124 ALEXANDER L. BOUNCE 



son et aZ."" This is the finding that the mean quantity of DNA per cell 

 nucleus is constant within a given species for normal resting diploid somatic 

 cells and that half this value is found in spermatozoa. The amount of DNA 

 per nucleus may, however, vary widely from one species to another. 



These findings would certainly identify DNA as a chromosomal constitu- 

 ent, even if the chromosomal localization of DNA were not known from 

 direct analysis and histochemical procedures. Furthermore, they lead, 

 when taken together with work on DNA turnover, to the selection of DNA 

 as the only known material to be a logical candidate for gene substance. 

 Recent work by Zamenhof et al?^ on one of the bacterial transforming 

 agents (Chapter 27) greatly strengthens this hypothesis. The suggestion 

 that genes may be composed of DNA has already been made by Mazia.^^ 



The question of the amount of DNA per nucleus is discussed in detail 

 in Chapter 19 of this book. Attention should be called to a recent paper 

 which is concerned with errors of counting that interfere with accurate 

 estimates of the amount of DNA per nucleus, and also with a means of 

 overcoming these errors.^^ 



(3) State of DNA in Cell Nuclei. The question of gel formation by iso- 

 lated nuclei in relation to the state of the DNA in the nuclei has already 

 been mentioned in this chapter and elsewhere.'''®*" It is the firm conviction 

 of the reviewer that a chemical bond, of a type other than a salt linkage, 

 normally exists between DNA and some protein of the cell nucleus, and 

 that most of the phosphate groups of the DNA are not involved in this 

 bond. Since it seems unlikely from the physical properties of nuclear gels 

 that the DNA can be attached to protein by multiple bonding along the 

 length of the nucleotide chain, the possibility of attachment of one end of 

 the nucleic acid chain to protein must be considered. It should eventually 

 be possible to determine the type of chemical bond in question. As long as 

 the DNA is firmly bound to nuclear protein, the peculiar "unwinding" 

 type of gel previously described can be formed by adding alkali, or salt at 

 a neutral pH. When the attachment of the DNA to protein is broken, this 

 type of gel can no longer be formed. 



(4) Intracellular Distribution of DNA . It has been mentioned that DNA 

 is a chromosomal constituent. Until recently it was thought that nucleoli 

 contained PNA but not DNA but this point of view has now been ques- 

 tioned, ^^ and will be discussed later. 



" J. N. Davidson, I. Leslie, R. M. S. Smellie, and R. Y. Thomson, Biochem. J. 45, 



Proc. XV (1949). 

 ^8 S. Zamenhof, H. E. Alexander, and G. Leidy, J. Exptl. Med. 98, 373 (1953). 

 " S. Albert, R. M. Johnson, and R. R. Wagshal, Science 117, 551 (1953). 

 *" A. L. Bounce in "The Enzymes" (Sumner and Myrback, eds.), Vol. 1, Part 1, p. 



222. Academic Press, New York, 1950. 

 81 M. Litt, K. J. Monty, and A. L. Bounce, Cancer Research 12, Sci. Proc. 279 (1952). 



