Which DNA polymerase possesses both 5′→3′ and 3′→5′ exonuclease activities?

Introduction / Context:
Exonuclease activities provide proofreading and primer removal during replication and repair. Different polymerases carry distinct nuclease domains that determine their functional niches at the fork.

Given Data / Assumptions:

• 5′→3′ exonuclease removes RNA primers/nicked DNA ahead of the polymerase.
• 3′→5′ exonuclease provides proofreading by excising misincorporated bases.
• We compare E. coli Pol I (Kornberg enzyme), Pol III, and thermostable Taq polymerase.

Concept / Approach:
E. coli DNA polymerase I has both 5′→3′ and 3′→5′ exonuclease activities, enabling nick translation and proofreading. Pol III, the main replicase, has strong 3′→5′ proofreading but lacks an intrinsic 5′→3′ exonuclease. Taq polymerase lacks 3′→5′ proofreading and therefore has relatively higher error rates; it also does not provide the robust 5′→3′ exonuclease used for primer removal in vivo.

Step-by-Step Solution:
List activities: Pol I = polymerase + 3′→5′ exonuclease + 5′→3′ exonuclease.Pol III = polymerase + 3′→5′ exonuclease; no 5′→3′ exonuclease.Taq = polymerase largely without 3′→5′ proofreading; not used for nick translation.Therefore, the enzyme with both exonuclease directions is Pol I (Kornberg enzyme).

Verification / Alternative check:
Biochemical assays of nick translation and primer removal in E. coli require Pol I’s 5′→3′ exonuclease; mutants in this domain abolish the activity.

Why Other Options Are Wrong:

• Pol III: lacks 5′→3′ exonuclease domain.
• Taq Pol: lacks 3′→5′ proofreading and is error-prone.
• None of these: incorrect because Pol I has both activities.
• Ligase: joins nicks; no exonuclease activity.

Common Pitfalls:
Assuming the main replicase must have all nuclease functions; in reality, tasks are partitioned among enzymes.

Final Answer:
Kornberg enzyme (E. coli DNA polymerase I)