The Watson-Crick Model of DNA

The polynucleotide strands of DNA seldom occur as single strands. Usually, two DNA single strands are found wound together in a double-stranded spiral. That this is so was realized by Watson and Crick in 1953. They proposed a model of the structure of DNA which bears their names, a proposal which was inspired, to some extent, by the x-ray photographs of DNA obtained by Wilkins. 

The important points may be summarized as follows according to the Watson-Crick model, DNA usually consists of two complementary, single strands of DNA of opposite polarity that are coiled around each other and held together by hydrogen bonds betweeaspecific base pairs. This double coil is called the DNA double helix. 

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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).

After two generations, there were equal amounts of two bands of DNA. One was hybrid DNA, and the other was DNA. Meselson and Stahl concluded from these incisive experiments "that the nitrogen in a DNA molecule is divided equally between two physically continuous subunits that following duplication, each daughter molecule receives one of these and that the subunits are conserved through many duplications." Their results agreed perfectly with the Watson-Crick model for DNA replication (Figure 5.16). 

Groove in DNA The structures of DNA have grooves between the lines of phosphodiester backbone. In the Watson-Crick model (B-DNA) the major groove is deep and wide and the minor groove is shallow and narrow. Different functional groups on the bases are accessible in the two different grooves of the same DNA molecule. 

Replication of viral RNA involves the synthesis of a complementary strand, in agreement with the Watson-Crick model of replication of DNA. 

The heart of the Watson-Crick model of the structure of DNA is the postulate that a molecule of DNA consists of two antiparallel polynucleotide strands coiled in a right-handed manner about the same axis to form a double helix. 

Primary and Secondary Structure. The DNA double helix was first identified by Watson and Crick in 1953 (4). Not only was the Watson-Crick model consistent with the known physical and chemical properties of DNA, but it also suggested how genetic information could be organized and rephcated, thus providing a foundation for modem molecular biology. 

In 1953, James Watson and Francis Crick made their classic proposal for the secondary structure of DNA. According to the Watson-Crick model, DNA under physiological conditions consists of two polynucleotide strands, running in opposite directions and coiled around each other in a double helix like the handrails on a spiral staircase. The two strands are complementary rather than identical and are held together by hydrogen bonds between specific pairs of... 

Watson-Crick model for DNA replication. The double helix unwinds at one end. New strand synthesis begins by absorption of mononucleotides to complementary bases on the old strands. These ordered nucleotides are then covalently linked into a polynucleotide chain, a process resulting ultimately in two daughter DNA duplexes. 

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