Gene Action and Amino Acid Coding 



429 



tion is RNA which is also a nucleic acid and 

 also has a four symbol code, A, U, C, G, in 

 which uracil (U) occurs in place of thymine 

 (T). We would then require the four symbol 

 code of DNA to be translated directly into 

 the four symbol code of RNA. There are 

 several questions we may ask, whose answers 

 will serve to test the hypothesis that DNA 

 nucleotide sequence specifies RNA nucleotide 

 sequence which, in turn, specifies amino acid 

 sequence. 



1. Where is RNA synthesized? Several 

 experiments support the view that much, if 

 not all, RNA is synthesized in the nucleus, 

 after which it can be detected, by radioactive 

 tracer studies, to enter the cytoplasm. There 

 is no evidence, on the other hand, of a flow of 

 RNA from the cytoplasm to the nucleus. 

 These results are consistent with the hypothe- 

 sis under consideration. 



2. What happens to nucleus-synthesized 

 RNA? Since the typical chromosome con- 

 tains RNA, chromosomal RNA, some of the 

 newly synthesized RNA must be retained in 

 the nucleus where it appears as part of 

 daughter chromosomes. Most of the newly 

 made RNA leaves the nucleus, and a con- 

 siderable portion is believed to be used in the 

 manufacture of new ribosomes. (In Neuro- 

 spora, ribosomal RNA is known to be syn- 

 thesized in the nucleus.) Already formed 

 ribosomes do not accept large quantities of 

 newly formed RNA, since it is found, by 

 labeling experiments, that only a relatively 

 small amount of a ribosome's RNA shows 

 turnover. Accordingly, the greater part of 

 the RNA in ribosomes, ribosomal RNA, is 

 usually incorporated at the time of ribosome 

 formation. Note that the mechanism by 

 which such nuclear RNA becomes incor- 

 porated as part of new ribosomes is still 

 unknown. 



3. What is the relationship between the 

 RNA synthesized in the nucleus and the DNA 

 in the nucleus? Bacteria manufacture RNA, 

 and continue to do so even after being in- 



fected with phage. The base ratio in the 

 DNA of a particular phage is known to differ 

 from that of the DNA of its host cell. After 

 phage infection it is found that the RNA 

 manufactured is diiferent from that manu- 

 factured prior to infection, having, in fact, 

 the base ratio of phage (substituting U for 

 T). Moreover, only the RNA synthesized 

 after infection can base pair in vitro with 

 phage DNA (made single-stranded by an- 

 nealing) to form a double strand — one 

 strand being RNA and one DNA. 



It is known that, under normal circum- 

 stances, RNA is first synthesized in the chro- 

 mosomes and is then transferred to the 

 nucleolus. Yeast cells can be fed radioactive 

 phosphorus so that the RNA synthesized 

 shortly thereafter is labeled. When such 

 labeled RNA is analyzed for its base ratio 

 it is found to have the same one as has yeast 

 DNA (making the substitution of U for T). 

 Other work "* demonstrates that, in normal 

 cells, freshly made nuclear RNA forms a 

 complex with chromosomal DNA and pro- 

 tein. (RNA in such a complex is resistant 

 to RNAase.) Such results support the 

 view that there is a direct base-for-base 

 correspondence between DNA and nucleus- 

 synthesized RNA. We can call this RNA, 

 which has the base ratio equivalent of DNA, 

 informational or template RNA. 



4. How is informational RNA synthe- 

 sized? In experiments dealing with the in 

 vitro synthesis of DNA (Chapter 35), it was 

 found that DNA can replicate in the absence 

 of RNA. This is very likely to be true also 

 in the nucleus, although there might be 

 subtle secondary interactions with RNA. 

 There is evidence, already mentioned in 

 answer to the previous question, in favor of 

 the view that RNA synthesis is intimately 

 related to DNA. It is found ^ that the 



■* By J. Bonner, R. C, Huang, and N. Maheshwari 



(1961). 



^ From the work of J. Hurwitz, of A. Stevens, of 



S. B. Weiss, of their colleagues and of others. 



