Watson-Crick DNA model

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. 

Marked changes occurred in the infrared spectra when polyuridylic acid and polyadenylic acid were mixed, and the changes were considered to result from hydrogen-bonding interaction between them. These quantitative measurements supported the conclusion that infrared spectra of such mixtures provide experimental evidence that the uracil units are present in the keto form and that the adenine units are probably present in the amino form, as in the Watson-Crick DNA model. Earlier it had been demonstrated (Miles, 1956 1958o) by means of infrared spectra of appropriate model compounds that the band at 1692 cm is characteristic of the keto form of uridine and would be absent in the enol form. It had also been shown that the 1630 cm band is probably indicative of the amino form of adenosine. If interaction of the polymers involved the enol form of uridine, there would be no band at 1695 cm in the mixture. 

FIGURE 28 2 Molecular modeling—1953 style James Watson (left) and Francis Crick (right) with their DNA model A Barrington Brown/Science Source Photo Researchers Inc... 

The structure proposed by Watson and Crick was modeled to fit crystallographic data obtained on a sample of the most common form of DNA called B DNA Other forms include A DNA which is similar to but more compact than B DNA and Z DNA which IS a left handed 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... 

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. 

---