88 Cell Interactions during Growth and Morphogenesis 



S-cells have produced enough alanine to be poisoned by it. In the mean- 

 time, a few R-cells have arisen by mutation. Since the S-cells cannot grow, 

 the R-cells do and eventually dominate the population. The proportion 

 of R- and S-cells is controlled at any time by the levels of alanine. 



The interaction between the two cell types, therefore, is not simply 

 the result of an exchange of materials between the two. The S-cells, by 

 acting upon themselves (i.e., committing suicide), permit the R-cells to 

 do something they ordinarily could not have done, namely grow. (Had 

 the S-cells not died out, there would have been no opportunity for the 

 R-cells to grow. ) 



TRANSFORMATION OF CELL TYPES DIPLOCOCCUS 

 PNEUMONIAE 



The bacterium D. pneumoniae gives rise to mutant types that are 

 different in many ways from the parental stock. For example, we can 

 isolate mutants that are resistant to penicillin or streptomycin (when the 

 parent type is sensitive to these antibiotics), mutants that can degrade 

 sugars that the parent type is incapable of degrading, and mutants that 

 require various compounds for growth which the parent type can make 

 for itself. 



Highly purified deoxyribose nucleic acid (DNA) has been pre- 

 pared from penicillin-resistant mutant cells. When the sensitive parental 

 type is exposed to this DNA, a large proportion of the cells are trans- 

 formed into the penicillin-resistant type, and what's more, they pass on 

 this new capacity to their offspring. The DNA can only cause the exposed 

 cells to acquire capacities that are possessed by the cells from which the 

 DNA was obtained (i.e., DNA from penicillin-sensitive bacteria never 

 transforms the exposed cells to penicillin-resistant varieties). In essence, 

 then, it is possible to extract genetic information from one cell and intro- 

 duce it into another. The macromolecule bearing this information can be 

 incorporated into the genetic apparatus of the treated cell and thus the 

 information is passed on to the progeny. 



SYNERGISTIC GROWTH BY BIOCHEMICALLY DEFICIENT MUTANTS 



We deal here indirectly with some of the most important genetic 

 experiments of our time, which were conducted upon the bread mold 

 Neurospora crassa. Among other things, they provided convincing proof 

 that genes act by controlling the synthesis of specific enzymes. Neurospora 

 crassa can be grown on a relatively simple medium containing glucose as 

 a source of carbon and energy, ammonium chloride as a source of nitrogen, 

 a few mineral salts, and two vitamins. Mutants were isolated, that, unlike 



