When a yeast cell is engineered to lack the enzyme Dna2, double-strand breaks in its DNA (1), collect stray sequences from all over the genome. Authors of a new paper suggest these insertions arise because Dna2 normally degrades excess DNA created during replication, such as so-called 5’ flaps (2a). Another possibility is that the rogue DNA is shed from dying cells (2b), although it is unclear whether Dna2 could be involved in that process. The excess bits can be integrated into breaks via nonhomologous end joining, in which repair enzymes weld the ends of severed DNA back together (3).
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