DNA polymerases

DNA Polymerases

DNA polymerases are essential enzymes that play a critical role in the process of DNA replication and DNA repair. These enzymes are responsible for synthesizing new strands of DNA by adding nucleotides to a pre-existing chain, using a template strand as a guide.

Structure and Function

DNA polymerases are complex proteins that have several key functional domains. The primary function of DNA polymerases is to catalyze the addition of deoxyribonucleotide triphosphates (dNTPs) to the 3' end of a growing DNA strand. This process is guided by the complementary base pairing rules, where adenine pairs with thymine and cytosine pairs with guanine.

Active Site

The active site of DNA polymerase is highly conserved and is responsible for the enzyme's catalytic activity. It typically contains a "palm," "fingers," and "thumb" domain, which together facilitate the binding of the DNA template and the incoming dNTPs.

Proofreading Activity

Many DNA polymerases possess a 3' to 5' exonuclease activity that allows them to remove incorrectly paired nucleotides, thereby enhancing the fidelity of DNA replication. This proofreading function is crucial for maintaining the integrity of the genetic information.

Types of DNA Polymerases

DNA polymerases are classified into several families based on their sequence homology and functional characteristics. The major families include:

Family A

Family A polymerases are primarily involved in DNA repair and replication in prokaryotes. An example is DNA polymerase I from Escherichia coli, which is involved in the removal of RNA primers and filling in the gaps during DNA replication.

Family B

Family B polymerases are found in eukaryotes and archaea. They are responsible for the bulk of DNA replication. DNA polymerase δ and DNA polymerase ε are examples of Family B polymerases in eukaryotes.

Family C

Family C polymerases are the primary replicative polymerases in bacteria. DNA polymerase III is a well-known example, responsible for the synthesis of the leading and lagging strands during bacterial DNA replication.

Family X

Family X polymerases are involved in DNA repair processes. DNA polymerase β is a key enzyme in base excision repair in eukaryotic cells.

Family Y

Family Y polymerases are specialized for translesion synthesis, allowing DNA replication to occur past damaged sites. These polymerases have a lower fidelity and are used as a last resort to bypass lesions that stall replication forks.

Mechanism of Action

DNA polymerases require a template strand and a primer with a free 3' hydroxyl group to initiate DNA synthesis. The enzyme catalyzes the formation of a phosphodiester bond between the 3' hydroxyl group of the primer and the 5' phosphate group of the incoming dNTP, releasing pyrophosphate as a byproduct.

Role in DNA Replication

During DNA replication, DNA polymerases work in concert with other proteins to ensure accurate and efficient duplication of the genome. The leading strand is synthesized continuously, while the lagging strand is synthesized in short fragments known as Okazaki fragments.

Clinical Significance

Mutations in DNA polymerase genes can lead to various genetic disorders and contribute to the development of cancer. Inhibitors of DNA polymerases are used as chemotherapeutic agents to target rapidly dividing cancer cells.

See Also

• DNA replication
• DNA repair
• Enzyme
• Nucleotide

References

• Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular Biology of the Cell. Garland Science.
• Kornberg, A., & Baker, T. A. (1992). DNA Replication. W.H. Freeman and Company.