As chromosomes synapse, the axial elements of homologous chromosomes have been described as alternately diverging and converging at multiple association sites in humans (Rasmussen and Holm 1978), and plants (Hasenkampf 1984; Albini and Jones 1987; Anderson and Stack 1988). Electron dense nodules have been observed at these association sites, either at matching sites on homologous axes, or bridging the distance between converging axes.These nodules are always present at newly formed stretches of the synaptonemal complex, but disappear in early pachynema. Sometimes nodules found at association sites are connected by a fiberous bridge. The presence of a nodule-fiber complex at association sites between adjacent axial elements, suggests that these nodules are involved in matching corresponding sites on homologous chromosomes and initiation of synapsis (Albini and Jones 1987; Anderson and Stack 1988). The zygotene nodules are referred to as "early" recombination nodules. Before the discovery of early recombination nodules, Carpenter (Carpenter 1975) observed electron dense structures distributed along the synaptonemal complexes of Drosophyla oocytes in serial section reconstruction's. The number and position of these nodules have been found to correlate with the frequency and distribution of recombination events, and are called late recombination nodules(Carpenter 1979).
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