The Watson-Crick DNA Structure

The sugar-phosphate backbones of the double helix follow helical paths at the outer edge of the molecule. 

The two strands in the double helix are complementary because of the base pairing rules that dictate A = T and G = C. Because of the base 

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DNA is a remarkably flexible molecule. Considerable rotation is possible around a number of bonds in the sugar-phosphate (phosphodeoxyribose) backbone, and thermal fluctuation can produce bending, stretching, and impairing (melting) of the strands. Many significant deviations from the Watson-Crick DNA structure are found in cellular DNA, some or all of which may play important roles in DNA metabolism. These structural variations generally do not affect the key properties of DNA defined by Watson and Crick strand complementarity,... 

Implicit in the functioning of the Watson-Crick DNA model is the idea that the strands of a DNA molecule must separate and new daughter strands must be synthesized in response to the sequence of bases in the mother strand. This is called semiconservative replication. Still, conservative replication, in which both strands of a daughter molecule are newly synthesized, could not be ruled out by consideration of the structure of DNA alone. 

One of the most intriguing questions is whether the Watson-Crick (WC) structures that dominate in DNA are intrinsically the most stable structures, even in the absence of the backbone and the solvent. In other words is the biological context required for these structures to be preferred It is noteworthy that theory predicts the WC structure for AT in the gas phase not to be the lowest in energy [41],... 

List the important features of the Watson-Crick DNA double helix. Relate the base pairing of adenine "with thymine and of cytosine with guanine to the duplex structure of DNA and to the replication of the helix. Explain the molecular determinants of the specific base pairs in DNA. 

There may be no end but there was a beginning. It happened around midcentury when, within the scope of a few years, three momentous discoveries were made. The first was the construction of the a-helix and jS-sheet structures as models for stable secondary structures in proteins the second was the Watson-Crick model structure for DNA and the third was Perutz s discovery that heavy atoms, such as mercury, could be introduced into protein crystals without destroying the crystalline structure, thus making it possible to obtain information about the missing phases. At this point I indulge in some personal reminiscences, which some of my readers may choose to skip, having heard or read of them already. 

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. 

The breakthrough came in 1953 when James D. Watson and Francis H. C. Crick proposed a structure for DNA. The Watson-Crick proposal ranks as one of the most important in all of science and has spurred a revolution in our understanding of genetics. The structure of DNA is detailed in the next section. The boxed essay It Has Not Escaped Our Notice. .. describes how it cane about. 

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... 

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. 

The well-known double-helical structure of DNA (deoxyribonucleic acid) is derived from the specificity of the Watson-Crick base pairing.154 Yanagawa and co-workers first addressed the issue of whether mononucleotide units could be... 

The most famous stracture in aU chemistry is the Watson-Crick double helix for DNA (figure 12.3). The discovery of this structure by James Watson and Francis Crick in 1953 was the beginning of molecular biology. An amazing number of insights about the nature of life have been derived from this structure. 

Here is a summary of some of the details of the Watson-Crick structure, called B-DNA ... 

The structure of DNA is a double helix, the Watson-Crick structure. 

The Watson-Crick structure is also referred to as B-form DNA, or B-DNA The B form is the most stable structure for a random-sequence DNA molecule under physiological conditions and is therefore the standard point of reference in any study of the properties of DNA Two structural variants that have been well characterized in crystal structures are the A and Z forms. These three DNA conformations are shown in Figure 8—19, with a summary of their properties. The A form is favored in many solutions that are relatively devoid of water. The DNA is still arranged in a right-handed double helix, but the helix is wider and the number of base pairs per helical turn is 11, rather than 10.5 as in B-DNA The... 

DNA can exist in several structural forms. Two variations of the Watson-Crick form, or B-DNA, are A- and Z-DNA. Some sequence-dependent structural variations cause bends in the DNA molecule. DNA strands with appropriate sequences can form hairpin/cruciform structures or triplex or tetraplex DNA... 

Today s understanding of information pathways has arisen from the convergence of genetics, physics, and chemistry in modern biochemistry. This was epitomized by the discovery of the double-helical structure of DNA, postulated by James Watson and Francis Crick in 1953 (see Fig. 8-15). Genetic theory contributed the concept of coding by genes. Physics permitted the determination of molecular structure by x-ray diffraction analysis. Chemistry revealed the composition of DNA. The profound impact of the Watson-Crick hypothesis arose from its ability to account for a wide range of observations derived from studies in these diverse disciplines. 

Figure 5-2 A distorted (flattened) view of the Watson-Crick structure of DNA showing the hydrogen-bonded base pairs.

In the Watson-Crick structure the two strands are antiparallel, an essential for replication. However, stable segments of double-stranded DNA with parallel strands can also be formed and may occur in specialized regions of the genome.125-128... 

Figure 5-34 (A) Two conformations of a segment of the yeast phenylalanine tRNA gene. The segment shown codes for the 3 end of the tRNA molecule shown in Fig. 5-30, including the T /C loop. (B) Formation of H-DNA (Fig. 5-24) proposed for a sequence in plasmid pGG32. The major element of the structure is the triplex, which is formed from the Watson-Crick duplex ( ) associated with the homopyrimidine loop through Hoogsteen base pairing (o, +). One of the two possible "isomeric" forms is shown. See Mirkin et al.378...

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