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.