incorrect base, creating a correct 3'-OH terminus so replication can resume with the proper nucleotide. That precise removal of a single nucleotide from the end is what the option describes. Other processes listed are handled by different activities: simply extending the strand is the polymerase’s main synthesis activity (adding nucleotides in the 5'→3' direction); ligating DNA fragments is done by DNA ligase; initiating primer synthesis is the job of primase.

3' to 5' exonuclease activity provides proofreading during DNA synthesis. When a wrong nucleotide is incorporated, the polymerase uses this exonuclease function to back up from the 3' end of the growing strand and remove the mispaired nucleotide. This action breaks the phosphodiester bond at the end of the nascent strand and excises the incorrect base, creating a correct 3'-OH terminus so replication can resume with the proper nucleotide. That precise removal of a single nucleotide from the end is what the option describes.

Other processes listed are handled by different activities: simply extending the strand is the polymerase’s main synthesis activity (adding nucleotides in the 5'→3' direction); ligating DNA fragments is done by DNA ligase; initiating primer synthesis is the job of primase.