# DNA Repair and Recombination The repair of double stranded breaks (DSBs) requires the presence of homologous dsDNA (i.e. a second copy of the chromosome or a sister chromatid). ## The DNA Repair Process ### Identification of DNA Lesions DNA damage is *most commonly noticed during DNA replication*. ![[Pasted image 20250128031454.png|475]] - **Translesion Synthesis:** Replication simply continues over the lesion - “Eh, close enough” - **Fork Stall:** - Leads into fork regression - **Fork Collapse:** aaa - Leads into double-stranded break repair (DSBR) ### Key Steps to DSB Repair ### Holliday Junctions A Holliday junction is a four-stranded DNA crossover structure formed during homologous recombination ![[Pasted image 20250128090142.png|500]] ## Homology Directed Repair (HDR) - Homologous recombination (HR): a type of HR ### DNA Recombinational ## Non-Homologous End Joining (NHEJ) NHEJ is present in all [[Taxonomy|eukaryotes and some bacteria]]. This is primarily due to: - NHEJ being a mutagenic process that is tolerated better in larger genomes In yeast, most repair is done via recombination, while in mammals most repair is done by recombination during S and G2 phase, and NHEJ during G1 and G0 - In mammals, most repair is done Steps that occur: - The Ku70-Ku80 complex binds to the broken chromosome ends - A kinase and nuclease bind the the Ku70-Ku80 complex on the DNA - Degradation of any single-stranded overhangs or hairpins occurs from the broken ends of the chromosome being repaired