Biochemistry of meiosis.

The process of meiosis in Lilium falls into four physiological stages - prezygotene, zygotene, pachytene, and post-pachytene. Each of these stages has distinctive metabolic characteristics. Commitment to meiosis occurs during the prezygotene interval at about the time when S-phase replication is completed. The activities following commitment are essential to synapsis inasmuch as perturbations of cells during that interval have subsequent effects on synapsis and crossing over. Just before the initiation of synapsis, a distinctive lipoprotein complex appears in the nucleus. The complex most probably functions in the process of pairing. Zygotene is marked by the delayed replication of specific intercalary segments of chromosomal DNA (Z-DNA), the replication being a necessary condition for ongoing synapsis. The replication occurs in the lipoprotein complex in the presence of a reassociation protein (r-protein). Z-DNA segments would appear to have other meiotic functions inasmuch as the replicated segments remain unligated to the body of chromosomal DNA until the beginning of chromosome disjunction. The pachytene interval is marked by an activation of endonucleolytic activity. The enzyme produces single-stranded nicks in the DNA at specific loci. These loci consist of moderately repeated segments; about 100-200 base pairs long. Extracellular agents, such as radiation, cause random nicking regardless of the meiotic stage at which they are applied. Localized nicking and repair are thus unique features of meiosis. The temporal segregation of metabolic activities concerned with pairing and crossing over and their operation in special chromosome regions constitute the most prominent features of the biochemical events associated with meiosis.