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The Brilliance of Watson, Crick and Chargaff
« on: January 18, 2008, 09:02:15 AM »
All the facts about DNA and RNA  were known by the end of the 1940s. By that time it was also becoming clear that DNA was a genetic material and that it therefore stood at the very center of the study of life. Yet the three dimensional structure of DNA was unknown. For these reasons, several researchers dedicated themselves to finding this structure.

One of the scientists interested in DNA structure was Dr. Linus Pauling, a theoretical chemist at the California Institute of Technology. He was already famous for his studies on chemical bonding and for his elucidation of the alpha helix, held together by hydrogen bonds, laid the intellectual groundwork for the double-helix model of DNA proposed by Watson and Crick. Another group trying to find the structure of DNA included Maurice Wilkins, Rosalind Franklin, and their colleagues at King's College in London. They were using x-ray diffraction to analyze the three-dimensional structure of DNA. Finally, James Watson and Francis Crick entered  the race. Watson, still in his early twenties, but already holding a Ph.D degree from Indiana University, had come tot he Cavendish Laboratories in Cambridge, England, to learn about DNA. There he met Crick, a physicist who at age 35 was retraining as a molecular biologist. Watson and Crick performed no experiments themselves, their tactic was to use other groups' data to build a DNA model.

Erwin Chargaff was another very important contributor. We have already seen how his 1950 paper helped identify  DNA as the genetic material, but the paper contained another piece of information that was even more significant. Chargaff's studies of the base composition of DNAs from various sources revealed that the content of purines was always roughly equal to the content of pyrimidines. Furthermore, the amounts of adenine and thymine were always roughly equal, as were the amounts of guanine and cytosine. These findings, known as Chargaff's rules, provided a valuable foundation for Watson and Crick's model.

Perhaps the most crucial piece of the puzzle came from an X-RAY diffraction picture of DNA taken by Franklin in 1952- a picture that Wilkins shared with James Watson in London on January 30, 1953. The x-ray technique worked as follows:

The experimenter made a very concentrated, viscous solution of DNA, then reached in with a needle and pulled out a fiber. This was not a single molecule, but a whole batch of DNA molecules, forced into side-by-side alignment by the pulling action. Given the right relative humidity in the surrounding air, this fiber was enough like a crystal that it diffracted x-rays in an interpretable way. In fact, the x-ray diffraction pattern in Franklin's picture was so simple--a series of spots arranged in an X shape--that it indicated that the DNA structure itself must be very simple. By contrast, a complex, irregular molecule like a protein gives a complex x-ray diffraction pattern with many spots, rather like a surface peppered by a shotgun blast. Because DNA is very large, it can be simple only if it has a regular, repeating structure. And the simplest repeating shape that a long, thin molecule can assume is a corkscrew, or helix

Franklin's x-ray work strongly suggested that DNA was a helix. Not only that, it game some important information about the size and shape of the helix. In particular, the spacing between adjacent bands in an arm of the X is inversely  related to the overall repeat distance in the helix, 33.2 angstroms, and the spacing from the top of the X to the bottom is inversely related to the spacing between the repeated elements (base pairs) in the helix. However, even tough the Franklin picture told much about DNA, it presented a paradox:
DNA was a helix with a regular, repeating structure, but for DNA to serve its genetic function, it must have an irregular sequence of bases.

Watson and Crick saw a way to resolve this contradiction and satisfy Chargaff's rules at the same time:



DNA must be a double helix with its sugar-phosphate backbones on the outside and its bases on the inside. Moreover, the bases must be paired, with a purine in one strand always across from a pyrimidine in the other. This way the helix would be uniform; it would not have bulges where two large purines were paired or constrictions were two small pyrimidines were paired. Watson has joked about the reason he seized on a double helix, "I had decided to build two-chain models. Francis would have to agree. Even though he was a physicist, he knew that important biological objects come in pairs."



dna_image - The Brilliance of Watson, Crick and Chargaff - Science and Research
Watson and Crick and their brainchild. The Double Helix DNA strand.
The pioneers of genetic molecular biology. One of the greatest discoveries of man, bar none.

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Lorenzo

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Re: The Brilliance of Watson, Crick and Chargaff
« Reply #1 on: February 07, 2009, 08:42:26 PM »
And so came to life the understanding of the DNA Super Helix concept!

The Divinity of DNA!


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Re: The Brilliance of Watson, Crick and Chargaff
« Reply #2 on: February 12, 2009, 03:56:50 PM »
The human genome project, identifies about 30,000~ genes in the human body that code everything. From the color of your hair, to your facial formation, to your laryngeal apparatus, to the size of your genitals, to your length of stature, to your build, to your psychological, to diseases etc.

The plan, now, is to find a way to code, to inhibit, to express certain genes.

In theory, scientists, claim that further unraveling of these codes can, unlock cellular immortality in genetic transcription, elongation, replication, in unwinding, in segregation and in partition and even excision and nucleotidic repair.

In layman's terms, "To Live Forever"

Interesting, isn't it?



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