Chapter *34 



ORGANIZATION, REPLICATION, 

 AND TYPES OF DNA IN VIVO 



I 



"ndirect evidence, consistent with 

 the view that DNA can serve as 

 -the chemical basis of chromo- 

 somal genetic material, was presented in the 

 last Chapter. That Chapter described what 

 may be called the primary structure oj DNA, 

 as being a single, long, unbranched, polarized 

 chain of nucleotides. In accord with the view 

 that the DNA polymer is genetic, we would 

 expect it to be linearly differentiated so that 

 different portions of it could represent dif- 

 ferent units of genetic material. This differ- 

 entiation cannot be attributed to either the 

 deoxyribose sugar or the phosphate, since 

 one of each is present in every nucleotide. 

 Differences in genetic information along the 

 length of the DNA strand must be due, there- 

 fore, to the bases present. Since species differ 

 by numerous point mutations, we would ex- 

 pect to find differences in the DNA's of dif- 

 ferent species. 



Histochemical analyses have been made of 

 the organic bases in DNA extracted from 

 various species. Considering the total amount 

 of the bases in an extract as 100%, Figure 

 34-1 gives the percentages of this found as 

 adenine (A), thymine (T), guanine (G), and 

 cytosine (C). Note that there is considerable 

 variation in base content, ranging from organ- 

 isms Relatively rich in A and T and poor in 

 C and G (sea urchin), to those showing the 

 reverse, where A and T are considerably less 

 abundant than C and G (tubercle bacillus). 

 These results demonstrate that the relative 

 amounts of the four bases are different in 

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the DNA's from radically different species. 



Can these data tell us whether a shift in the 

 sequence of bases also could produce differ- 

 ences in the properties of the genetic material? 

 That different orders of the same bases may 

 also be involved, in specifying different genetic 

 units, is suggested by the fact that the chicken, 

 salmon, and locust, which must be very dif- 

 ferent genetically, all have very similar base 

 ratios. One might suggest as an alternative 

 explanation that these species are molecular 

 polyploids, differing only in the number of 

 the same types of DNA molecules which they 

 possess. This possibility can be eliminated 

 from serious consideration in the light of our 

 knowledge of the limited contribution which 

 chromosomal polyploidy has made to evolu- 

 tion, at least in the animal kingdom (Chapters 

 18,29). 



So long as histochemical analyses are made, 

 of the total DNA of cells having a high DNA 

 content, approximately the same base ratios 

 would be expected to be obtained from dif- 

 ferent members of a single species. This has 

 proven true. Moreover, as expected, the 

 same base ratios have been found in different 

 normal and neoplastic tissues of the same and 

 different human beings. Nevertheless, it is 

 expected that a genome would contain many 

 molecules of DNA which differ from each 

 other in base sequence and content. 



The variation, found in different species, 

 in the ratio A + T/G + C (which is about .4 

 for the tubercle bacillus and is about 1.8 in 

 the sea urchin), is understandable in terms of 

 our chemical knowledge, since the DNA 

 strand imposes no limitation upon which base 

 may be present, or how often it may appear 

 along the length of the fiber. There is, how- 

 ever, a remarkable equality in the amount of 

 A and the amount of T in the DNA within a 

 species, and also an equivalence in the 

 amounts of G and C (refer to Figure 34-1). 

 Since, in each species, A = T, and G = C, 

 it is also apparent that A + G = T + C, or, 

 in other words, the total number of DNA 



