Helical structure of DNA

The [Co(phen)3]3+ complex is photoactive and a powerful oxidant in its excited state. The ion has no H-bonding groups and hence is considerably more hydrophobic1279 than hexaamine relatives. These properties have proven particularly useful. Aryl and alkyl substituted [Co(phen)3]3+ complexes have received a great deal of attention due to their ability to intercalate within the helical structure of DNA through a combination of electrostatic and hydrophobic forces. The chirality of the tris-chelate complex is crucial in determining the degree of association between the complex and... 

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... 

Figure 13.5 The helical structure of DNA. Bases on the opposite strands pair. The pairing is always between thymine and adenine and cytosine and guanine.

We encountered the properties of hydrophilic and hydrophobic molecules in our thoughts about driving forces for formation of three-dimensional protein structures. Specifically, proteins fold in a way that puts most of the hydrophobic amino acid side chains into the molecular interior, where they can enjoy each other s company and avoid the dreaded aqueous environment. At the same time, they fold to get the hydrophilic amino acid side chains onto the molecular surface, where they happily interact with that enviromnent. The same ideas are important for the double-stranded helical structure of DNA. The hydrophobic bases are localized within the double hehx, where they interact with each other, and the strongly hydrophilic sugar and phosphate groups are exposed on the exterior of the double helix to the water environment. Now, we need to understand something more about structural features that control these properties. 

DNA is a structurally polymorphic macromolecule which, depending on nucleotide sequence and environmental conditions, can adopt a variety of conformations. The double helical structure of DNA (dsDNA) consists of two strands, each of them on the outside of the double helix and formed by alternating phosphate and pentose groups in which phosphodiester bridges provide the covalent continuity. The two chains of the double helix are held... 

The cis-syn thymine dimer from DNA is believed to be formed from intra-strand dimerization of adjacent thymine residues. Photoaddition of two unsaturated molecules in the solid state can arise only if they are initially located in proximity in the crystal lattice [545, 546]. The formation of interstrand dimers would require gross distortion of the helical structure of DNA in order for the bases to approach the limiting distance (c. 4 A.) Hence, such dimers would be formed in only very small amount. However, the composition of the photoproducts may differ under varying experimental conditions [547-550]. 

Electron transfer reactions are among the most widespread and significant in all of chemistry. Electron transfer (ET) within the double helical structure of DNA exhibits an extremely broad range of mechanistic behavior, and its exploration has become a focal point within the chemical community since the key studies of Barton and collaborators [1-3]. 

Two major discoveries in 1953 were of crucial importance in the history of biochemistry. In that year James D. Watson and Francis Crick deduced the double-helical structure of DNA and proposed a structural basis for its precise replication (Chapter 8). Their proposal illuminated the molecular reality behind the idea of a gene. In that same year, Frederick Sanger worked out the sequence of amino acid residues in the polypeptide chains of the hormone insulin (Fig. 3-24), surprising many researchers who had long thought that elucidation of the amino acid sequence of a polypeptide would be a hopelessly difficult task. It quickly became evident that the nucleotide sequence in DNA and the amino acid sequence in proteins were somehow related. Barely a decade after these discoveries, the role of the nucleotide... 

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 5-3 The double-helical structure of DNA. The structure shown is that of the B form and is based on coordinates of Amott and Hukins.10 The major and minor grooves, discussed on p. 213, are marked.

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. 

The double-helical structure of DNA and the ability of the molecule to unravel in order to template the formation of copies of itself and transcribe RNA is clearly connected intimately with the supramolecular interactions that bind the two nucleotide strands together. It is the information encoded within the individual nucleobases that tells the molecule to form a double helix. This is an... 

The self-assembling double-helical structure of DNA has provided the inspiration for a further area of supramolecular self-assembly, namely the use of metal ions to template the assembly of organic threads into... 

In an early work on the compositions of DNA, Chargaff (1955) noted that the ratios of adenine to thymine and of guanine to cytosine are very close to unity in a very large number of DNA samples. This observation has been used to support the helical structure of DNA proposed by Watson and Crick (1953). From the base compositions of DNA and RNA given in the following tables, deduce the statistical significance for the statement that the base ratios (A/T(U) and G/C are unity for DNA but vary for RNA (Note Calculate the ratios first and then perform statistical analysis on the ratios). 

A counterion condensation model has been generalized to include the helical structure of DNA. A single helix is defined by the position vector r = (acos), where the helical rise and radius are h and a, respectively, and < ) is the rotational angle. The length between a pair of charges along the helix is given by [41, 42]... 

While the complementary double helical structure explained how particular sequences of bases could be used to store a genetic instruction it was not immediately clear how replication occurred or, indeed, how these instructions were used. Later work by Gamow linked DNA base pair sequences to protein synthesis [15] but it was not until 1961, when Nirenberg and Matthaei demonstrated that cell-free protein synthesis relied upon synthetic or natural polynucleotides [16], that the final link was made. The information held within the linear DNA sequence is replicated every time a cell divides. Replication is possible because of the unique double helical structure of DNA as shown in Fig. 2.7. 

By any measure, Sir Francis H. C. Crick is a smart man. Over forty years ago, while still a graduate student at Cambridge University, Crick and James Watson used X-ray crystallographic data to deduce the double helical structure of DNA, an accomplishment for which they later received the Nobel prize. Crick went on to contribute to the elucidation of the genetic code and to pose provocative conceptual questions on the function of the brain. Well into his seventies, he is pushing science further and faster than most of us will at the pinnacle of our powers. 

The structure of the dodecamer retained the normal B-DNA form with very little distortion, indicating that, at these low levels of Pt-binding, the basic double-helical structure of DNA is not greatly disrupted. The only detectable distortion was a movement of the platinated guanine rings slightly outward, by about 1 A, towards the Pt-site into the major groove. The three... 

Nucleic acid structures also involve assembly events determined by differential solubilities of different molecular constituents. The stacking of purine and pyrimidine bases in the helical structures of DNA relies on hydrophobic effects, whereas the positioning of phosphate groups in contact with solvent reflects their hydrophilic nature. Secondary structures of RNA likewise are influenced by differential solubilities of polar and nonpolar constituents. 

Why is the double-helical structure of DNA more stable in solution at higher rather than lower ionic strength ... 

The field of molecular biology is usually held to have begun with the description of the double helical structure of DNA by Watson and Crick in 1953, followed by the... 

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