346 H. H. HOHKRTS, H. .1. HHirilN, AM) H. ,1. M (■CAHIII Y 



hand, tlu'if iiiiiiht he a specific type of rihusunic iov each pi'otciii ol llic 

 cell. There are a few indications of sonic heterogeneity in the ribosomes. 

 The nunil)cr of diffcMcnt protein coniponents .suggests that lihosonies of 

 different prott'in coinpleinent exist (Section II,C). If the rihosonie.s 

 contain a component approximating DNA in conii)osition, many (Hfferent 

 types of ribosonie aiv needed to contain e\-en one coniph'te copy of the 

 cell's DNA. 



More direct evidence of heterogeneity comes from chiomatography 

 of ribosomes. Samples taken from different parts of the elution pattern 

 have different chromatographic jiroperties. These differences maj'' be due 

 to the protein i)ortion of the particles as thei'e is little spread in the 

 elution pattern of the RNA obtained from ribosomes. Slight differences 

 between the RNA's from SOS and 50S ribosomes have been observed both 

 by chromatography and density separations. These differences indicate 

 that both techni(iues are sensitive to the composition of the RNA and 

 that RNA's of widely different compositions would be detected easily if 

 they had been ])resent. 



The most promising techniciue of isolating a si^ecific class of ribo- 

 somes is precipitation by an antibody. At present the only analysis of 

 such RNA shows a marginal difference from the average ribosomal 

 RNA. 



Templates for different proteins would not be expected to vary 

 appreciably in their base compositions as the amino acid distribution 

 is relatively constant. An RNA forming the structure of the ribosomes 

 might also be expected to be constant. Thus any separation of ribosomes 

 based on differences in their RNA composition may be difficult to 

 achieve and the heterogeneity of the RNA of ribosomes separated by 

 other techniques (such as antibody precipitation) may be difficult to 

 demonstrate. 



E. SITE OF RIBOSOME SYNTHESIS 



Mammalian cells are large enough for radioautographs to show that 

 newly .synthesized RNA is localized in the nucleus. Bacteria are too small 

 to allow such direct methods, but Caro and Forro have analyzed the 

 distribution of radioactivity in slices of bactei'ia. These studies have 

 shown that newly synthesized RNA, unlike the bulk of the RNA, is 

 localized in a central region of the cell (Caro and Forro, 1961). At cor- 

 responding times the radioactivity is found in the eosome fraction (see 

 Section I V,B) . 



The composition of the newly formed RNA suggests that a consider- 

 able fraction has a composition like that of the cell's DNA. An enzyme 

 has been isolated which in the i)resence of a DNA primer catalyzes the 



