GENETICS EST MODERN BIOLOGY — BEADLE 407 



chainlike through peptide linkages. The resulting protein then peels 

 off the template and the process is repeated. For hemoglobin, for 

 example, there are assumed to be two DNA segments, one for each of 

 the two protein chains, and two UNA templates. 



Protein synthesis can be carried out in a cell-free system in %vhich 

 are present ribosomes, template KNA, carrier RNA, amino acids, the 

 requisite enzymes, and other necessary components. By means of 

 radioactive labels it can be shown that proteins are synthesized step- 

 wise beginning at the free amino end and ending up with a free 

 carboxyl group in the last amino acid to be added to the chain. Thus 

 there is a good experimental evidence suggesting that the translation 

 process does occur as postulated. 



A large number of proteins serve as enzymes or essential com- 

 ponents of enzymes. Enzymes catalyze chemical reactions that would 

 otherwise occur at rates so low that life processes would essentially 

 cease. For each enzyme protein there is supposedly a segment of DNA 

 information in the nucleus — a gene — and corresponding microsomal 

 RNA templates in the cytoplasms of those cells active in synthesis 

 of that particular enzyme protein. An important question of present- 

 day biology is concerned with the nature of the mechanism by which 

 the 4-symbol code of DNA is related to the 20-symbol code of pro- 

 teins. It is obvious that single symbols of DNA cannot stand for 

 amino acid, for there are only 4. Likewise pairs of DNA symbols 

 will not do, for there are only 16 such pairs if the DNA molecule is 

 read in one direction. If one reads in one direction and uses 3 

 symbols per amino acid, there are 64 possibilities. 



Recent genetic evidence strongly suggests that the code is indeed a 

 triplet code read in one direction from a starting point on the tem- 

 plate RNA. Each successive triplet normally encodes an amino acid. 

 Thus if we assume a segment of a template beginning: UCG UUU 



UGA UUA UGU UUG , where U (uracil nucleotide) replaces 



A in the DNA code, the amino acids arginine, phenylalanine, glutamic, 

 tyrosine, valine, and cysteine might be mcorporated by the successive 

 underline triplets. The order of nucleotides is determined by making 

 use of synthetic template RNAs. Thus such an RNA template con- 

 sisting of . . . UUUUUUUIJA incorporates only phenylalanine 

 and tyrosine. Tyrosine appears to be at the carboxyl end of the 

 polypeptide so synthesized (preliminary reports from S. Ochoa's 

 laboratory). Thus the triplet encoding tyrosine must be read UUA. 

 Now from mutational replacement of amino acids in proteins such as 

 occurs in hemoglobin on mutation from normal hemoglobin to sickle 

 cell hemoglobin in which one glutamic acid unit in the chain is changed 

 to glutamic acid, one can deduce the nucleotide sequence in the code 



672-174 — 63 30 



