Animal Cells and Their Nutrition - 1 35 



and uracil) bases possess chemical affinity for 

 one another: cytosine for guanine (C — G), 

 adenine for thymine (A — T), and adenine 

 for uracil (A — U) — as is shown in Figure 7-5. 

 This important generalization was first pre- 

 dicted by F. H. C. Crick, of Cambridge Uni- 

 versity, in England, working in conjunction 

 with J. D. Watson, now at Harvard Univer- 

 sity. From the base-pair rule it follows that 

 the line-up of bases in a newly synthesized 

 unit of RNA is predetermined by the line-up 

 of bases in the DNA master template in a 

 chromosome of the cell. 



At this point it is not possible even to sum- 

 marize the long trail of evidence that has led 

 to two important conclusions: (1) that RNA 

 plays a dominant role in the synthesis of 

 enzymes and other protein components in the 

 cell and (2) that the specificity of each pro- 

 tein produced is determined by the base 

 sequence of the particular RNA unit that 

 participates in the synthesis (Fig. 7-5). The 

 code of this system — which popularly has 

 been called the code of life — is currently in 

 process of being solved, as will be explained 

 more fully later (Chap. 27). ft is, apparently, 

 a fairly simple code, constituted of the four 

 bases (A, C, G, and U) arranged in varying 

 order in groups of three (Fig. 7-5). Each 

 triplet — of which there are 64 (4X4X4) 

 variations — appears to exert an attractive 

 force upon a particular amino acid. How- 

 ever, some of the triplets may be neutral and 

 some amino acids may display affinity for 

 more than one triplet. Thus it has been 

 found that triplet AAA is capable of picking 

 up the amino acid phenylalanine, UUA picks 

 up asparagine, and AAU picks up isoleucine 

 — as is shown in Figure 7-5. At present the 

 system has not been fully decoded, but in- 

 tensive current research will doubtless soon 

 yield more complete information. Already it 

 is known, indeed, that the code is widely if 

 not universally employed in nature and that 

 it enables each cell to transmit its own genie 

 instructions from the chromosomes to the 

 sites where enzymes and other important pro- 

 tein components are being synthesized. 



The processes of protein synthesis are not 

 very simple, however. Undoubtedly, the ribo- 

 somes (Fig. 2-13) are very active and essential 

 parts of the system. These particles, which 

 are generally affiliated with the endoplasmic 

 reticulum, display a very high (40 to 60 per- 

 cent) content of RNA, and the ribosomal 

 RNA is of very high molecular weight (3 to 

 4 million). There are, however, two other 

 fractions of RNA in the cytoplasm, as will 

 be explained more fully in Chapter 27. One 

 of these, called transport RNA, is of low 

 molecular weight (2 to 3 thousand) and this 

 fraction can migrate freely throughout the 

 cytoplasm. Moreover, much recent evidence 

 indicates that this micromolecular fraction of 

 RNA is first to affiliate with the separate 

 amino acids, according to the triplet code. It 

 seems highly probable that these freely dif- 

 fusible RNA units serve to pick up and to 

 transport the different amino acids to the 

 ribosomal surface (Fig. 7-5). Coming to the 

 surface of a ribosome, presumably, the trans- 

 port RNA becomes aligned, according to the 

 base-pair rule, with a template fraction of 

 .RAM (Chap. 27). Thus the template RNA 

 serves to determine the amino acid sequence 

 in the finished protein product, and this 

 sequence in turn determines the specific na- 

 ture of the final protein. 



But many problems still remain. Energy 

 must be provided for the synthesis, and this 

 appears to be derived from the ATP reserves 

 (p. 143) of the cell. In any case, protein syn- 

 thesis does not occur in preparations of dis- 

 integrated cells, unless ATP is present. More- 

 over, the rate of protein synthesis falls oft 

 rapidly when ribonuclease, an enzyme that 

 destroys RNA, is added to the medium. It is 

 not known, however, how the serially aligned 

 amino acids become linked together, form- 

 ing an elongate peptide chain. It is, to be 

 sure, essentially a process of dehydration syn- 

 thesis; that is, a molecule of water is split 

 away each time a peptide bond is formed. 

 Furthermore, the efficiency of the process 

 must be high, as has been demonstrated by 

 the recent work of Richard Schweet, of the 



