Chromosome organization in the regulation of meiotic prophase

The prophase stages of meiosis, particularly zygotene and pachytene, are organized to sustain the events required for recombination between homologous chromosomes. Three categories of DNA sequences are believed to function in the control of these events. One category comprises the sites of transcription for meiosis-specific proteins. The other two categories are related to the structural organization of chromosomes at meiotic prophase. Sequences that are delayed in replication until zygotene may provide the sites for chromosome alignment in securing the synapsis of homologues. General chromosome synapsis is presumed to be functionally distinct from the synapsis of DNA strands that occurs in localized regions at which recombination may take place. Recombinational synapsis probably involves families of moderately repeated sequences, here designated as PDNA. PDNA segments have a compound organization. Each of their ends is occupied by a moderately repeated sequence that belongs to one of several hundred families designated as 'PsnDNA'. The latter range from 150-300 bp in length and do not hybridize with the internal PDNA regions. PsnDNA sequences are the sites at which most of the programmed nicking, gapping and repair syntheses occur during pachytene. They are also the sites at which histones are displaced by a meiotic prophase-specific protein that somehow renders the PsnDNA accessible to the action of meiotic endonuclease. This structural change in the chromatin is partly controlled by a meiosis-specific small nuclear RNA (PsnRNA) that is homologous with PsnDNA and also has a specific affinity for the histone-replacing protein. The complex of events associated with the transformation in PsnDNA chromatin regions is also subject to control by homologous chromosome pairing.