Polypeptide Synthesis and RNA 



425 



tioning of DNA genes (Chapter 32), it is 

 necessary to understand how a DNA tem- 

 plate that remains in the nucleus of a retic- 

 ulocyte orders the amino acid sequence of 

 hemoglobin manufactured in the cytoplasm. 

 Clearly, if the DNA functions as a template 

 in this respect, it must be doing so indi- 

 rectly. 



DNA might be used to make another tem- 

 plate, neither DNA nor protein, which can 

 leave the nucleus and enter the cytoplasm 

 where it will be used for protein synthesis. 

 RNA fits this description, being a nucleic 

 acid which also has a four-symbol code (A, 

 U, C, and G) in which uracil (U) occurs 

 in place of thymine (T) . Such a mechanism 

 would require the four-symbol code of DNA 

 to be transcribed directly into the four- 

 symbol code of RNA — that is, it would in- 

 volve a problem of transcription. It would 

 also require RNA to carry information trans- 

 lated into polypeptide sequences — that is, it 

 would involve a problem of translation. 

 Therefore, the multiple hypothesis is sug- 

 gested that DNA nucleotide sequence is 

 transcribed into RNA nucleotide sequence 

 which, in turn, is translated into amino acid 

 sequence. 



Messenger RNA 



Under normal circumstances, a considerable 

 amount, if not all, of the RNA in higher or- 

 ganisms is synthesized in chromosomes and 

 then transferred to the nucleolus. Subse- 

 quently, using radioactive tracers, RNA can 

 be detected entering the cytoplasm. On the 

 other hand, no evidence is found for a flow 

 of RNA from the cytoplasm to the nucleus. 

 These results are consistent with the hy- 

 pothesis under consideration. 



The relationship between RNA synthesis 

 and DNA can be studied in bacteria. RNA 

 is synthesized in bacteria after infection with 

 a DNA phage whose base ratio differs from 

 that of the host DNA. The RNA manufac- 

 tured after phage infection is different from 



the RNA manufactured prior to infection; 

 its base ratio depends upon that of phage, 

 since only the RNA synthesized after infec- 

 tion can base pair in vitro with strand-sepa- 

 rated phage DNA to form a hybrid double 

 strand — one RNA and one DNA. (Hybrid 

 RNA-DNA molecules have a unique specific 

 density and, therefore, can be identified in 

 the ultracentrifuge tube; they are also rela- 

 tively resistant to RNase.) Also, freshly- 

 made nuclear RNA from normal cells can 

 form a complex with chromosomal deoxy- 

 ribonucleoprotein. ! Such results suggest 

 the existence of a direct base-for-base de- 

 pendence of nucleus-synthesized RNA and 

 nuclear DNA. 



As mentioned, RNA complementary to 

 phage DNA is made after a DNA phage in- 

 fects its host. This phage-specific RNA is 

 found to attach to a small percentage of 

 already-formed ribosomes, suggesting that 

 at least some ribosomes do not permanently 

 carry a template of RNA (obtained from the 

 DNA template) containing information for 

 the specification of an amino acid sequence. 

 Such ribosomes are capable of receiving seg- 

 ments of RNA which carry the information 

 for making phage-specific polypeptides. 

 Thus, a type of RNA, called messenger RNA 

 or mRNA, is synthesized. mRNA carries 

 information for gene action from phage 

 DNA to the ribosome. Presumably the mes- 

 senger RNA causes the assembly of various 

 amino acids at the ribosome where they are 

 joined to form polypeptides. Messenger 

 RNA is also found and functions in normal, 

 uninfected cells. 4 The RNA genetic material 

 of MS<f)2 which infects E. coli is conserved 

 during all replications (by RNA synthetase) 

 and translations (by serving as a messenger 

 RNA) which occur during a lytic cycle. 5 



3 As shown by J. Bonner, R. C. Huang, and N. 

 Maheshwari (1961). 



4 M. Hayashi and S. Spiegelman (1961); see S. 

 Spiegelman (1964). 



5 As shown by A. H. Doi and S. Spiegelman 

 ( 1963). 



