POLYNUCLKOTIDt SYNT1II:SIS IN NUCLEOLUS AND CI IROMOSOM i;s l8l 



hours there is little net change in the amoinit of nucleolar RNA. Theretore, the 

 change in specific activity has only two possible interpretations. I'.ither the RNA 

 is synthesized rapidly and then broken down at the same site, or it is being 

 transported from the nucleolus to some other site in the cell. 



About the fifth hour there is an abrupt change in the rate of loss of isotope from 

 the nucleolus. After that time the decrease in specific activity can be accounted 

 for by the increase in net amount of RNA. The result can be explained if the 

 nucleolus contained two types of RNA with quite different metabolic properties. 

 Vincent ( 19) has reported two such types in the nucleolus of starfish oocytes. 



Two metabolic types are also indicated by the specific activity curves for the 

 chromosomal RNA (fig. 5). As in the nucleolus, one fraction disappears in 2 or 

 3 hours while the other fraction changes specific activity at about the rate at 

 which new RNA would dilute the labeled material during growth of the nu- 

 cleus. The cytoplasm does not contain a detectable amount of the RNA with a fast 

 turnover. The decreases in specific activity can be accounted for by net synthesis, 

 as revealed by the increase in volume and RNA content. This increase is cor- 

 related with a proportional drop in specific activity of RNA (fig. 5). 



An examination of the changes in specific activity indicates that neither cyto- 

 plasmic nor chromosomal RNA is likely to be the source of all of the rapidly 

 labeled RNA which appears in the nucleolus. We reach the conclusion that part 

 of it must be synthesized there. Therefore, we have an answer to the question 

 originally posed. Some of the nuclear RNA is synthesized at a site where there 

 are few if any DNA templates. However, we should point out that part of the 

 RNA of the nucleolus, especially the fraction with a slow turnover, could come 

 from the chromosomes. This could be the template RNA of the nucleolus, as sug- 

 gested by Vincent (20), while the RNA with a fast turnover may be the product 

 of these templates which is being transported to the cytoplasm. 



Now let us consider the fraction with a high rate of turnover, which appears in 

 the chromosomes or in that region of the nucleus occupied by chromosomes. It 

 may not necessarily be attached to or associated with chromosomes. This RNA 

 could be material from the nucleolus in route to the cytoplasm, perhaps in the 

 form of relatively large particles; i.e., molecular aggregates which are below the 

 resolving power of the light microscope. On the other hand, it could be a fraction 

 produced in the chromosome for transport to the cytoplasm or even to the nu- 

 cleolus. Autoradiographs made 30 minutes after the larvae begin to eat food con- 

 taining cytidine-H^ show label in both the nucleolus and among the chromosomes 

 (figs. 3 and 4). Quantitative data are not yet available for the relative rates of 

 incorporation. However, no label is yet detectable in the cytoplasm, even when 

 the exposure is long enough to make the film above the nucleolus much blacker 

 than in the photograph. The same result is obtained in root tip cells of the bean, 

 Vicia jaba. These observations favor the view that the differences between cyto- 



