A mutant DNA polymerase was found to have higher error rate and synthesized only short DNA fragments. In the statements below, potential explanations are given.

A. The 5′ to 3′ exonuclease activity is compromised.
B. The 3′ to 5′ exonuclease activity is compromised.
C. The polymerase tends to frequently dissociate from the template.
D. The polymerase is unable to unwind the DNA template during replication.

Which one of the following options represents the combination of all correct statements?

Correct option is D

Problem Analysis:

• Mutant DNA polymerase has a higher error rate and synthesizes only short DNA fragments.

• We need to identify the correct explanation for this behavior based on the four provided statements.

Explanation of Statements:

1. Statement A: "The 5' to 3' exonuclease activity is compromised."

   • Incorrect: The 5' to 3' exonuclease activity is responsible for proofreading during DNA replication, ensuring errors are corrected by removing nucleotides from the 5' end. However, this activity typically does not affect the synthesis of short DNA fragments or cause an increased error rate directly. Therefore, Statement A is unlikely to explain the issue.

2. Statement B: "The 3' to 5' exonuclease activity is compromised."

   • Correct: The 3' to 5' exonuclease activity is essential for proofreading and correcting mistakes during DNA replication. If this activity is compromised, the polymerase would not be able to correct mismatched bases, leading to a higher error rate. Additionally, this might also contribute to shorter DNA fragments because the polymerase could stall without proper proofreading. Statement B is correct.

3. Statement C: "The polymerase tends to frequently dissociate from the template."

   • Correct: If the polymerase dissociates frequently from the template, it would result in shorter DNA fragments, as the polymerase does not stay attached to the template long enough to elongate the strand properly. This would explain the short DNA fragments synthesized by the mutant polymerase. Statement C is correct.

4. Statement D: "The polymerase is unable to unwind the DNA template during replication."

   • Incorrect: Unwinding of the DNA template is done by helicases, not by the polymerase itself. If the polymerase were unable to unwind the DNA template, the replication would likely be completely stalled rather than resulting in a higher error rate or short fragments. Therefore, Statement D is not a correct explanation.

Conclusion:

• Statements B and C are correct because:

  • B explains the higher error rate due to loss of proofreading (3' to 5' exonuclease activity).

  • C explains the synthesis of short DNA fragments due to frequent dissociation of the polymerase from the template.