The Relation of Chromosome Compaction to DNA Synthesis During Meiosis in Lilium

Abstract

Three intervals of DNA synthesis in Lilium microsporocytes-premeiotic S-phase, delayed zygotene replication and pachytene repair synthe-sis-were analyzed with respect to chromosome compaction. Chromatin was prepared from isolated nuclei and fractionated into a condensed and a diffuse component. Glutaraldehyde was used in a parallel procedure to crosslink chro-matin that might be compacted but unstable in the fractionation procedure. Nuclei in premeiotic S-phase had a much larger fraction of condensed chro-matin rescued by glutaraldehyde than those in any other stage. Virtually no DNA replication occurred in this fraction until the last interval of S-phase. Zygotene replication occurred mainly in the diffuse chromatin fraction and the distribution remained so after completion of replication. Pachytene repair syn-thesis occurred mainly in condensed chromatin. Generally, sequences with high C₀t value fractionated preferentially with diffuse chromatin but the opposite was the case for pachytene nuclei. Digestion with staphylococcal nuclease revealed no significant differences in nucleosome organization of condensed and diffuse chromatin.