Question

Which of the following activity is not present in Taq polymerase?

• A. 5'–3' polymerase
• B. 5'–3' exonuclease
• C. 3'–5' exonuclease
• D. Both 5'–3' polymerase and 5'–3' exonuclease

Hint:

Taq polymerase lacks proofreading ability because it does not have 3'–5' exonuclease activity, making it prone to errors during amplification.

Answer 1

The Correct Option is C

Taq polymerase is built for speed and heat stability in PCR, not for accuracy. It has 5'-3' polymerase activity (to extend the primer) and 5'-3' exonuclease activity (used in probe-cleavage-based detection methods), so both of these are present, ruling out options that claim otherwise.

What Taq lacks is 3'-5' exonuclease activity, the proofreading function that checks and removes mismatched bases -- this is why Taq-amplified DNA has a higher error rate than DNA amplified with proofreading enzymes such as Pfu.

So the activity not present in Taq polymerase is 3'-5' exonuclease.

Answer 2

A comparative angle: line Taq up against a proofreading polymerase such as Pfu and see which activity separates them.

1. 5'-3' polymerase: Every DNA-synthesizing polymerase, proofreading or not, has this activity, since it is the basic chemistry of extending a strand. This does not distinguish Taq from anything.
2. 5'-3' exonuclease: This is a feature of Taq-family (Pol I-type) polymerases used for nick translation and probe degradation; it is present in Taq.
3. 3'-5' exonuclease: This is the activity that proofreading polymerases like Pfu possess but Taq does not. Its presence or absence is the actual dividing line between "high-fidelity" and "standard" thermostable polymerases, and Taq falls on the side that lacks it.
4. Both 5'-3' polymerase and 5'-3' exonuclease: Grouping these two together as absent contradicts the fact that both are individually present in Taq, so this option cannot be right.

The comparison confirms that the one activity separating Taq from proofreading enzymes, and therefore the one missing from it, is 3'-5' exonuclease activity.

The correct answer is 3'-5' exonuclease.