What are the different DNA repair mechanisms?

The five main DNA repair mechanisms include: base excision repair (BER), nucleotide excision repair (NER), homologous recombination (HR), non-homologous end joining (NHEJ), and mismatch repair (MMR). 

1. Base excision repair is a mechanism that fixes DNA damage resulting from oxidation, alkylation and deamination. These types of damage cause minimal alteration to the structure of the DNA helix. BER plays a crucial role in maintaining the stability of the genome, which helps prevent the onset of conditions and diseases like premature aging, cancer, and neurodegenerative disorders. 
2. Homologous recombination repairs DNA double-strand breaks (DSBs) and interstrand crosslinks, primarily during the S and G2 phases of the cell cycle. It starts with the resection of the DSB ends to generate ssDNA overhangs. The ssDNA invades a homologous DNA sequence, forming a displacement loop, and DNA synthesis occurs using the intact homologous strand as a template. The newly synthesized strand is then used to repair the DSB. 
3. NHEJ repairs DSBs throughout the cell cycle, particularly during the G1 phase. The broken DNA ends are bound by DNA-dependent protein kinase (DNA-PK), which gathers other NHEJ factors to the site of the break. The processed DNA ends, prepared by nucleases and polymerases to remove damaged nucleotides, are then ligated together by DNA ligase IV. 
4. NER targets bulky DNA lesions such as those induced by UV radiation and chemical carcinogens. It involves the recognition of lesions by NER protein complexes, followed by dual incision of the damaged DNA strand by endonucleases, creating a gap. DNA polymerase then repairs the gap (using the undamaged strand as a template) and DNA ligase seals the nick, completing the repair process.   
5. Mismatch repair is a dna repair mechanism that occurs when newly synthesized dna has been made. This mechanism fixes mismatched bases that were not removed and replaced during proofreading.