436 CHAPTER 47 



of one DNA strand is rich in successive trip- used to make the same type of informational 



lets containing at least one A, this portion RNA (messenger RNA or sRNA). For it is 



is used to make U-containing messenger possible, via mutation, to invert a segment of 



RNA, and that (2) there is a corresponding double-stranded DNA; sometimes the length 



segment in the complementary DNA strand, of the inverted segment would correspond 



each of whose triplets contains T, which is exactly with a protein-specifying cistron or 



used not to make messenger RNA but A-con- operon. If this occurs, some parts of a given 



taining sRNA. It is not necessary to assume DNA strand may be used to make messenger 



that all portions of a given DNA strand are RNA, and other parts to make sRNA. 



SUMMARY AND CONCLUSIONS 



The translation of DNA nucleotide sequence into amino acid sequence involves the following 

 events: Each of the single strands of double-helix DNA serves as template for RNA poly- 

 merase, which synthesizes two complementary single strands of informational RNA. Seg- 

 ments of informational RNA which are composed primarily of U-containing nucleotide 

 triplets attach to ribosomes in the cytoplasm and function as messenger RNA. Segments 

 of informational RNA which are complementary to messenger RNA, and hence have at 

 least one A per triplet, are used to make transfer (soluble or s) RNA. Each kind of amino 

 acid is individually activated and attached to a different kind of sRNA in the cytoplasm. 

 The sRN A molecules, each carrying an amino acid, apparently base pair with complementary 

 regions of messenger RNA, so that the transported amino acids are arranged in a specific 

 linear sequence on the ribosome. The amino acids are then linked enzymatically to form a 

 polypeptide which is freed from sRNA, after which the sRNA molecules are liberated from 

 pairing with messenger RNA, and each is free to receive another amino acid for transfer. 



It is assumed that sRNA possesses a nucleotide triplet, containing at least one A, and that 

 this triplet serves both to specifically attract a particular amino acid and to pair with its 

 complementary triplet in messenger RNA. The code letters in this sRNA triplet and in 

 messenger RNA would be complementary. The code letters in messenger RNA have been 

 determined for almost all amino acids (each triplet contains at least one U), although the 

 sequence of the letters within the triplet and the order in which they are read are still largely 

 undetermined. 



The comma-free, triplet RNA code for specifying amino acids is essentially solved, al- 

 though some of the details are unknown at this time. 



REFERENCES 



Allfrey, V. G., and Mirsky, A. E., "How Cells Make Molecules," Scient. Amer., 205:74-82, 

 1961. 



Bautz, E. K. F., and Hall, B. D., "The Isolation of T4-Specific RNA on a DNA-Cellulose 

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Brenner, S., Jacob, F., and Meselson, M., "An Unstable Intermediate Carrying Information 

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Crick, F. H. C, "Nucleic Acids," Scient. Amer., 197:188-200, 1957. 



Crick, F. H. C, "On Protein Synthesis," Symp. Soc. Exp. Biol., 12:138-163, 1958. 



Furth, J, J., Hurwitz, J., and Goldman, M., "The Directing Role of DNA in RNA Synthesis," 

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Garen, A., "Genetic Control of the Specificity of the Bacterial Enzyme, Alkaline Phos- 

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