28 CELL HEREDITY 



The question of protein as hereditary material 



Until recently, it was the prevailing view that of all the cellular 

 constituents, onlv proteins were complex enough to qualifv as hereditary 

 material. The work with bacteria and phages has provided convincing 

 evidence that nucleic acids can play this role. But the question still 

 remains whether proteins also are represented among the hereditary 

 determinants of the cell. There is virtually no evidence in direct sup- 

 port of such a possibility, but neither is there anything decisively 

 against it. The best negative evidence comes from the experiments 

 with synthetic plant viruses, in which the nature of the protein coat 

 had no apparent genetic effect upon the progeny viruses. Protein trans- 

 formations in bacteria do not work, but in this field negative results 

 have little weight. 



GENES AND GENE ACTION 



In this chapter we have discussed experiments in which the chemical 

 nature of hereditary material was demonstrated. Two levels of analysis 

 were necessary in these experiments, one dealing with the chemistry of 

 the material, and the other with its biological role. Transformation 

 experiments have given us a general correlation of nucleic acid mole- 

 cules with genetic determinants, but have told us nothing of the de- 

 tailed chemical composition of individual genes, nor of the manner in 

 which they exert their manifold effects upon cell development. 



To specify the detailed chemistry of a gene will require advances 

 both in chemistry and in genetics. The chemical problem, involving 

 isolation of individual nucleic acid molecules and analysis of their 

 nucleotide sequences, appears formidably difficult. On the other hand, 

 the genetic problem of identifying the biological unit is advancing 

 rapidly at the present time, and is even providing material to aid in 

 unraveling the chemistry of the gene. Genetics began with the demon- 

 stration by Mendel that hereditary material could be analyzed into sets 

 of genetic units by means of breeding experiments. Recent advances 

 in genetics have provided refinements in the concept of the genetic unit 

 and new tools for its study. 



What is a gene? We define the gene in the broadest sense: a hered- 

 itary determinant which in its alternative forms is responsible for differ- 

 ences in a particular trait. We do not specify either its location in the 

 cell (chromosomal, nonchromosomal) or its constitution (DNA, RNA, 

 protein) because flexibility and generality are necessary in the face of 



