14 The Life Cycle of the Single Cell 



ascospores and are contained within a large sac or ascus. The ascus bursts, 

 the ascospores are released and then germinate, yielding small round 

 yeast cells, still haploid. Two haploid cells of the proper sexes ^ can come 

 together and fuse once again to yield a large diploid individual. The cycle 

 is complete at this point and can be repeated. 



Many questions come to mind. For example, what specific environ- 

 mental conditions induce a diploid cell to undergo meiosis and produce 

 ascospores? What causes the ascospores to germinate into haploid cells? 

 What happens when the haploid cells are kept separate and not allowed 

 to meet other haploid gametes of the correct sexual type? Can they grow 

 by budding just Hke diploid cells? How is it that the haploid gametes can 

 be of diflFerent sexes and yet look alike? What is the physiological basis 

 of sex in these organisms? How is sex inherited in yeast? Some of these 

 questions have been answered through research. Many still remain un- 

 answered. 



The Life Cycle of Paramecium Aurelia 



Figure 5 is a drawing of Paramecium, a favorite subject of study in 

 elementary biology classes. Its large size (usually 150 microns x 50 

 microns - ) and intricate structure attracted the attention of early biolo- 

 gists. In recent years its life cycle and genetics have been rigorously 

 defined. We can do no more here than briefly summarize these findings. 



SOME FACTS ABOUT PARAMECIUM 



P. aurelia is a fresh-water organism that preys upon bacteria and 

 algae. It is grown in the laboratory in association with a single bacterial 

 species. In recent years a bacteria-free nutrient medium has been devised 

 in which Paramecia can grow at a good rate. This medium is complex, 

 consisting as it does of proteins and many amino acids, eleven vitamins 

 including a derivative of vitamin D which is a requirement that is char- 

 acteristic of higher animals, and purines and pyrimidines ( needed for the 

 synthesis of nucleic acids ) . P. aurelia possesses three nuclei. One is large 

 ( macronucleus ) and two are small (the micronuclei). It has a complex, 

 organized gut consisting of a mouth leading into a gullet lined with cilia 



1 There are two sexes amongst yeast gametes, called -f and — . Two + cells or 

 two — cells cannot fuse. Only a + and — cell can. The two sexes look identical (in 

 contrast to those of higher organisms) and can only be distinguished by their mating 

 reaction. In each ascus two ascospores are of sex + and two are of sex — . From your 

 knowledge of genetics, can you deduce the manner in which sex is inherited by yeast? 



2 A micron is 0.000001 meter. The average bacterium is about 2 X 0.5 microns. 



