lS2 SUBCELLULAR PARTICLES 



plasm and nucleolus are not just differences in rates, but that a relatively long 

 interval compared to diffusion rates of most molecules may be required for the 

 labeled RNA to reach the cytoplasm. This would happen if the nucleus were 

 the site of synthesis and the material mo\ed out as large molecules or aggregations 

 of molecules. 



HYPOTHESIS AND CONCLUSIONS 



If one makes the reasonable assumption that the chromosomes are composed 

 of many different templates, the products of which may be required in greatly 

 different quantities by the cell at certain periods in differentiation, an hypothesis 

 may be formulated. For gene products required in greatest quantity, RNA mas- 

 ter templates could be deposited in the nucleolus during periods of cellular dif- 

 ferentiation. In this special environment, they either would be self-duplicating or 

 would produce complementary molecules for transport to the cytoplasm. Other 

 templates required in smaller amounts could be produced primarily or exclusively 

 at the genetic locus, where extra RNA templates might also be temporarily de- 

 posited. Other types of RNA such as that in chloroplasts and viruses could be, 

 under proper conditions, self-duplicating. The principal difficulty with such an 

 hypothesis is that it will explain any result on turnover rates and base ratios which 

 may be found, and no evidence for or against the hypothesis would appear to be 

 obtainable by cytochemistry or biochemistry unless specific RNA's could be 

 isolated. Since there might be thousands of these on this hypothesis, this would 

 also appear to be an impossible task. For example, to try to find precursor rela- 

 tionships by examining base ratios of RNA's is probably hopeless. If one makes the 

 assumption that both template and product are in the nucleus or nucleolus, and 

 that different RNA's are duplicating at different rates in addition to their destruc- 

 tion in the cytoplasm at varying rates, any relationship found could be predicted. 

 Certainly statements that nuclear RNA cannot be the precursor of cytoplasmic 

 RNA based on such evidence is meaningless. Likewise, proof of the hypothesis 

 with the evidence appears impossible. Nevertheless, additional circumstantial 

 evidence is of value and a greater understanding of RNA metabolism can surely 

 be gained by these various techniques. 



Other clues concerning the mechanism of synthesis of RNA might be obtained 

 by determining the sites of synthesis in various types of cells. The experiments of 

 Goldstein and Plant (5) indicate a transfer of labeled RNA from the amoeba 

 nucleus when it is transferred into an unlabeled cell. However, the question of 

 whether only a part or all of the RNA of this cell is made in the nucleus remains 

 unclear. Prescott (7) reported no incorporation in anucleate pieces with C"-uracil 

 as precursor. On the other hand Plant and Rustad (6) found uptake of adenine-C^* 

 in amoeba fragments. Further experiments appear to be necessary, and if the rate 

 of incorporation is low, one must distinguish between synthesis of new molecules 

 and the addition of nucleotides to the end of RNA chains. 



