526 - Herediiy and Evolution 



plate forms of RNA. Thus the base-pair rule 

 plays a crucial role in determining the pre- 

 cise order of amino acids in the peptide chain 

 of the synthesized protein (Fig. 4-20). In the 

 last analysis, therefore, the base-pair sequence 

 of the genie DNA eventually determines the 

 amino acid sequence in the protein compo- 

 nents of each organism. 



The situation is even more complex, how- 

 ever. Not only is a specific enzyme required 

 for each amino acid as it affiliates with trans- 

 fer RNA, but energy, provided by ATP, must 

 be available. Moreover, the formation of pep- 

 tide bonds, which link the amino acids to- 

 gether after they have been aligned properly 

 upon the template, requires a further fund of 

 energy. This is provided by another high- 

 energy phosphate compound, namely gua- 

 nine triphosphate (GTP). The fact remains, 

 however, that the base order of the genie ma- 

 terial finally determines the amino acid se- 

 quence in the proteins of the organism. Thus 

 the specific enzymes of a species are a product 

 of its genes. 



THE GENETIC CODE 



Deciphering the code of biochemical in- 

 struction, which is built into the DNA inher- 

 itance of an organism, represents a truly 

 formidable problem. Considerable progress 

 has recently been made, however, mainly as 

 a result of experiments by Crick, Ochoa, 

 Nirenberg, Ben/er, Hurwitz, and others. Two 

 principal methods have been valuable: (1) 

 studies on protein synthesis in cell-free bac- 

 terial preparations (p. 524) to which are 

 added artificially synthesized samples of (tem- 

 plate) RNA — in which the sequence of bases 

 is known; and (2) studies on the transmission 

 of hereditary defects in the DNA component 

 of bacteriophage viruses in doubly infected 

 bacteria (see below). 



Since there are only four bases — cytosine 

 (C), guanine (G), thymine (T), and adenine 

 (A) — present in a DNA molecule, and since 

 there are some 20 amino acids to be coded, it 

 is not possible for each amino acid to be 



coded by a single base. In fact, two bases per 

 amino acid are not enough, because this sys- 

 tem would yield only 16 (4x4) combina- 

 tions. The simplest possible code, therefore, 

 must consist of three bases, which provides 

 for a system in which 64 (4 X 4 X 4) "words" 

 are available. 



Such a triplet code is shown in Table 27-1. 

 Almost beyond doubt this is the code utilized 

 by organisms generally. In this system, some 

 amino acids are coded by more than one 

 triplet, as has been verified in a number of 

 experiments. In other words, using the spe- 

 cial language of the code maker, the genetic 

 code is a degenerate one. 



Simplest Concept of a Gene. In order to 

 code the synthesis of an average protein, 

 which is a peptide chain consisting of some 

 500 to 1000 amino acid units, the DNA mole- 

 cule must carry 500 to 1000 triplets, arranged 

 precisely in a specified sequence. The simplest 

 concept of a gene, accordingly, would be a 

 segment of a DNA molecule in which, on the 

 average, some 1500 to 3000 base pairs are rep- 

 resented. This is not an unduly large num- 

 ber, however. Most DNA molecules are 

 exceedingly long. The DNA of the T 4 bac- 

 teriophage, for example, encompasses more 

 than 200,000 base pairs, which would repre- 

 sent about 200 genes. 



DNA in a chromosome is always combined 

 with protein material. This, presumably, sta- 

 bilizes the morphology of the chromosome. 

 In any event, when chromosomes are sub- 

 jected to digestion with deoxyribonuclease, 

 the Feulgen-positive fraction disappears, 

 leaving a sort of "ghost." But if chromo- 

 somes are treated with proteolytic enzymes, 

 they tend to break into fragments of Feulgen- 

 positive material. 



BASIC NATURE OF MUTATION 



As a by-product of the decoding studies of 

 Crick and of Benzer, some insight has been 

 gained as to the nature of mutational change. 

 A heavy treatment of bacteria with two kinds 

 of phage DNA (p. 524) often results in a 



