Watson-Crick model of DNA

C13-0102. hi the 1950s, Edwin Chargaff of Columbia University studied the composition of DNA from a variety of plants and animals. He found that the relative amounts of different bases changed from one species to another. However, in every species studied, the molar ratios of guanine to cytosine and of adenine to thymine were found to be very close to 1.0. Explain Chargaff s observations in terms of the Watson-Crick model of DNA structure. 

A -DNA The Watson-Crick model of DNA is based on the x-ray diffraction patterns of B-DNA. Most DNA is B-DNA however, DNA may take on two other conformations, A-DNA and Z-DNA. These conformations are greatly favored by the base sequence or by bound proteins. When B-DNA is slightly dehydrated in the laboratory, it takes on the A conformation. A-DNA is very similar to B-DNA except that the base pairs are not stacked perpendicular to the helix axis rather, they are tilted because the deoxyribose moiety puckers differently. An A-DNA helix is wider and shorter than the B-DNA helix. 

The features of the Watson-Crick model of DNA deduced from the diffraction patterns are ... 

The Chemistry of the DNA Molecule 97 The Watson-Crick Model of DNA 99 DNA Replication 100 Semiconservative Replication Enzymic Mechanism of Replication Problems of the Replication Model... 

In addition to speculations about the form of water in proteins, workers have turned their theoretical considerations to other biological macromolecules. As early as 1953, Jacobsonshowed a remarkable fitting of the ice lattice to the Watson-Crick model of DNA. The same thing can be shown with a polysaccharide such as hyaluronic acid (Figure 1). 

Analogs of the natural deoxynucleoside triphosphates can be used for DNA synthesis, both in vivo and in vitro, but substitution of unnatural for natural deoxynucleotide must conform with the requirements for base-pairing in the Watson-Crick model of DNA. Thus with the purified enzyme system, thymine could be replaced by uracil or 5-bromouracil, 5-methyl- and 5-bromocytosine for cytosine, and hypoxanthine for guanine ( 6). Although chemically synthesized deoxyuridine triphosphate can be incorporated into DNA, there is apparently no kinase in nature which phosphorylates deoxyuridylate to the triphosphate stage. This may account for the absence of uracil nucleotides in DNA ( 6). 

Fig. 4. The Watson-Crick model of DNA. P = phosphate, D = deoxyribose. For abbreviations of the bases, see Table 1 (modified from Bennett 1970).

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