224 



ERNST FREESE 



other iiuilaiits willi tlii' .^aiuc hiocliciiiical rccniiivnu'iit sliowed a con- 

 tinued increatsc of revertants throughout several generations, indicating 

 that the reverse mutation was induced by a mistake in replication. P'or 

 a given mutant the time at which the first induced mutants showed up 

 was difterent for AP tiian for BI), in the case of Sabnonella, while it was 

 the same in the case of A', coli; this may be caused by a difference in the 

 methods used. But the o\er-all pattern of either cessation or continued 

 increase of mutation induction was, for a given mutant, the same with 

 both mutagens, in agreement with the expectations. The precise timing, 

 however, of the appearance of revertants could also depend on the func- 

 tional activity of the DNA strand in wliicli the first pairing mistake 

 occurs. 



C. DYES 



Man}' different basic dyes have been used for the cytological staining 

 of nucleic acids. Some of them, whose structures are shown in Fig. 8, 

 have also been tested for their effect on living oi'ganisnis; proflavin and 



H,N 



Proflavin 



NH, 



HoCv 



N 

 I 

 H,C 



Acridine orange 



N 



I 



CH 



CH, 



HoCv 



I 



N' 

 I 

 CH, 



^CH.j 



HoCn 



N 

 I 

 H,C 



CH, 



NH, 



Methylene blue Toluidine blue 



Fig. 8. Structure of dyes often rmi)loyed in biological experiments. 



acridine orange are two examples of "acridine dyes." Undoubtedly many 

 other compounds with structures similar to those in Fig. 1 have biological 

 effects. For exam])le, the similarity to some aromatic carcinogens is 

 noteworthy. 



A direct attachment of these dyes to nucleic acids is the probable 

 cause of mutations. However, there are some other biological effects 

 wliich will be mentioned briefly. The i)resence of acriflavin in the growth 

 mediinn of various organisms eliminates ncjn-chromosomal elements, e.g., 



