AZT is an example of which class of compounds used to terminate DNA synthesis?

AZT is an example of which class of compounds used to terminate DNA synthesis?

• A. A nucleoside analog that terminates DNA synthesis.
• B. A transcription inhibitor that blocks RNA polymerase.
• C. A DNA methyltransferase.
• D. A ligase enzyme used in cloning.

Answer: (A)

AZT works because it’s a nucleoside analog designed to halt DNA synthesis. It looks enough like thymidine to be used by viral reverse transcriptase, but after it’s converted inside the cell to its triphosphate form, it’s incorporated into the growing DNA strand. The crucial detail is that it has a 3' position missing a hydroxyl group, so once AZT is added, there’s no 3'-OH available to form the next phosphodiester bond. That stops the chain from extending, effectively terminating DNA synthesis. This is different from blocking transcription, which would stop RNA polymerase from making RNA rather than stopping DNA chain elongation. It’s also not a DNA methyltransferase or a ligase enzyme, which have distinct roles in modifying DNA bases and sealing nicks or joining fragments, respectively. So AZT is best described as a nucleoside analog that terminates DNA synthesis.

AZT works because it’s a nucleoside analog designed to halt DNA synthesis. It looks enough like thymidine to be used by viral reverse transcriptase, but after it’s converted inside the cell to its triphosphate form, it’s incorporated into the growing DNA strand. The crucial detail is that it has a 3' position missing a hydroxyl group, so once AZT is added, there’s no 3'-OH available to form the next phosphodiester bond. That stops the chain from extending, effectively terminating DNA synthesis.

This is different from blocking transcription, which would stop RNA polymerase from making RNA rather than stopping DNA chain elongation. It’s also not a DNA methyltransferase or a ligase enzyme, which have distinct roles in modifying DNA bases and sealing nicks or joining fragments, respectively. So AZT is best described as a nucleoside analog that terminates DNA synthesis.