Homologous genetic recombination during meiotic prophase I. Prophase I includes a directed recombination process. The double-strand breaks are created and processed early. Strand invasions and replicative extension produce double-crossover intermediates, and some of these mature into stable crossovers. Shown here are the early stages of meiosis in mouse spermatocytes. (Leptotene, zygotene, and pachytene are terms used to describe the subphases of meiosis prophase I.) The progress of the recombination reactions is shown in the upper panel, matched to the subphases of prophase in the microscopic images in the lower panel. The chromosomes are viewed by staining particular proteins through immunofluorescence. Chromosomes are stained red, and a few recombination proteins, fused to green fluorescent protein, appear as green foci. As prophase progresses, the homologous chromosome pairs, at first diffuse, become tightly aligned, producing rodlike structures. The chromosomes again become diffuse as prophase ends and the cells begin the first cell division.