Electron-microscopic study of the spindle and chromosome movement in the yeast Saccharomyces Cerevisiae

Abstract

Mitosis in the yeast Saccharomyces cerevisiae was investigated in thick (0·25–1 μm) serial sections with a high voltage electron microscope and in preparations of spheroplasts spread on a water surface. Spindle microtubules originate from a plaque-like structure called the spindle pole body (SPB) which is inserted within the persisting nuclear envelope. Early in mitosis the SPB duplicates and a set of long and short microtubules develops on each SPB. The spindle arises as the SPBs separate on the nuclear membrane and reorient so that the microtubules fall into line. Chromosomes do not condense and are not individually visible. Genetic studies, however, have indicated that there are 17 linkage groups. The number of microtubules was determined in diploid and haploid spindles on serial stereo micrographs. In diploid mitosis about 40 microtubules issue from a SPB. Most are non-continuous and often they are visibly associated with a chromatin fibre. The spindle in haploid cells is similar except that the number of microtubules is about half that in diploid cells and the SPB is smaller. The pole-to-pole microtubules vary in number from spindle to spindle, but in each case enough microtubules are present to account for each linkage group being associated with a single non-continuous microtubule. We conclude that mitosis in yeast is comparable in its general aspect to that observed in typical eukaryotes.