Significance of DNA Repair Systems in Bacteria
The bacterial genome requires robust protection against destructive environmental factors. Primarily, repair mechanisms counteract the consequences of ultraviolet (UV) irradiation, which induces the formation of thymine dimers within the nucleic acid molecule. The protective response relies on specific enzyme complexes that recognize these structural anomalies and correct them, restoring the normal functional conformation of the strand. Broadly, these restorative processes are divided into two main types based on their dependence on environmental conditions.
Light Repair (Photoreactivation)
This pathway is activated exclusively in the presence of light, providing a direct and rapid mechanism for defect correction.
The mechanism of light repair relies on a specialized photoreactivating enzyme (photolyase). When the bacterium is exposed to light, this enzyme acts directly on the damaged site, cleaving the thymine dimers formed by UV radiation. Consequently, the nucleic acid structure is restored without requiring the excision or synthesis of new fragments.
Dark (Excision) Repair
Unlike photoreactivation, dark repair is entirely independent of light and can occur in total darkness. It is a more complex, multi-step mechanism known as nucleotide excision repair.
This process physically removes defective fragments of the strand and fills in the resulting gap. Successful completion of this cycle requires the coordinated action of several enzymes, each performing a strict function at a specific stage.
Steps of Excision Repair
Excision repair of the genome proceeds through four sequential steps:
- Incision. An enzyme complex, specifically a repair endonuclease, scans the molecule and recognizes the lesion (thymine dimer). Upon locating the defect, the endonuclease makes a targeted incision in the DNA strand 5' to the damage.
- Excision. An exonuclease enzyme takes over, completely removing the damaged segment containing the pathological dimer, thereby clearing the site for a new structure.
- Resynthesis (Gap Filling). A gap remains where the segment was removed. DNA polymerase synthesizes a completely new, healthy segment. The intact complementary strand serves as a template for error-free restoration.
- Ligation. The final step connects the newly synthesized fragment to the main strand. DNA ligase restores the phosphodiester bond, sealing the ends of the molecule securely.