Helix structure of DNA

The double helix structure of DNA has the hydrophobic bases pointing to the centre of the helix in an almost planar arrangement. These base-pairs are closely stacked perpendicular to the long axis of the chain, and are attracted to each other by Van der Waals forces. The hydrophilic phosphates are negatively charged at the pH of the cell and point to the outside. 

Fio. 5.A8. The double-helix structure of DNA. D, deoxyribose P, phosphate ester bridge A, adenine C, cytosine G, guanine and T, thymine... 

Watson and Crick and Wilkins determined the double-helix structure of DNA. 

Fig. 1 The double helix structure of DNA in its B -form, with its relevant dimensions and the main interactions...

The molecule of DNA is like a coded message. This message, the genetic information contained in and transmitted by nucleic acids, depends on the sequence of bases from which they are composed. It is somewhat like the message sent by telegraph, which consists only of dots, dashes, and spaces in between. The key aspect of DNA structure that enables storage and replication of this information is the famed double helix structure of DNA mentioned above. 

The structure of the DNA molecule is basically determined by nucleic acid base interactions. Although the three-dimensional double helix structure of DNA is influenced by various contributions, the hydrogen bonding in DNA base pairs is of particuar importance. Because it is difficult to obtain gas phase experimental data for isolated base pair characterisation (only a limited number of experimental studies are available [21]) quantum chemical calculations can represent a useful tool to obtain reference data on the structure, properties and interactions of nucleic acid pairs. Theoretical studies can help us to understand the properties of nucleic acids and they are fundamental for verification... 

Figure 1.5 Diagrammatic representation of (a) a nucleic acid and (b) double helix structure of DNA. Illustrations, Irving Geiss. Rights owned by Howard Hughes Medical Institute. Reproduction by permission only.

But the distance between the laboratory and the marketplace is often long. Space engineers didn t invent rockets one day and send men to the moon the next. Some twenty years elapsed between the discovery of the double-helix structure of DNA and the first transplantation of a gene from one organism to another. The atomic bomb came forty years after Albert Einstein gave us the theory behind it and his formula that linked energy and matter E = me2. As Donna Fitzpatrick of the Department of Energy recalls ... 

Largely because of advances in biotechnology, the initial working draft of the human genome sequence was completed in 2000, well ahead of schedule. In April of 2003,50 years after James Watson and Francis Crick described the double-helix structure of DNA and over 2 years ahead of schedule, researchers announced the completion of the Human Genome Project. The final version contains 99% of the gene-containing sequence, with 99.9% accuracy. 

The folded, three-dimensional structure, or conformation, of a protein is vital to its function. To understand the relation between the function of a protein and its form, we need to know both what it does and its detailed structure. The most widely used method for determining the complex structures of proteins, DNA, and RNA is x-ray crystallography. Computer-assisted analysis of the data often permits the location of every atom in a large, complex molecule to be determined. The double-helix structure of DNA, which is key to its role in heredity, was first proposed based on x-ray crystallographic studies. Throughout this book you will encounter numerous examples of protein structures as we zero in on how proteins work. 

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