Fig. 4

(a) Recombination mechanism proposed by Meselson and Radding (1975). (a) A nick (hydrolyzed phosphodiester bond) is made in one DNA strand. (b) The resulting 3′-OH primes DNA synthesis (dashed line), displacing one DNA strand. (c) The displaced strand invades a homologous DNA duplex by base pairing to complementary sequence to form a D-loop (displacement loop) and heteroduplex DNA. (d) Continued DNA synthesis with strand displacement and cleavage of the D-loop gives a Holliday junction. The heteroduplex DNA is asymmetric since red is paired with black in only one of the duplex molecules. (e) The Holliday junction can move (branch migrate). (f) Resolution of the Holliday junction gives crossover or non-crossover products. (b) Double-strand break repair mechanism proposed by Szostak (Szostak et al. 1983). (g) A double-strand break is formed during meiosis or from DNA damage. The 5′-terminated strands are removed by an exonuclease (end resection). (h) One 3′-terminated strand invades a homologous DNA to form a D-loop. (i) The invading 3′-end primes DNA synthesis (dashed line). The displaced strand is extended until it eventually base pairs to the other end of the broken chromosome (second-end capture). (j) The captured 3′-end primes DNA synthesis. (k) The DNA ends are ligated to form a double Holliday junction. (l) Resolution of the Holliday junctions gives crossover or non-crossover products