Double-helix model

One of the most thoroughly investigated examples of polymeric biomolecules in regard to the stabilization of ordered structures by hydration are the DNAs. Only shortly after establishing the double-helix model by Watson and Crick 1953 it became clear, that the hydration shell of DNA plays an important role in stabilizing the native conformation. The data obtained by the authors working in this field up until 1977 are reviewed by Hopfinger155>. 

The secondary structure of DNA is shown in Figure B. This "double helix" model was first proposed in 1953 by James Watson and Francis Crick, who used the x-ray crystallographic data of Rosalind Franklin and Maurice Wilkins. Beyond that, they were intrigued by the results of analyses that showed that in DNA the ratio of adenine to thymine molecules is almost exactly 1 1, as is the ratio of cytosine to guanine ... 

The double helix model provides a simple explanation for cell division and reproduction. In the reproduction process, the two DNA chains unwind from each other. As this happens, a new matching chain of DNA is synthesized on each of the original ones, creating two double helices. Since the base pairs in each new double helix must match in the same way as in the original, the two new double helices must be identical to the original. Exact replication of genetic data is thereby accomplished, however complex that data may be. 

The nucleic acids are among the most complex molecules that you will encounter in your biochemical studies. When the dynamic role that is played by DNA in the life of a cell is realized, the complexity is understandable. It is difficult to comprehend all the structural characteristics that are inherent in the DNA molecules, but most biochemistry students are familiar with the double-helix model of Watson and Crick. The discovery of the double-helical structure of DNA is one of the most significant breakthroughs in our understanding of the chemistry of life. This experiment will introduce you to the basic structural characteristics of the DNA molecule and to the forces that help establish the complementary interactions between the two polynucleotide strands. 

In the case of 10-fold helices, the two chains in a double helix would be related by 2-fold symmetry coincident with the helix axis. Starting from several grid points, the intra-and inter-molecular short contacts within a double helix were minimized. Only the double helix model with 10 chains could be constructed without any fatal contacts. However, we could not pack this model successfully in the unit cell because of the large helical radii for several atoms. The results of these calculations are summarized in Table II. 

Although the chemical nature of single-stranded DNA was well known by 1950, it was Watson and Crick who finally solved the structure of double-stranded DNA in 1953 and proposed a double helix model of DNA based on x-ray diffraction data [2], This concept eventually earned them a Nobel prize in 1962. They proposed that DNA consists of two independent strands, each having alternate pentose sugar (deoxyribose) and phosphate units linked via ester linkage (phosphodiester) as part of their backbone... 

Another prediction of the DNA double helix model is the partition function of the condensed layer [41, 42]... 

O—Na+—O bridges. No double-helix model, as originally proposed for this structure, has been found to be in acceptable agreement with the observed data. The R factor is 29%. 

A left-handed 4(—0.85) helix was proposed for hyaluronate. From attempts to refine the model structures, and calculation of Fourier synthesis, it was shown that two hyaluronate chains pass through the tetragonal unit-cell, with a = 0.99 nm and c = 3.39 nm. The chains are antiparallel, but not coaxial. The double-helix model was excluded. 

Fig. 6.6. Schematic representation of the Watson and Crick double helix model of DNA. The radius of the double helix is 10 A, the vertical rise per base pair is 3.4 A, and one complete turn of the double helix traverses 10 base pairs of 34 A.

It is instmctive that Watson and Crick built their double helix model to fit the very limited information derived from the sparse diffraction pattern of the non-crystalline B-form of DNA, which Rosalind Franklin had obtained... 

With this Planning for the Future example set forth, this chapter will focus on describing the three competency-based outcomes categorized from the data presented in Chapter 2. Each competency, systems, sustainability and ethics, is defined based on recent theories, contextualized based on recent research, and finally synthesized based on assessment rubrics. This is done to address the above Statement of the Problem, Paradigms and pedagogy regarding the need for competency mastery in mechanical engineering education need to be created and/or enriched so that the DNA double helix model of content and competency development can be enacted widely. With that Statement of the Problem in mind, the first competency, systems competency, is presented next. 

The middle of the twentieth century marked the end of a long period of determining the building blocks of chemistry chemical elements, chemical bonds, and bond angles. The lists of these are not definitely closed, but future changes will be more cosmetic than fundamental. This made it possible to go one step further and begin to rationalize the strucmre of molecular systems, as well as to foresee the structural features of the compounds to be synthesized. The crucial concept is based on the Bom-Oppenheimer approximation and on the theory of chemical bonds and resulted in the spatial structure of molecules. The great power of such an approach was first proved by the construction of the DNA double helix model by Watson and Crick. The first DNA model was built from iron spheres, wires, and tubes. 

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