Question

Retroviruses, like HIV which causes AIDS, have their genetic information in the form of RNA. Reverse transcriptase synthesizes a DNA copy of the viral genome. One drug used in treating AIDS is AZT, an analog of deoxythymidine, which has an azido group at the \(3^{\prime}\) position of the sugar. It can be phosphorylated and competes with dTTP for incorporation into the reverse transcript. Once incorporated, its presence terminates chain elongation. There is a window in which the effect is primarily on viral replication since AZT is much less effective at competing with dTTP for incorporation by cellular DNA polymerases because of the proofreading ability of DNA polymerases. Proofreading activity to maintain the fidelity of DNA synthesis A. occurs after the synthesis has been completed. B. is a function of \(3^{\prime}\) to \(5^{\prime}\) exonuclease activity intrinsic to or associated with DNA polymerases. C. requires the presence of an enzyme separate from the DNA polymerases. D. removes mismatched bases in the interior of the chain. E. does not occur in prokaryotes.

Short answer

Answer: Proofreading activity in DNA synthesis is a function of \(3^{\prime}\) to \(5^{\prime}\) exonuclease activity intrinsic to or associated with DNA polymerases.

Step-by-step solution

1. Understanding Proofreading Activity in DNA synthesis

Proofreading activity is an essential mechanism employed by DNA polymerases during the process of DNA replication. It maintains the fidelity of DNA synthesis by checking for any errors that might have occurred during the addition of nucleotides to the growing DNA strand. If a mismatched base is found, it is removed and replaced with the correct base.

2. Analyze the Options

Let's analyze each option to understand which answer is the most accurate description of proofreading activity in DNA synthesis: A. Proofreading activity does not occur after the synthesis is completed but rather takes place during the DNA replication process. B. DNA polymerases contain a \(3^{\prime} \to 5^{\prime}\) exonuclease activity domain that recognizes and removes mismatched bases during DNA synthesis. This function helps maintain the fidelity of DNA replication. C. Although some separate enzymes can repair DNA errors, the proofreading activity discussed here is an intrinsic function of DNA polymerases or enzymes directly associated with them. D. Proofreading activity does remove mismatched bases, but it does not have the capability to remove them from the interior of the chain. It removes and corrects them during the replication process as the polymerase progresses. E. Proofreading activity occurs in both prokaryotes and eukaryotes, helping maintain the fidelity of their genetic information.

3. Choose the Correct Option

Based on the analysis of all options, option B seems to be the most accurate description of proofreading activity in DNA synthesis. It states that proofreading activity is a function of \(3^{\prime}\) to \(5^{\prime}\) exonuclease activity intrinsic to or associated with DNA polymerases. So, the correct answer is: B. is a function of \(3^{\prime}\) to \(5^{\prime}\) exonuclease activity intrinsic to or associated with DNA polymerases.