Organization, Replication, and Types of DNA in Vivo 



311 



FIGURE 34-5. The opposite direction oft/ie sugar-phospluite lin/<ages 

 in the tyro cliains of a DNA double helix. 



single-stranded at certain places or at certain 

 times. Such data merely prove that a very 

 appreciable part of the chromosomal DNA, 

 in the wide variety of organisms studied, is 

 not single-stranded. The content and organi- 

 zation of DNA in viruses attacking bacteria 

 have also been studied by chemical analysis 

 and by X-ray diffraction. In the varieties T2 

 and T7, for example, the data are entirely 



consistent with the DNA's being present in 

 the Watson-Crick double helix configuration. 

 In two other smaller varieties, however, 

 called 0X174 and SI 3, the DNA is definitely 

 single-stranded. 



Whenever the DNA is in the double helix 

 configuration we can consider one strand to 

 be the complement of the other, so that if we 

 know the sequence of bases in one strand we 



