Synthesis-dependent strand annealing

Synthesis-dependent strand annealing (SDSA) is a major mechanism of homology-directed repair of DNA double-strand breaks (DSBs).

[6] In yeast, the D-loop is formed by strand invasion with the help of proteins Rad51 and Rad52,[7] and is then acted on by DNA helicase Srs2 to prevent formation of the double Holliday junction in order for the SDSA pathway to occur.

Therefore, although SDSA produces non-crossover products because flanking markers of heteroduplex DNA are not exchanged, gene conversion may occur, wherein nonreciprocal genetic transfer takes place between two homologous sequences.

Research in Drosophila melanogaster identified the Bloom syndrome helicase (Blm) as the enzyme promoting dissassembly of the D-loop.

[17] Sgs1(BLM) may disassemble D-loop structures analogous to early strand invasion intermediates and thus promote NCO formation by SDSA.

[17] The Sgs1 helicase forms a conserved complex with the topoisomerase III (Top3)-RMI1 heterodimer (that catalyzes DNA single strand passage).

A current model of meiotic recombination, initiated by a double-strand break or gap, followed by pairing with an homologous chromosome and strand invasion to initiate the recombinational repair process. Repair of the gap can lead to crossover (CO) or non-crossover (NCO) of the flanking regions. CO recombination is thought to occur by the Double Holliday Junction (DHJ) model, illustrated on the right, above. NCO recombinants are thought to occur primarily by the Synthesis Dependent Strand Annealing (SDSA) model, illustrated on the left, above. Most recombination events appear to be the SDSA type.