Meiotic cytology of Saccharomyces cerevisiae in protoplast lysates.

This report describes cytological features of meiosis in Saccharomyces cerevisiae prepared for electron microscopy by lysis of protoplasts or nuclei on an aqueous surface. Whereas the chromatin of cells lysed before or after meiotic prophase was widely dispersed, pachytene bivalents appeared as discrete, elongate masses of compact chromatin. These bivalents were of nearly uniform thickness; they ranged in length from about 0.6 micrograms to 4.0 micrograms, with a median of 1.6-1.8 micrograms. Enzymatic digestion of chromosomal DNA removed the chromatin to reveal the underlying synaptonemal complex. The lysis of partially purified nuclei was less disruptive and thereby revealed the regular association of the telomeres with fragments of the nuclear envelope. In tetraploid cells, pachytene lysates contained quadrivalents characterized by the close apposition of chromatin masses of similar length. One or more points of intimate association appear to represent sites of exchange between pairing partners. The departure of the diploid cells from pachytene was accompanied by the renewed association of spindle microtubules with the chromosomes shortly before the diplotene chromosomes decondensed. Later, the successive meiotic divisions were identified by the appearance of a single spindle for meiosis I and of two spindles for meiosis II.