MICROBIOLOGY - M.H. Serhiichuk - 2008

Chapter 4. BACTERIAL GENETICS

Repair of Damaged DNA

DNA Repair refers to the restoration of Introduction/20.html">DNA Structure damaged by mutagens.

Investigating the lethal and mutagenic effects of ultraviolet rays, Kelner and Dulbecco as early as 1949 described the ability of bacterial Cells to restore their genome. Because bacterial cells were exposed to visible light after ultraviolet irradiation, resulting in a dramatic decrease in lethality and mutation frequency, this phenomenon was named photoreactivation. It was later discovered that photoreactivation targets damages caused by ultraviolet radiation, such as pyrimidine dimers and other single-strand DNA Lesions. Visible light activates a photoactivating enzyme that carries out the repair process.

In the late 1950s and early 1960s, evidence emerged for the existence of dark Repair of DNA Damage caused by all known mutagens. Dark repair is a multistep process involving various Enzymes, including specific endonucleases, exonucleases, DNA polymerases, and DNA ligases. Dark repair can be divided into two types: Excision Repair and post-Replication repair. During excision repair, Genetic information is fully restored, primarily following single-strand damage to the DNA molecule; in this regard, this type of repair is information-preserving. The First stage of dark repair involves the recognition and incision of the DNA strand at the lesion site by a specific endonuclease. Next, the excised damaged region is removed by an exonuclease or a DNA polymerase exhibiting exonuclease activity. In the subsequent stage, the deleted DNA segment is resynthesized on the template of the complementary strand by DNA polymerase I or DNA polymerase III. In The final stage, the newly synthesized segment is joined to the main DNA strand by DNA ligase. During post-replication repair, the double-stranded DNA structure is restored by filling gaps that arise in the newly synthesized strand during replication opposite the damaged sites. This process can occur via two pathways: recombination and de novo DNA Synthesis mediated by a modified polymerase. Along with exonucleases, polymerases, and ligases, the participation of regulatory genes recA and lexA is essential for post-replication repair. Dark repair processes are known to be genetically controlled; for instance, more than 20 genes are known to mediate this control in E. coli. Mutations in these genes disrupt DNA repair systems, leading to a sharp increase in the sensitivity of Bacteria to DNA-damaging Mutagenic Factors.