Electron dense structures seen in electron micrographs of synaptonemal complexes (q.v.). In Drosophila, since the number of RNs in mid pachytene oocytes is about the same as the number of chiasmata observed at diplonena, it is assumed that RNs participate in the exchange process. The mei-W68 mutation abolishes meiotic exchange in the oocytes of homozygotes, and it also suppresses the formation of RNs. The SPO11 gene of Saccharomyces cerevisiae is an ortholog of mei W68. SPO11 encodes topoisomerase II, an enzyme that produces double-strand breaks in DNA. See Chronology, 1975, Carpenter; centromeric coupling, topoisomerase.
Subjects: Genetics and Genomics.