211 K \1)1 AI'lON HI()I,<)(;V 



■J. Thcic arc specific enzymes, rihoiiuclease and deoxyrihonuclease, 

 which act to detiiadc each type of nncieic acid. 



3. The total nucleic- acid content may he renio\c(l specifically hy chemi- 

 cal ajj;ents such as trichloroacetic acid. 



Thus, sites of nucleic acid are readily identifiable a.s parts of the 

 cell with basophilia which is removable by acid extraction, and the type 

 of nuclease susceptibility shows which of the two nucleic acids is present 

 (cf. Figs. 6-2C, F and 5a, /;). 



Such specific nucleic acid basophilia adds to cell morphology an impor- 

 tant chemical datum, showiiif^ that, in a cell such as that of Fig. G-1, for 

 example, there is undoubtedly considerable nucleic acid in the basal zone 

 and nucleolus as well as in the chromatin. More precisely, from such a 

 cytological preparation as Fig. (3-20 it can be concluded that, in these 

 parts of the cell, the nucleic acid concentration is so high that the dye 

 bound as dye nucleate is in high enough concentration to appear as strong 

 visible color in structures no more than o ij. thick (p. 209). To what extent 

 does this approach a complete picture of the distribution of major polynu- 

 cleotide concentrations within the cell? It must be emphasized that basic 

 staining does not lead to localization of nucleic acid by any of its natural 

 physical properties in the same manner as the natural green color indicates 

 the sites of chlorophyll. Instead, visualization by basophilia depends 

 on the capacity of nucleic acid to bind the cations of basic dyes, which 

 may be mainly through displacement of protein from its natural combina- 

 tion with the residual phosphoric acid valencies of the polynucleotide. 

 This staining reaction may therefore be very complex, and the relation 

 between color and amount of substrate may by no means necessarily be 

 a simple one. From basophilia alone it is impossible to answer such 

 questions as: How strict is the proportionality between basophilia and 

 nucleic acid concentration? Is this proportionality constant or highly 

 variable? What interpretations may be assigned to negative basophilia? 

 Can there be considerable accumulations of polynucleotide which are 

 unaccompanied by any basophilia? Is an increase or decrease of baso- 

 philia due to change in amount of nucleic acid or to change in the number 

 of phosphoric acid valences which are available for dye binding? The 

 transition from a cytological to a cytochemical viewpoint poses all such 

 questions and immediately reveals the danger inherent in uncontrolled 

 cytochemical use of staining reactions. Only by an independent method 

 which measures nucleic acid directly can these questions be answered, and 

 in natural ultraviolet absorption of nucleic acid such a method is available 

 (see Sect. 3-4). In general, regions of strong nucleic acid ultraviolet 

 absorption have been found to coincide with those of pronounced baso- 

 philia (see Figs. 6-2C, D), but there are indications that the amount of 

 dye bound in cells for a given amount of pentose polynucleotide is variable 

 (M. H. Flax, unpublished data), and one extreme case has been reported 



