GENETIC CONTROL 27 



cells to certain viruses depend on specific interactions between the surface 

 of the cell and the virus protein (De Somer, et al. 1959 ; Holland et al., 1959 ; 

 Gordon and Smith, 1960). Only primate cells are susceptible to polio- 

 myelitis virus. But cells of rabbit, pig, mouse and chicken can be infected 

 by RNA extracted from the virus. It is striking that the virus thus produced 

 in non-primate cells shows the typical host range and serological specificity 

 of poliomyelitis virus. Since the structure of the protein coat is not in- 

 fluenced by the cell in which it is produced, it must have been strictly con- 

 trolled by information contained in the virus RNA. 



There is a complete analogy between virus infection by RNA and 

 bacteriophage infection, which normally occurs by injection of DNA into 

 the recipient cell. Virus RNA is the carrier of information for the pro- 

 duction of at least the virus protein, just as DNA is the carrier of the 

 information which controls the synthesis of an array of phage proteins. 



3. Genetic Material of Higher Organisms 



Sperm heads and chromosomes are two structures known to contain 

 mendelian genes. Comparison of their constitution provides a first indica- 

 tion about the nature of the genetic material in animals. Chromosomes are 

 made of DNA, histones, 'residual' acidic protein and RNA (Brachet, 1942). 

 Residual protein may be absent from metaphase chromosomes (Casperson, 

 1941 ; Bloch and Godman, 1955). Sperm head in fishes contains no RNA 

 and no histones; it is made of DNA associated with protamines. The only 

 constituent that fish sperm head and chromosomes have in common there- 

 fore seems to be DNA which accordingly must be the carrier of genetic 

 determinants. 



Other indications in the same direction are found in the constancy of the 

 DNA content per cell in any given species. Moreover, many mutagenic 

 agents act upon animals and plants in the same way as on bacteria and most 

 of them are known to react easily with DNA. The ultraviolet action 

 spectrum for the production of mutations in higher organism as well as in 

 micro-organisms resembles very much a nucleic acid absorption spectrum. 

 DNA from different species haVe different com.positions : the adenine/ 

 guanine ratio seems to be characteristic of the species (Vischer and Char- 

 gaff, 1948; Chargaff et al, 1949; Chargaff, 1956). DNA is also one of the 

 most stable compounds of the animal cell and this quality is well suited to a 

 guardian of hereditary characters. 



DNA therefore appears as the most probable carrier of genetic informa- 

 tion in higher organisms and although as direct an evidence as in bacterial 

 transformation has not been clearly obtained so far, it is the common belief 

 that DNA is at least the main genetic material in higher organisms. 



Extensive discussions about the nature of the genetic material will be 

 found in reviews by Hotchkiss (1955) and by Brachet (1957). 



