DNA Polymerases and Synthesis

DNA polymerases are the enzymes that synthesize new DNA strands by adding nucleotides to a primed template, and they are central both to copying the genome and to repairing it. Their template-directed chemistry, processivity, and built-in error correction together determine how accurately genetic information is transmitted.

Definition

A DNA-directed DNA polymerase is an enzyme that catalyzes the template-directed addition of deoxyribonucleotides to the 3' end of a growing strand, copying a DNA template in the 5'-to-3' direction.

Scope

The entry covers the catalytic mechanism of template-directed nucleotide addition, the 5'-to-3' polarity and requirement for a primer, proofreading by 3'-to-5' exonuclease activity, processivity factors, and the existence of distinct polymerase families specialized for replication, repair, and translesion synthesis. It is a mechanistic reference topic.

Key concepts

• Template-directed nucleotide addition
• Requirement for a primer and a template
• 5' to 3' synthesis polarity
• Proofreading (3' to 5' exonuclease)
• Replication fidelity
• Processivity and sliding clamps
• Polymerase families (replicative, repair, translesion)

Mechanisms

DNA polymerases add deoxyribonucleoside triphosphates one at a time to the free 3'-hydroxyl of a primer strand, selecting each incoming nucleotide by Watson-Crick complementarity to the template, which gives synthesis its defined 5'-to-3' direction and its dependence on both a template and a primer. Many replicative polymerases carry a separate 3'-to-5' exonuclease (proofreading) activity that excises misincorporated nucleotides, greatly lowering the error rate; remaining mismatches are corrected by mismatch repair, and together these mechanisms determine overall replication fidelity. Processivity factors such as sliding clamps keep the polymerase bound so it can copy long stretches without dissociating. Cells contain multiple polymerase families with distinct roles: high-fidelity enzymes for genome replication, specialized polymerases for filling gaps and repair, and lower-fidelity translesion polymerases that can copy past damaged template bases.

Clinical relevance

Polymerase fidelity and the repair pathways that back it up are central to how genome integrity is maintained, and defects in these activities are linked to genome instability. The entry is reference biology and not a basis for individual clinical decisions.

History

Arthur Kornberg's isolation of a DNA-synthesizing enzyme in the late 1950s established that DNA replication is enzyme-catalyzed and defined the basic catalytic requirements. Later work distinguished multiple eukaryotic polymerases, characterized proofreading and mismatch repair as determinants of fidelity, and described specialized translesion polymerases, giving the modern picture of a family of enzymes with division of labor.

Key figures

• Arthur Kornberg
• Thomas Kunkel
• Ulrich Hübscher
• Wei Yang

Seminal works

• kornberg-1969
• hubscher-2002
• kunkel-erie-2005

Frequently asked questions

Why does a DNA polymerase need a primer?

DNA polymerases can only extend an existing strand by adding nucleotides to a free 3'-hydroxyl; they cannot start a new strand from scratch, so a short primer (usually RNA, made by primase) provides the starting 3' end.

How do polymerases keep replication accurate?

They select nucleotides by base-pairing to the template and many can proofread, using a 3'-to-5' exonuclease to remove a wrong nucleotide before continuing; mismatch repair then corrects errors that escape proofreading.