Chapter 20 



ORGANIZATION AND REPLICATION 

 OF DNA IN VIVO 



T: 



|hat DNA serves as the chem- 

 ical basis of chromosomal ge- 

 netic material was supported 

 by the indirect evidence presented in the 

 last chapter. The primary structure of DNA 

 was described as a single, long, unbranched, 

 polarized chain of nucleotides. If the DNA 

 polymer were genetic material, one would 

 expect it to be linearly differentiated so that 

 successive portions could represent different 

 genes. This differentiation cannot be based 

 upon either the deoxyribose sugar or the 

 phosphate, since one of each is present in 

 every nucleotide. Therefore, all differences 

 in genetic information along the length of 

 the DNA strand would have to be due to 

 the organic bases present. Since species 

 differ genetically, one might expect them to 

 differ in DNA quantity and/or base content. 

 Figure 20-1 gives the per genome DNA 

 content of various types of organisms. It is 

 generally true that the higher an organism 

 is on the evolutionary scale, the larger is its 

 genomic DNA content. Perhaps it would 

 be more meaningful to say that the DNA 

 content per genome increases as the number 

 of functions controlled by genes increases. 

 Histochemical analyses reveal the organic 

 base content in DNA extracted from various 

 species. Considering the total amount of 

 the bases in an extract as 100%, we see in 

 Figure 20-2 the portions found as adenine 

 (A), thymine (T), guanine (G), and cyto- 

 sine (C). There is considerable variation 

 in the relative frequency of bases, ranging 

 265 



from organisms relatively rich in A and T 

 and poor in C and G (sea urchin) to those 

 in which A and T are much less abundant 

 than C and G (tubercle bacillus). The 

 DNA samples taken from radically different 

 species contained relatively different amounts 

 of the four bases. 



Do these data suggest that a shift in the 

 sequence of bases can produce genetic dif- 

 ferences? The assumption that different 

 orders of the same bases might be involved 

 in specifying different genetic units is con- 

 sistent with the fact that the chicken, salmon, 

 and locust — certainly all very different ge- 

 netically — have very similar base ratios. An 

 alternative explanation would be that these 

 species are molecular polyploids which differ 

 only in the multiples of a basic set of DNA 

 molecules they contain. This possibility can 

 be eliminated from serious consideration in 

 light of our knowledge that chromosomal 

 polyploidy has made a limited contribution 

 to evolution, at least in the animal kingdom 

 (Chapters 11, 18). 



As long as relatively crude histochemical 

 analyses are made of the total amount of 

 DNA in cells with a large DNA content, 

 we should expect to find roughly the same 

 base ratios among the different members of 

 a single species. This expectation has 

 proved true. Moreover, the same base ratios 

 are found in different normal and neoplastic 



figure 20-1. DNA nucleotide pairs per ge- 

 nome in various organisms. 



