Electron microscopical studies of frozen-dried yeast: II. The nature of basophile particles and vesicular nuclei in Saccharomyces

Abstract

Haploid and diploid vegetative nuclei of Saccharomyces were stained selectively for basophilia after post-fixation in vacuo with anhydrous, vapor reagents specifically effective for proteins and polypeptides and studied in conjunction with enzymatic and other controls. The nuclei cleave simply by elongation and medial constriction of hour-glass forms with no indications of a spindle apparatus or resolvable chromosomes with classical morphological attributes, or a breakdown of the nuclear boundary at any time until final cleavage. Instead, the nuclear contents are particulate, measuring about 50–80 Å and seemingly dispersed at random in most sectioning planes, but frequently detectable as short rows streaming into the developing daughter nucleus. Association of these particles with faintly stainable submicroscopic cords of anastomotic form in sections is detectable; one or two peripheral clusters of similar particles, of hitherto undetermined function, occur in the dividing nucleus regardless of haploidy or diploidy. No organization of obviously light microscopically resolvable dimension occurs in the nucleus. Integrity of the particulate matter in the nucleus is destroyed or very greatly reduced by DNA-ase, but preserved after RNA-ase. The latter abolishes basophily of the dense deposits of ribosomes in the cytoplasm with the exception of a small resistant fraction mostly in the vicinity of the nucleus. These resistant particles are not seen in specimens subjected successively to both the nucleic acid depolymerases. The cytogenetical implications of these findings are discussed very briefly in view of Mendelian heredity established in yeast, and in terms of an evolution of cytogenetical mechanisms.