92 CONTROL MECHANISMS IN CELLULAR PROCESSES 



It is difficult to reconcile these results with the hypothesis of the 

 nuclear origin of cytoplasmic RNA. For the time being, it must be 

 conceded that in cells as used by Harris, nuclear RNA is continu- 

 ously formed and degraded, and that some of the cytoplasmic RNA 

 is formed inside the cytoplasm, but there is still a margin allowing 

 for some nuclear RNA to pass into the cytoplasm. The quantitative 

 autoradiography with tritium may be less precise than the author 

 claims. It is extremely difficult to avoid personal bias in the choice 

 of material to be counted, since the labeling of the cells is usually 

 highly irregular and cell sizes vary appreciably. In the case of tri- 

 tium, the yS-rays are absorbed in very short distances, about 90 per 

 cent in 1.2 ix of water (Fitzgerald et al., 1951), so that self absorp- 

 tion of preparations of uneven thickness can give erroneous quan- 

 titative estimates. The results with fil^roblasts could indicate a 

 much larger flow of RNA from nuclei into the cytoplasm, if the error 

 was substantially higher than assumed. 



Intranuclear Site of RNA Production. There may remain doubt 

 about the exclusive synthesis of RNA in the nucleus in all cells, but 

 it can be considered as proved that at least part of the cytoplasmic 

 RNA does originate in the nucleus. This does not necessarily mean 

 that RNA is produced by genes. For a long time, the nucleolus was 

 considered by many as the RNA synthetic site since it was the part 

 of the nucleus richest in RNA. If that had been the case, the theory 

 of genie origin of RNA could not be maintained. Cytochemical ob- 

 servations showed an accumulation of RNA inside the nucleolus in 

 cells with a high rate of protein synthesis ( Caspersson, 1941 ) . The 

 available methods could not discern if the nucleolus were the real 

 site of RNA formation; only the use of autoradiography with radio- 

 active precursors could provide the refinement necessary. 



Thus, Taylor et al. (1955) found an accumulation of P''-, incor- 

 porated into RNA, in the nucleoli of Drosopliila salivary gland cells. 

 Later, those experiments were repeated by using C^^ adenine and H^ 

 cytidine as a precursor ( McMaster-Kaye and Taylor, 1959). RNA 

 appeared in the nucleoli before it was noticeable in the chromosomes. 

 It was found in the chromosomes at about the same time as it ap- 

 peared also in the cytoplasm. New RNA was also observed to 

 accumulate in puffs of salivary gland chromosomes of Rhijnchosciara 

 (Ficq et al, 1958) and Drosophila (Rudkin and Woods, 1959). 

 These and other similar experiments (Woods and Taylor, 1959) 



