POLYNUCLEOTIDE SYNTHESIS IN NUCLEOLUS AND CHROMOSOMES 183 



Brachet ct al. ( i) have shown that in anucleate pieces ol the alga, Acetabularia, 

 there is a net synthesis of RNA extending over a period of 30 days. This peculiar 

 organism with a very large cytoplasmic to nuclear ratio may he a poor one from 

 which to generalize, hut it obviously does synthesize RNA in the absence of a 

 nucleus. One wonders if this may result from the presence of chloroplasts which 

 are increasing, or if this organism has certain specialized sites other than the 

 nucleolus and chromosomes for the RNA master templates which enable it to 

 get along with the peculiar nucleocytoplasmic ratio. 



Evidence that can be cited in favor of the view that all or essentially all ot the 

 synthesis of RNA in cells without chloroplasts or viruses occurs in the nucleus 

 is still rather limited. Four points are worth repeating. /) Some cells appear to 

 have little synthesis of RNA, except perhaps in the nucleolus, during DNA 

 synthesis. 2) Synthesis of RNA stops during late stages of division when the 

 chromosomes are condensed and the nucleolus is disappearing. ^) There is a lag 

 of 30 minutes or more, after the cell begins incorporation of an isotope, before 

 label appears in cytoplasmic RNA. 4) Specific activity time curves are compatible 

 with the hypothesis that RNA of the nucleolus and chromosomes could be trans- 

 ported to the cytoplasm. 



What turnover rates would be required for the nucleolus and nucleus if they 

 were the sites of synthesis of all of the RNA in the salivary gland cell? From 

 table I we see that the nucleolus has between one twenty-seventh and one-fiftieth 

 the volume, and therefore the RNA content, of the cytoplasm. Assuming no ap- 

 preciable turnover for cytoplasmic RNA, which doubles in amount in about 

 9-10 hours, we see that fewer than 50 nucleolar volumes of RNA would be re- 

 quired. If one-half of the RNA present in the nucleolus at any given moment 

 were template RNA and one-half the material being transported out, this would 

 mean that the template RNA would have to duplicate or produce a comple- 

 mentary particle 100 times in 10 hours, or 10 times an hour. This rate is perhaps 

 too fast, but if we consider the chromatin also the rate is more reasonable. The 

 volume of the nucleus is about one-fifth that of the cytoplasm. Correcting for 

 concentration differences and assuming one-half is template RNA, the turnover 

 rate would need to be only i per hour, not counting the nucleolus. Therefore if 

 we consider both places sites of RNA synthesis for cytoplasm the required turn- 

 over rate becomes reasonable for cells with this nucleocytoplasmic ratio of RNA. 



su^I^L\RY 



Evidence for a duplex type of template for DNA synthesis has been obtained 

 by autoradiography. All of the chromosomes of a cell appear labeled at the first 

 division following a period of chromosome duplication (DNA synthesis) in a 

 medium containing tritium-labeled thymidine. However, these chromosomes re- 

 veal their hybrid nature; i. e., their content of original unlabeled and new labeled 



