Fig. 1. Development of meiotic chromosome core-associated recombination protein complexes in mouse spermatocytes [adapted from Moens et al. (Moens et al., 2002)]. About 250 to 300 ENs (green), become associated with the chromosome cores (blue) prior to synapsis. They are spherical structures, about 100 nm in diameter, and are identified by antibodies to RAD51 and DMC1 protein (green). They are the sites of programmed, double-strand DNA breaks at meiotic prophase and they are associated with phosphorylated histone H2AX (yellow; this report). The ENs are transformed into 200 synaptonemal complex (SC)-associated, RPA-defined TNs (SCs, parallel blue lines; TN, red) by the acquisition of several proteins – RPA, BLM, MSH4, MSH5 and topoisomerases – while they loose the RAD51-DMC1 and H2AX components. In the mouse spermatocyte nucleus, about 25 of the TNs acquire MLH1 protein (blue) that mark the sites of chiasmata. The remaining TNs are resolved as non-crossovers and they relocate to the outside the SCs (this report).