What are the major DNA repair mechanisms?

Cells employ multiple DNA repair mechanisms to maintain genomic integrity. Base excision repair (BER) removes damaged bases via glycosylases. Nucleotide excision repair (NER) removes bulky lesions like thymine dimers by excising a ~30 nucleotide stretch. Mismatch repair (MMR) corrects replication errors using MutS/MutL proteins. Double-strand break repair occurs via non-homologous end joining (NHEJ), which is error-prone but fast, or homologous recombination (HR), which is accurate but requires a template. Defects in repair pathways cause cancer predisposition syndromes like xeroderma pigmentosum (NER) and Lynch syndrome (MMR).