10 The Life Cycle of the Single Cell 



each granule forms a new flagellum. If by mischance one of the daughter 

 cells does not receive a basal granule, it and its descendants will never 

 again form a flagellum. Thus the basal granule is independent of the 

 nucleus to the extent that the latter cannot replace the former after it 

 once has been lost. 



Other organelles (the gullets of ciliate protozoa, the chloroplasts of 

 some algae, etc.) act in the same fashion, i.e., they duplicate at cell divi- 

 sion and if lost from a cell cannot be regained. A most interesting case 

 involves the mitochondria of yeast. You will recall that mitochondria are 

 small bodies enclosed in membranes and distributed in the cytoplasm. 

 They contain organized collections of enzymes involved in respiration. 

 Recently it has been found that normally large yeast cells (which can 

 respire) occasionally give rise by budding to dwarf cells (which cannot 

 respire). And the descendants of these so-called "petite" cells are in- 

 variably petite even after hundreds of generations of growth. Thus, once 

 lost, the capacity to respire can never be regained. During budding, the 

 mitochondria concerned with respiration are normally shared between 

 mother and daughter cell, but once in a while by mistake the bud re- 

 ceives none of them. Because these mitochondria seem to be independent, 

 in the same sense as are flagellar granules, a cell bereft of them cannot 

 respire nor can its descendants do so, the loss being irretrievable. 



The nucleus itself, of course, is an autonomous organelle. That is, 

 a cell deprived of its nucleus cannot create a new one. The loss of the 

 nucleus is lethal for the cell; when anucleate, it can no longer reproduce. 



In summary, the cell is not simply a bag of homogeneous material 

 parcelled out in imprecise fashion during cell division. It is an assembly 

 of highly structured organelles. Many of these (nucleus, basal granules, 

 etc. ) are autonomous in the sense that they control their own duplication. 

 Thus, for cell division to leave the daughters with a fair share of all the 

 constituents, the cell must make coordinated preparations, and organelles 

 must be moved to the right places in the mother cell. 



The Life Cycle 

 of a Typical Unicellular Organism — Yeast 



We choose common Baker's yeast, Saccharomyces cerevisiae, to ex- 

 emplify the life cycle of simple plants and animals because it displays 

 three features that are present in the cycles of most organisms within 

 these groups : 



1. The single cell as the unit of existence: In contrast to higher ani- 

 mals (i.e., organized assemblies of specialized cells that are dependent 



