Fundamentals of Molecular Biology - V.I. Rezyapkin 2009

DNA Repair
Direct Repair

In direct repair, the restoration of the original Introduction/20.html">DNA Structure occurs in a single step.

Photoreactivation

In this type of repair, a key role is played by an enzyme known as photolyase. Activated by light, photolyase recognizes a thymine dimer in the DNA, forms a complex with it, and cleaves the covalent bonds formed between the pyrimidine rings:

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Following this, photolyase loses its affinity for DNA and dissociates. The DNA structure is thereby restored.

Repair of alkylated bases

This type of repair is carried out by methyltransferase. This enzyme removes methyl or ethyl groups from modified bases, thereby restoring the original DNA structure:

Methyltransferase transfers the methyl or ethyl group to one of its own Cysteine residues, which inactivates the enzyme.

Repair of single-strand DNA breaks

The repair of single-strand breaks is mediated by DNA ligase. It rejoins the severed strands in the DNA molecule (Fig. 3.4).

Fig. 3.4. Repair of single-strand DNA breaks

Repair of AP sites

AP sites resulting from purine loss can be repaired by insertases. These Enzymes can insert the correct base—identical to the one present before the damage—into the gap by attaching it to deoxyribose (Fig. 3.5). As a result, the DNA structure is restored.

Fig. 3.5. Repair of AP sites



Last update: 12/08/2026

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