352 BACTERIOPHAGES 



by the Visconti-Delbriick theory and in part because of "high" 

 negative interference of a second kind that characterizes loci 

 close enough together to fall within the same heterozygous region. 

 Different ;II jnutations are often distinguishable by other proper- 

 ties such as the rate of spontaneous mutation to wild type, or the 

 "transmission coefficient" defined as the fraction of infected 

 Kl 2 cells that produce T4r progeny. Many of the closely linked 

 mutants that give less than 0.001 per cent recombination are 

 alike in transmission coefficient and rate of reverse mutation 

 and therefore are presumed to be recurrences of identical muta- 

 tions. 



segment A 



47 104 101 103 105 106 



ll W I l li i I — Mi l m l III 1 1 I m ill I 



segment B 



51 102 



I II H I I I Ii l l llll I 



Figure 12. Linkage map of the rll mutants of phage T4. Segments A and 

 B are different cistrons. Reproduced from S. Benzer, 1955, Proc. Natl. Acad. 

 Sci. U. S., 41, 344, with permission. 



a. The Gene as a Functional Unit 



It is generally assumed that the function of the gene is to 

 determine the specific chemical structure of a protein molecule 

 such as an enzyme. If it is to transmit enough information for 

 this purpose the gene must be a complex structure. As proteins 

 contain some 20 kinds of amino acid, and nucleic acids usually 

 contain only 4 kinds of nucleotide, it seems probable that 3 or 4 

 nucleotides are necessary to specify one amino acid. Therefore it 

 would take a DNA strand some 1,000 nucleotides in length to 

 determine the structure of a protein of 30,000 molecular weight. 

 The functional unit of the gene may then involve a DNA segment 

 of the order of 1,000 nucleotides in length. 



A mutation may involve a chemical change any place in this 

 functional unit, providing only that the chemical change results 

 ultimately in a modified phenotype. The chemical change in 



