Chapter *47 



GENE ACTION 



AND AMINO ACID CODING 



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N THE last Chapter, we found that 

 genes can act functionally, as 

 .operator genes, or structurally, to 

 cause the production of particular sub- 

 stances. Restricting our attention to DNA, 

 which seems to be the genetic material in 

 most kinds of organisms, the question may 

 be asked relative to structural gene action: 

 What can DNA do, or have done to it, 

 which would result in the formation of par- 

 ticular polypeptide chains? Since we are 

 dealing with conserved DNA, that is, DNA 

 which remains part of a polynucleotide, 

 whatever it does must be done in situ. Since 

 DNA is not protein, it is probably not an 

 enzyme, and probably does not act as a 

 catalyst in producing its cistronic effect. 

 Accordingly, being inactive in this respect, 

 DNA is thought to serve as a kind of tem- 

 plate, so that things are done to it or with it. 

 (In this respect DNA serves as a better inert 

 template than does RNA, whose ribose sugar 

 is more reactive than the deoxy-D-ribose in 

 DNA, so that, as a substance, RNA is less 

 stable than DNA.) 



We know that following strand separation, 

 each DNA strand apparently serves as a 

 template for the formation of a complemen- 

 tary chain. If DNA is also used as a tem- 

 plate for cistron functioning, we may ask 

 what kind of template information it may 

 contain. This information must be con- 

 tained in the fact that in a linear sequence 

 of DNA, there are usually only four differ- 

 ent base pairs, A : T, T : A, C ; G, G : C. 

 427 



What information are these base pairs sup- 

 posed to contain? Since their "primary" 

 product is, at least in some cases, a specific 

 polypeptide chain, consider what makes a 

 polypeptide chain specific. Almost all poly- 

 peptide chains contain one or more of each 

 of the twenty amino acids commonly found 

 in organisms. These amino acids are shown 

 in Figure 32-2, p. 285. Polypeptides usually 

 differ only in the number and sequence of 

 these amino acid building blocks. Clearly, 

 then, our problem is to understand how 20 

 amino acids and their sequence can be speci- 

 fied by DNA. Both the polypeptide and 

 DNA are linearly arranged, so this common 

 trait helps us visualize the relation between 

 the two. The sequence of nucleotides must 

 be meaningful in specifying amino acid se- 

 quence. But how can a linear template of 

 nucleotides, of which there are usually only 

 four kinds, determine the linear sequence of 

 amino acids, of which there are 20 kinds? 

 We are presented with a problem of DNA 

 coding. 



Let us postpone further discussion of the 

 nature of the genetic code, until we have 

 completed a consideration of certain evi- 

 dence regarding protein synthesis. 



Given the template of conserved DNA, 

 the question can be asked: Where in the 

 cell does the information contained in DNA 

 become translated into a polypeptide se- 

 quence? You might think that this specifi- 

 cation takes place in the nucleus, using the 

 DNA template directly. If so, it ought to be 

 possible to demonstrate that the nucleus is 

 the main or only site of protein synthesis. 

 There is very good evidence, however, not 

 only that some protein synthesis takes place 

 in the nucleus isolated from its cytoplasm,^ 

 but that protein synthesis also occurs in the 

 cytoplasm in the absence of the nucleus. 

 The evidence even suggests that the cytoplasm 

 is the major site of protein synthesis. Let us 



1 From work of A. E. Mirsky, V. G. Allfrey, and 

 others. 



