Final answer:
The three major DNA repair mechanisms in E. coli are Base Excision Repair, which fixes small base lesions; Nucleotide Excision Repair, which repairs bulky DNA damage; and Direct Reversal, which chemically reverses certain types of base modifications.
Step-by-step explanation:
Major DNA Repair Mechanisms in E. coli
The three major DNA repair mechanisms in E. coli include Base Excision Repair (BER), Nucleotide Excision Repair (NER), and Direct Reversal. Each targets specific types of damage to ensure the integrity of the DNA.
- Base Excision Repair targets small, non-helix-distorting base lesions caused by deamination, oxidation, and alkylation. Specific DNA glycosylases recognize and remove the damaged bases, followed by end-processing and resynthesis of the excised DNA strand.
- Nucleotide Excision Repair addresses bulkier, helix-distorting lesions such as thymine dimers caused by UV light and larger chemical adducts. This process involves recognition of the distortion, removal of a short single-stranded DNA segment containing the lesion, and synthesis of a replacement strand.
- Direct Reversal repairs damage through chemical reversal of certain modified bases without breaking the phosphodiester backbone. This includes repair of thymine dimers by photolyase and reversal of methylated guanine bases by OGT, the bacterial equivalent of methyl guanine methyl transferase (MGMT).