WHAT IS A GENE? 



169 



large polypeptide chains. That part of the cistron that specifies the loca- 

 tion of a single amino acid is in a sense a functional unit and points up 

 again the series of hierarchies of organization which exist within the 

 aperiodic DNA crystal. If the function we measure is the result of some 

 enzyme action, the functional unit which underlies it must have an ex- 

 tent and complexity sufficient to determine the order of amino acids in a 

 polypeptide large enough to show that enzyme activity. The best esti- 

 mate from cis-trans tests of the rll mutants in phages indicates that the 

 cistron has dimensions of the order of 10'^ nucleotide pairs. 



The base sequence within a cistron has not yet been analyzed, but 

 an internal differentiation has been revealed indirectly. Multiplying T4 

 phages have been treated with mutagens such as 5-bromouracil, 2- 

 aminopurine, nitrous acid, heat and low pH. The sites of the muta- 

 tions produced at the rll locus show a spectrum peculiar to each 

 mutagen and different from the distribution of sites that mutate spon- 

 taneouslv (Figure 6.7). This arrangement of so-called hot spots indicates 

 a nonhomogeneity along the length of the cistron. It is known that 5- 

 bromouracil specifically replaces thymine in DNA. On the other hand, 

 2-aminopurine seems to replace both adenine and guanine. The pat- 

 tern of mutations induced by these substances is a reflection of the 

 chemical heterogeneity along the two DNA polynucleotides that consti- 

 tute the cistron. This method, in principle, promises a way to a base 

 analysis of genetic material. 



Another example of heterogeneity is found in the rates at which 



FIGURE 6.7. Genetic maps showing the locations of spontaneous, 5-bromouracil- 

 ond 2-aminopurine-induced mutational alterations in cistron 6 of the rll region of 

 phage 74. The height of each column indicates the number of mutants. The occur- 

 rence of one mutant in the induced sets reflects a much higher mutability than in the 

 spontaneous set. As a consequence, the background of spontaneous mutations and 

 mutations occurring with low frequency do not show up on the maps of induced 

 mutants (from Freese, 1959, Proc. Nafl. Acad. Sci., 45:622). 



