330 



H. H. RllRKRTS. H. J. IJRITTKN. AND H. J. MCCARlin 



I']\'i(l('iitly such results ai'c not. couipatihlc with any >iin|)lc ])i'ccur.sor 

 product relationship. 



Phenol-extracted UNA may also be fractionated by chromatography 

 on columns of methylated scrum albumin adsorbed on kieselguhr (Man- 

 dell and Ilei-sliey, 19()()). A clear sejjaration is obtained Ix'tween soluble 

 KNA. DNA, and hiyh molecular wei(2;ht RNA. Analyses of pulse-labeled 



Bottom 



20 Top Bottom 5 

 Fraction number 



cr 



20 Top 



Fig. 22. Sedimentation analysis of RXA extracted from ribosomi>s («) from cells 

 given a 30-second exposure to C'^-uracil, (b) from cells given a 30-second exposure 

 to C"-uracil followed by 20 minutes growth in C'"-uracil at a 200-fold excess con- 

 centration. Cent rifugat ion 230 minutes at 37,000 rpm, 4°C. 



RNA slio\y two main features. At early times almost all of the radioac- 

 tivity appears in the I'cgion of high moUn-ular weight RNA in three 

 peaks, none of which corres])onds exactly to the two ])eaks of ribosomal 

 RNA (Fig. 23). JVIoreover, the over-all specific radioactivity of the high 

 molecular weight region is greater than that of soluble RNA (e.g., a 

 ratio of 3-4 at 1 minutes) (Fig. 23). 



Analysis of a number of time points in this way allows deterniinalion 

 of the delay of entry of label into S-RNA and DNA relative to the 

 high molecular weight RNA. Parallel experiments carried out with C^'- 

 uracil and P'- as the label lead to a delay time of 1-2 minutes. The 

 interpretation has been made (Midgley, 1962) that the entry of label 

 into both S-RNA and DNA results from nucleotide matei'ial produced 



