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The structure of DNA
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Over view of DNA structure and function
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Over view of DNA structure and function
DNA is the molecule that is the basis for heredity.
It contains the patterns for constructing proteins in the body,including the various enzymes.
A new understanding of heredity and hereditary disease was possibleonce it was determined that DNA consists of two chains twistedaround each other.
The two chains are composed, of alternating phosphate and sugargroups, which are held together in a double helix structure byhydrogen bonds between pairs of organic bases.
The bases areAdenine (A)Thymine (T)
Guanine (G)Cytosine (C).
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The DNA story
In 1962 James Watson (1928present) Francis Crick (19162004), and Maurice Wilkins (19162004) jointly received the NobelPrize in medicine or physiology (in1962) for their determination in1953 of the structure of
deoxyribonucleic acid (DNA).
Because the Nobel Prize can beawarded only to the living,Wilkins's colleague RosalindFranklin (19201958), who died of
cancer at the age of 37, could notreceive the award.
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The DNA story
The background for the work of the four scientists wasformed by several scientific breakthroughs
progress made by X-ray crystallographers in studyingorganic macromolecules;
the growing evidence supplied by geneticists that itwas DNA, not protein, in chromosomes that wasresponsible for heredity
http://www.chemheritage.org/classroom/chemach/pharmaceuticals/watson-crick.html
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The DNA story
Of the four DNA researchers only RosalindFranklin had any degrees in chemistry - herundergraduate and graduate degrees fromCambridge University.
During World War II she gave up her researchscholarship to contribute to the war effort atthe British Coal Utilization ResearchAssociation
After the war she joined the LaboratoireCentrale des Services Chimiques de l'Etat inParis, where she was introduced to thetechnique of X-ray crystallography and rapidlybecame a respected authority in this field.
In 1951 she returned to England to King's
College, London, where her charge was toupgrade the X-ray crystallographic laboratorythere for work with DNA.
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The DNA story
Already at work at King's College was MauriceWilkins, a New Zealandborn but Cambridge-educated physicist.
During World War II he was shipped out to theUnited States to work on the Manhattan Project.Like many other nuclear physicists he became
disillusioned with his subject when it was appliedto the creation of the atomic bomb
It was Wilkins's idea to study DNA by X-raycrystallographic techniques, which he hadalready begun to implement when Franklin wasappointed.
The relationship between Wilkins and Franklinwas unfortunately a poor one and probablyslowed their progress.
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In 1951 James Watson, an American, arrived at the Cavendish Laboratory inCambridge.
Watson had two degrees in zoology: abachelor's degree from the University ofChicago and a doctorate from the Universityof Indiana, where he became interested ingenetics.
At a conference Watson heard Wilkins talkon the molecular structure of DNA and sawX-ray crystallographic photographs of DNA.
The DNA story
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Watson and Crick rapidly put together several models of DNA
and attempted to incorporate all the evidence they could gather.
Franklin's X-ray photographs, to which they had gained access
without her permission, were critical to the correct solution.
The four scientists announced the structure of DNA in articles
that appeared together in the same issue ofNature.
The DNA story
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Then they moved off in differentdirections.
Franklin - X-ray crystallographic analysisof the structure of the tobacco mosaic
virusa landmark in the field.
Wilkins applied X-ray techniques to thestructural determination of ribonucleic
acid (RNA)
Watson's subsequent - Cold SpringHarbor - director from 1968. From 1988
The DNA story
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DNA Structure
DNA is a very stable molecule
This stability is provided by the
robust repetitive phosphate-sugarbackbone in each DNA strand
Phosphate links the 5 position of
one sugar with the 3 position of
the next sugar.
DNA backbone
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DNA Structure
In addition to the sugar 2 deoxyribose and the phosphate, DNAmolecules contain four nitrogen containing bases.
Two Pyrimidines, Thymine (T) and Cytosine, (C) and two purines,Guanine (G) and Adenine (A).
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DNA DoubleHelix
Complimentarypairing
Gene
Purines are larger than pyrimidinestherefore a regular double helix requires apurine in one strand to be matched by apyrimidine in the other strand
Specific hydrogen bonding required between the bases to fit
opposite one another.
This is called complimentary pairing
DNA strands run in opposite directions therefore DNA isoften represented as one strand in the 5 to 3 direction.
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Complimentary pairing
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http://www.umass.edu/molvis/tutorials/dna/dnapairs.htm