Mechanics of meiosis.

Abstract

The present paper reviews our observations on the premeiotic DNA replication in Lilium and on the morphology and behaviour of chromosomes during the meiotic prophase in Schizophyllum, Lilium, Drosophila, Bombyx and Homo as revealed by three-dimensional reconstructions of synaptonemal complexes (SC). The main results and conclusions are the following: (1) Premeiotic DNA replication in Lilium is 6 times longer than the mitotic S-phase. (2) Presynaptic alignment of homologous chromosomes was not observed. (3) Initiation of synapsis was in Bombyx and Homo restricted to telomere regions while in Lilium complex formation was generally initiated interstitially. (4) Interlocking of chromosomes was frequent at zygotene. (5) Direct evidence for resolution of interlockings by breakage and reunion of chromosomes was obtained in Homo and Bombyx. (6) Two phases of chromosome pairing and SC formation were revealed in Bombyx. (7) A karyotype based on SC length, centromere index and chromatin markers was constructed for human spermatocytes. (8) The number and distribution of recombination nodules were in agreement with the hypothesis that these structures are involved in crossing over and/or gene conversion. (9) Fragments of the SC were found in prometaphase I bivalents of human spermatocytes corresponding in number to the number of chiasmata. (10) In the absence of crossing over, compensatory mechanisms have evolved in female Bombyx substituting for chiasmata in the retention of homologues.