BIOLOGICAL ORDER 



Finally, the ratio (adenine + thymine)/(guanine + cytosine) 

 varies with the bacterial species from 0.6 to 2.8. How this is re- 

 flected in the constitution of enzymes is not yet known. 



Anyhow, for the time being, the coding problem is considered 

 as entirely open.* 



Mutation 



The nucleic acid. Thus: (a) the genetic information is contained 

 in the nucleic acid; (b) the code is a sequence of nucleic bases. 

 Moreover, it has been learned that the nucleic acid is the substrate 

 of mutations, that is, of hereditary variation. The smallest unit of 

 gene variation can be measured by recombination experiments. When 

 two difl^erent chromosomes multiply in the same cytoplasm, they 

 mate, and recombinants are produced. It happens that the prob- 

 ability of recombinations is related to the distance between the two 

 factors. The longer the distance, the greater the probability of 

 recombination. Two closely linked structures have a high prob- 

 ability of remaining joined. It was found by Seymour Benzer, work- 

 ing with bacteriophage, that the unit of variation was of the order 

 of three to five nucleotides. 



By the use of mutagenic agents, even more refined data were ob- 

 tained. The spontaneous mutation rate in a bacterium or in a bac- 



* Since the Compton Lectures were delivered (March, 1960), our knowl- 

 edge concerning the synthesis of proteins in general and the code in par- 

 ticular has increased considerably. 



a. It has been learned that the structural gene, the specific DNA template 

 carrying the genetic information for the synthesis of a given protein, pro- 

 duces a specific RNA "messenger." This messenger, detached from the chro- 

 mosome, becomes attached onto a nonspecific cytoplasmic ribosome. It is the 

 system ribosome + messenger which assembles the amino acids into a given 

 sequence: diataxy (etymologically, to put in order), thus producing a specific 

 protein. See Jacob and Alonod: "Genetic regulatory mechanisms in the syn- 

 thesis of proteins," /. MoL Biol. (1961), 3, 318-356; and Jacob and Wollman: 

 Sexuality and the Genetics of Bacteria, Academic Press, New York, 1961. 



b. The code is now being deciphered: a synthetic poly-uridylic acid con- 

 tains the information for the synthesis of a protein having the characteristics 

 of poly-L-phenylalanine. A synthetic poly-cytydilic acid takes care of L-proline 

 [M. W. Nirenberg and J. H. Matthaei, Proc.'Nat. Acad. Sci. (1961), 47, 1588- 

 1602]. Whether a sequence of three or four nucleotides is involved is not 

 yet known. It is clear, however, that the whole code will soon be deciphered. 



[26] 



