184 SUBCELLULAR PARTICLES 



DNA, when they dupHcate one more time in a medium free of labeled precursors. 

 Each regularly produces one labeled and one unlabeled daughter chromosome. 

 Attempts are described to get circumstantial evidence for or against a template 

 mechanism for RNA synthesis. Direct evidence is not available and there seems 

 to be no feasible experiment by autoradiography to test the hypothesis directly, as 

 was done for DNA. On the other hand circumstantial evidence for the idea can 

 be cited. /) The synthesis of RNA and DNA, although not mutually exclusive 

 of each other, are interrelated, as might be expected if DNA were the template 

 for RNA synthesis as well as for its own duplication. In some types of cells RNA 

 synthesis appears to stop during DNA synthesis and during the periods when 

 the chromosomes are condensed in division stages. However, the nucleolus is ac- 

 tive in RNA metabolism at all stages in the root-tip cells of Vicia jaba. In these 

 and other cells it incorporates labeled precursors of RNA much more rapidly 

 than any other part of the cell. The conclusion is almost inescapable that syn- 

 thesis occurs in the nucleolus where there is no DNA. Therefore, RNA may be 

 self-duplicating. At least, in the nucleolus the possible templates available would 

 appear to be limited to RNA, protein or a nucleoprotein. The time course studies 

 of changes in specific activity of nucleolar, chromosomal and cytoplasmic RNA 

 indicate that there are two metabolically different fractions in both the nucleolus 

 and in the chromosomes. One fraction turns over very rapidly and could be the 

 RNA transported to the cytoplasm and continually replaced by newly synthesized 

 RNA. The fractions with slower turnover could be the master templates derived 

 from the genetic loci. These would be able to duplicate or produce complementary 

 molecules, which may be required in greater quantity than could be produced by 

 a single site of genetic material. 



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