Which enzyme listed lacks 3'→5' exonuclease (proofreading) activity under standard conditions used in PCR and DNA synthesis workflows?

Introduction / Context:
Proofreading improves fidelity by removing misincorporated nucleotides via 3'→5' exonuclease activity. Different polymerases vary in this capability, directly affecting error rates in amplification and synthesis applications such as PCR or cloning.

Given Data / Assumptions:

• Klenow fragment retains 3'→5' exonuclease.
• Taq polymerase (from Thermus aquaticus) lacks 3'→5' proofreading.
• Ligases do not have exonuclease activity and are not polymerases.

Concept / Approach:
Taq DNA polymerase performs 5'→3' polymerization but lacks 3'→5' exonuclease. Consequently, its base substitution error rate is higher than that of proofreading polymerases (for example, Pfu, Phusion). Klenow and E. coli Pol III possess proofreading; ligase is irrelevant to exonuclease discussion.

Step-by-Step Solution:
Identify which enzyme is classically known as non-proofreading: Taq.Eliminate Klenow (has 3'→5' exonuclease) and Pol III (proofreading core subunits).Ignore ligase for exonuclease properties; it seals nicks, not edits bases.

Verification / Alternative check:
High-fidelity PCR protocols replace Taq with proofreading enzymes specifically to gain 3'→5' exonuclease activity and reduce errors.

Why Other Options Are Wrong:

• Klenow: retains proofreading even though it lacks 5'→3' exonuclease of full Pol I.
• T4 DNA ligase: not a polymerase; exonuclease activity is not its function.
• “All of the above”: false because Klenow and Pol III proofread.

Common Pitfalls:
Assuming thermostable equals proofreading; thermostability and exonuclease capability are independent traits.

Final Answer:
Taq DNA polymerase