6-23 THE YEAST CELL 



DEMONSTRATION OF NUCLEOLUS, CHROMOSOMES, AND 

 CENTROSOME BY STANDARD CYTOLOGICAL TECHNIQUES 



Mrs. Rafalko and I (unpublished) made cytological examinations 

 of yeast cells by a variety of standard techniques. This involves 

 dehydration of cells and considerable shrinkage, but is compensated for 

 by better resolution and the possibility of comparison with conven- 

 tional preparations. Most of our data are on Saccharomyces bay- 

 anus. The nucleolus lies in the nuclear vacuole. When stained with 

 haemotoxylin, (Rafalko, unpublished) it is found to contain from one 

 to three internal plastin bodies, characteristic of the conventional 

 endosomes found in other forms. Haemotoxylin is effective in show- 

 ing internal structures in the nucleolus probably by revealing dif- 

 ferences in density. A halo of unstained material surrounds the 

 nucleolus, similar to the halo surrounding the endosome of protozoa. 

 Occasionally radiating threads can be seen extending from the nu- 

 cleolus through the surrounding halo into the nucleoplasm. Fig. 6-21 

 is a photograph of one of Dr. Rafalko's Feulgen-stained prepara- 

 tions showing four pairs of Feulgen -positive chromosomes in the 

 nucleus with a Feulgen -negative nucleolus attached to one pair. 



I 



Fig. 6-17 Hecently Copulated Cells Stained with Toluidine 

 Blue Showing Paired Chromosomes in the Zygote Nucleus. 



In S. cerevisiae, two round nucleoli are present in the early 

 stages of growth. Eventually both nucleoli fuse. 



In S. bayanus, a single nucleolus is observed in the chrome- 

 acetic -formaldehyde Feulgen preparations and in chrome- acetic 

 formaldehyde preparations stained with either methyl green and 

 pyronin or methyl green and acid fuchsin; cwin nucleoli have not 

 yet been found in S. bayanus. The nucleolus in S. bayanus is very 

 large, and centrally located, and much larger than that of S. cere- 

 visiae and more elongate, although in a young cell it may be rounder 



