4 CELL HEREDITY 



change from (f to a^ which results from the influence of the extract on 

 cellular metabolism and (2) a hereditary change from a to a^ which per- 

 sists indefinitely among all progeny of the treated cells in the absence of 

 the extract because of the presence of a new hereditary determinant. 

 Various compounds from a^ cells may assist oT cells to form pigment, and 

 thereby to look like a"*", but the progeny clones derived from such cells 

 are again oT . Such oT cells are called phenocopies; they are altered 

 physiologically, but remain genetically a". Here we are not concerned 

 with physiological effects, but rather with the question: Is there any 

 chemical constituent within the a^ cell which can change the heredity of 

 the a cell from a to a ? 



Suppose experiments could be performed successfully in which a par- 

 ticular molecular species extracted from one cell type, a"*", the donor, 

 could transform cells of a different cell type, a~, the recipient, so that 

 they became genetically a^ and yielded upon extraction the same active 

 compound. The transforming extract would then be acting either by 

 altering a hereditary determinant of an a~ cell (mutation) or by intro- 

 ducing a new hereditary determinant from the a^ clone. With more de- 

 tailed information, one could distinguish between these alternatives. If 

 the transforming extract were indeed introducing a new hereditary deter- 

 minant into the a~ cell, then this new determinant would be present in 

 the extract. Transformation experiments, therefore, provide a method 

 for the isolation and identification of hereditary material. 



Successful transformation experiments have been performed with 

 several strains of bacteria, in which the symbols a^ and a~ may be 

 taken to represent any one of many pairs of different cell traits. Only 

 one class of molecules has been found thus far with the ability to induce 

 hereditarv transformations. Analogous experiments have been carried 

 out with a number of viruses and there too the same class of com- 

 pounds has been found capable of transmitting viral properties. These 

 compounds are nucleic acids. 



Chemistry of the nucleic acids 



Nucleic acid molecules are long-chain polymers (polynucleotides) 

 composed of monomeric units called nucleotides, which are themselves 

 complex molecules, each consisting of three components: a nitrogen base, a 

 sugar, and a phosphoric acid radical. A nucleotide without the phosphate 

 is called a nucleoside. All nucleic acids so far studied fall into two 

 classes: the DNA's (deoxyribonucleic acids) and the RNA's (ribonucleic 

 acids), which differ from each other in the sugar moeity, in one of the 

 nitrogen bases, and in many physical properties. The chemical relation- 



