Biochemical Basis of DNA Repair

Introduction

DNA repair is a fundamental process that ensures the stability and integrity of the genetic material in all living organisms. The DNA in our cells is constantly under threat from various sources of damage, such as exposure to UV radiation, chemicals, and errors during DNA replication. Without efficient DNA repair mechanisms, these damages could lead to mutations, genomic instability, and ultimately diseases like cancer.

Types of DNA Damage

There are several types of DNA damage that can occur in our cells. These include single-strand breaks, double-strand breaks, base modifications, and crosslinks. Single-strand breaks involve the cleavage of one of the two strands of DNA, while double-strand breaks occur when both strands are broken. Base modifications can alter the chemical structure of individual nucleotides, leading to mismatches during DNA replication. Crosslinks are covalent bonds between two nucleotides on the same strand or between nucleotides on opposite strands.

DNA Repair Mechanisms

Cells have evolved several mechanisms to repair different types of DNA damage. The main DNA repair pathways include base excision repair (BER), nucleotide excision repair (NER), mismatch repair (MMR), homologous recombination (HR), and non-homologous end joining (NHEJ). BER is responsible for repairing single-base lesions, while NER removes bulky lesions caused by UV radiation. MMR corrects errors that occur during DNA replication, HR repairs double-strand breaks using a homologous DNA template, and NHEJ joins broken DNA ends without the need for a template.

Role of Enzymes in DNA Repair

Enzymes play a crucial role in DNA repair by catalyzing the chemical reactions involved in the process. For example, DNA glycosylases are enzymes that recognize and remove damaged bases during BER, while endonucleases cleave the DNA backbone at the site of damage. DNA polymerases then fill in the gaps left by the removal of damaged nucleotides, and DNA ligases seal the nicks in the DNA backbone. Other enzymes like helicases and topoisomerases help in unwinding and restructuring the DNA molecule during repair.