37. NUCLEIC ACID AND PROTEIN SYNTHESIS 407 



quirements of transfer RNA underlying the specificity with which it re- 

 acts with amino acids. Presumably the secret lies in the sequence of bases 

 in some part of the molecule. With the refinement of techniques for frac- 

 tionating RNA, and of techniques for sequential degradation of nucleic 

 acids, we may hope to find the answer. 



2. The Role of Deoxyribonucleic Acid 



Cell-free systems have shed little light on the role of DNA in protein 

 synthesis. Just as we can now be assured that RNA participates actively 

 in protein synthesis, we can state with equal certainty that DNA must in 

 some way ultimately control the process. The in vitro systems we have dis- 

 cussed and other experiments in whole cells (see particularly the review by 

 Chantrenne 8 and Chapter 38 by Gros) make it clear that protein synthesis 

 can proceed in the absence of DNA. It seems obvious, therefore, that DNA 

 must somehow be able to convey a stable pattern of information to the site 

 of protein synthesis. We have reviewed the evidence that the ribonucleo- 

 protein particle is the site of protein synthesis and have expressed the view 

 that RNA is the most likely candidate for the template. It, therefore, seems 

 highly probable that DNA must be involved in the manufacture of this 

 RNA template. This activity would most likely occur in the nucleus and as 

 we have seen there is accumulating evidence that RNA and probably ri- 

 bosomes are synthesized in the nucleus. DNA can replicate itself and it is 

 not unreasonable to expect that it might also be able to produce a molecule 

 of RNA, thus conferring to the latter a complementary copy of its base 

 sequence. 



We visualize particle RNA as relatively highly polymerized. The total 

 molecular weight of the RNA of the 80 S particle may be about 2 X 10 6 . 

 However, we have seen that such particles can be dissociated reversibly 

 into smaller units simply by lowering the Mg ++ concentration and we are 

 tempted to wonder what the true covalently continuous size of particle 

 RNA subunits really is. This has relevance to the template theory since we 

 conventionally picture genetic information residing in large molecules. If 

 it is found that particle RNA consists of relatively small subunits, then they 

 must be arranged in some kind of rigid structure by the protein of the par- 

 ticles. This would be all the more reason to suspect that the whole particle, 

 not just the RNA, is synthesized in the nucleus. 



The origin of transfer RNA then becomes an intriguing problem. Two 

 possibilities might be considered. Perhaps once the ribosomal (template) 

 RNA is synthesized, it in turn is able to synthesize strands of RNA com- 

 plementary to segments of itself which, under the proper conditions, would 

 be released as soluble RNA into the cytoplasm. Perhaps the indigenous 

 ribonuclease activity of particles has some role in releasing such copies 



