Crossing over takes place during pachytene stage of meiosis-I. ‚ÄčDuring pachytene stage, the homologous chromosomes are held closely together along their length by the synaptonemal complex (SC). The DNA of sister chromatids is extended into parallel loops. Under the electron microscope, within the center of the SC, a number of electron-dense bodies about 100 nm in diameter are seen. These structures are called recombination nodules because they correspond to the sites where crossing over (exchange of genetic material) is taking place between non-sister chromatids of the homologous chromosomes. Recombination nodules contain the enzyme recombinase that facilitates genetic recombination, which is completed by the end of pachytene. The chromosomes remain connected at the sites of crossing over.
The beginning of diplotene, the next stage of meiotic prophase I after pachytene is recognized by the dissolution of the SC, which leaves the chromosomes attached to one another at specific points by X-shaped structures, termed chiasmata. Chiasmata are formed by covalent junctions between a chromatid from one homologous chromosome and a non-sister chromatid from the other homologous chromosome. Chiasmata are located at sites on the chromosomes where crossing over between DNA molecules from the two chromosomes had taken place. The homologous chromosomes separate from one another at the diplotene stage. The diplotene can be an extremely long phase (extending up to months or years) of oogenesis in vertebrates during which the bulk of oocyte growth occurs. Thus diplotene can be a period of intense metabolic activity. 


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