52 HEWSON SWIFT 



arises from their intimate association with genes and chromosomes, and 

 their apparent involvement in protein synthesis. Cytochemical techniques 

 have played a dominant part in establishing these relations. 



All cytochemical reactions have their limitations and it is important to 

 investigate these as fully as possible. It should be remembered that, at 

 least for the present, relatively inaccurate or nonspecific methods may 

 often serve a useful purpose, particularly where the variables are of known 

 magnitude. With many biological problems, knowing whether the amount 

 of nucleic acid in a cell increases or decreases is fully as important as know- 

 ing the exact amount present. 



Cj'tochemistry, in treating the cells themselves as units, can supply in- 

 formation on individual cell variation that is obviously lost in the homogen- 

 ates often used in biochemical procedures. It thus constitutes practically 

 the only approach to many biological problems, for example, concerning 

 the chemical changes that accompany mitosis. The three major cytochemi- 

 cal methods for nucleic acid localization depend on the three major nucleo- 

 tide components. Basic dyes are associated with the phosphoric acid, the 

 Feulgen reaction with the sugar, and ultraviolet absorption with the purines 

 and pyrimidines. 



II. Basic Dyes 



It has been known for many years that the ability of fixed cells to bind 

 basic dyes was associated, at least in part, with their nucleic acid content. 

 Only comparatively recently, however, has there been any concerted at- 

 tempt to use dye binding as a technique for the cytochemical study of 

 nucleic acids. Among the early important studies were those of Mathews,^ 

 Hammarsten and Hammarsten,^ and Brachet.' Recently basic dyes have 

 been used in a large number of investigations, some in conjunction with 

 microphotometric determinations. Variation in dye intensity has been in- 

 terpreted as an indication of variation in nucleic acid concentration, or in 

 some cases of "polymerization." To evaluate these techniques it is neces- 

 sary briefly to examine the major factors affecting dye-nucleic acid inter- 

 action. 



Without doubt the most important factor in dye binding is the electro- 

 static charge on the dye molecule and on the nucleoprotein with which it 

 combines. Basic dyes are usually the salts of colored bases, and their com- 

 mon combining group, or auxochrome, is the amino group which exists in 

 a positively charged dissociated form ( — NH3+) except at high pH levels. 



» A. Mathews, Am. J. Physiol. 1, 445 (1898). 



* E. Hammarsten, G. Hammarsten, and T. Teorell, Acta Med. Scand. Suppl. 196, 



1 (1928). 

 3 J. Brachet, Compt. rend. soc. biol. 133, 88 (1940). 



