The Life Cycle of the Single Ceil 11 



upon each other for their collective existence ) , the single yeast cell is the 

 total organism. 



2. Growth by vegetative (asexual) reproduction: Yeast cells mul- 

 tiply asexually, a single cell giving rise to two by budding. Sexual fusion 

 of gametes does occur under special conditions ( see below ) but its occur- 

 rence is not obligatory to growth. 



3. Sexual reproduction: You will recall from your study of genetics 

 that the higher plant or animal, consisting mostly of diploid cells (two of 

 each kind of chromosome ) , contains a few haploid germ cells or gametes 

 (one of each kind of chromosome). Two haploid gametes of the appro- 

 priate sexes can come together and fuse to yield a new diploid organism. 



This process is called sexual fusion and serves a very important 

 function. The diploid individual receives a set of chromosomes from each 

 parent. By meiosis, it in turn produces gametes with a single chromosomal 

 set which, by random assortment, is generally made up of some chromo- 

 somes from each of the original parents. Thus, the genes of the parents 

 are shuffled together in the o£Fspring and are reshuffled in the gametes 

 produced by that offspring (Fig. 3). Sexual fusion, then, is a source of 

 genetic recombination, producing new varieties by reshuffling the old. 

 New varieties are important to a population, for they ensure that there 

 will always be types that can take advantage of new environments and 

 thereby make the species fit to survive under a variety of conditions. 



To clarify this point, consider a population of yeast cells living in a 

 moderately cool bit of dirt and plentifully supplied with glucose to grow 

 on. With the coming of summer, the soil becomes hot and the glucose is 

 replaced by a different sugar, galactose. Now suppose that the population 

 had originally consisted of two types: cells that were resistant to heat but 

 could grow only on glucose, and cells that were sensitive to heat but 

 could grow equally well on galactose or glucose. Under the new condi- 

 tions, type A could stand the heat but not the new sugar; type B could 

 grow on the new sugar but could not stand the heat. Of course, type A 

 might produce a mutant that could grow on galactose and type B a mutant 

 that could resist heat, but these would be exceedingly rare events that 

 might not come to pass, in which case the yeast population would be 

 wiped out. But if sexual reproduction could occur, the chromosomes 

 bearing the genes for heat resistance and the ability to grow on galactose 

 would be shuffled together in the zygote, and the recombinant type would 

 be able to survive. Figure 3 illustrates this. 



Microorganisms such as yeasts and molds, protozoa, algae, and some 

 bacteria are capable of sexual fusion. That is, diploid cells can by meiosis 

 produce haploid gametes. These can be of different sexes ( or mating types 

 as they are called ) . Haploid gametes of the appropriate sexes can fuse to 

 yield new diploid individuals. 



