OO') 



UADIA rioX UK )!.()(; Y 



aiul scattering of protein by making two photographs or absorption 

 measurements of the same cell, one before and the other after enzymatic 

 or chemical rem()\al of the nucleic acid (see p. 231, Table 0-4, and Fig. 

 6-5a, 6). Since the difference between the two is (hrectly dependent upon 

 the natural absorption of the purine and pyrimidine components of the 

 nucleic acid, the latter technicjue provides an easy and sure method of 

 obtaining evidence of intracellular nucleic acid distribution. The sensi- 

 tivity is comparable with that of the Feulgen reaction (Tables 6-1, 2). 

 In the specificity of the nucleases lies the possibility of overcoming the 



(a) (6) 



Fig. 6-5. Test, left, and blank for nucleic acid determination by ultraviolet absorption. 

 Photographs, at 254 mn, of a maize pollen mother cell (No. 14281'>3-54) taken before 

 (a) and after (b) the section had been subjected to hot 5 per cent trichloroacetic acid 

 to extract all polj'nucleotide. The change in density of the spherical nucleolus is 

 marked. By direct measurement of this nucleolus (Pollister and Leuchtenberger, 

 1949b) it was found that the extinction of a central cylinder through a was 0.750, that 

 through b was 0.405, the difference being 0.345. These three values are assumed to 

 represent, respectively: (1) total specific and nonspecific light loss in the part of the 

 nucleolus measured; (2) light loss due to protein, mainly nonspecific; and (3) the light 

 lo.ss due to polynucleotide, mainly specific absorption. 



major disadvantage that ultraviolet absorption alone does not discrimi- 

 nate between RNA and DXA (Davidson, 1947; Pollister, 1950). 



None of the ultraviolet absorption curves of cell structures is exactly 

 like the curve of pure nucleic acid; there is always distortion, certainly 

 due in part to the associated protein, which characteristically exhibits 

 specific absorption in the region of 2750 A owing to its content of aromatic 

 amino acids (Chap. 5, this volume). Since these constitute but a small 

 percentage of the total amino acid content, the specific absorption of 

 proteins is very low in comparison with that of nucleic acid (Table 6-1 

 and Fig. 6-3), and, within the region of nucleoprotein absorption which 

 has been most studied (2500-2800 A), protein must be present in 20-50 



