336 



K. U. ROBERTS, R. J. BRITTEN, AiNU B. J. MCCARTHY 



different composition cxij^t and only one of tliem is stable the conipoisi- 

 tion of newly foi-nicd inatei-ial will differ from the total. The newly 

 formed material must be distinu;uislied by its content of a tracer. Thus 

 the base ratios of newly formed KNA are usually estimated from the 

 P-^- content of the different mich'otides obtained by hydrolysis of the 

 RNA. This jirocedure, however, yields an "ap|)arcnt" compositit^n which 

 may differ from the true composition of newly formed RXA. 



If 5'-nucleotidcs are isolated and measured the apparent composition 

 will be determined partly by the specific radioactivities of the different 

 nucleotide precursors. Several authors have shown that considerable 

 differences in the specific radioactivities of the precursors do exist for 

 prolonged periods of time (Volkin and Astrachan, 1956; Yeas and 



T 3 



C\] 



X 



Q. 

 O 



E 



=3 



E 



"x 



o 



E 

 Ci 

 O 



Cytidylic Adenylic 



Guonylic 



3000 



2000 ^ 



- 1000 



in 



"c 



O 

 U 



rO 

 Q. 



Fraction number 



Fig. 28. Separation of nucleotides released by alkaline hydrolysis of RNA by 

 chromatography on Dowex-1. A small quantity of P"-labeled unknown material is 

 mixed with a much larger quantity of RNA of known composition before hydrolysis. 

 The composition of the unknown material can be calculated from the measured 

 specific radioactivities of tho nucleotide peaks. 



Vincent, 1960). When 2',3'-nucleotides are isolated, differences in the 

 specific radioactivities of the precursors will be averaged out provided 

 that the product nucleic acid has a random distribution of nearest 

 neighbors. Accordingly, alkaline hydrolysis to yield 2',3'-nucleotides is 

 customarily used. 



The neighbor distribution in ribosomes is random (Bolton, 1960) but 



