Watson and Crick(1953)Double helix model of DNA
Double helix model of DNA
Early models of DNA
A structure for nucleic acid has already been
proposed by Pauling and Corey. Their model
consists of three intertwined chains, with the
phosphates near the fibre axis, and the bases
on the outside.
Another three-chain structure has also been
suggested by Fraser. In his model the
phosphates are on the outside and the bases
on the inside, linked together by hydrogen
bonds.
On Feb. 28, 1953, Francis Crick walked into the Eagle pub in
Cambridge, England, and, as James Watson later recalled,
announced that "we had found the secret of life." Actually, they
had. That morning, Watson and Crick had figured out the
structure of deoxyribonucleic acid, DNA. And that structure — a
"double helix" that can "unzip" to make copies of itself —
confirmed suspicions that DNA carries life's hereditary
information.
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Francis Crick and James Watson with
Maurice Wilkins received the 1962
Nobel Prize for discovering the
molecular structure of
deoxyribonucleic acid (DNA).
Widely regarded as one of the most
important discoveries of the 20th
century it has led the way to the
mapping and deciphering of all the
genes in the human chromosomes
X-ray diffraction
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X-ray crystallography was originally used
to look at the structures of simple organic
minerals, but was progressively applied to
more and more complex molecules. It
aided in determining the structures of the
alpha helix, the beta sheet, hemoglobin,
and DNA
X-Ray Diffraction and the
Structure of DNA
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In 1951,, James Watson, joined the lab and the
two formed a close working relationship. They
were convinced that if the three-dimensional
structure of a molecule known to play a role in
passing genetic information -- DNA -- could be
determined,. They made models based on
research done in several fields. Crick and
Watson saw the result of Rosalind Franklin's xray diffraction studies, and a final piece of the
puzzle was fitted. In 1953 they created a visual
model of DNA
X-Ray Diffraction and the
Structure of DNA

They made models based on research
done in several fields. Crick and Watson
saw the result of Rosalind Franklin's x-ray
diffraction studies, and a final piece of the
puzzle was fitted. In 1953 they created a
visual model of DNA
X-Ray Diffraction and the
Structure of DNA
Watson was shown this picture by Wilkins in early 1953.
From the picture it was possible to calculate:
1) the distance between bases (3.4A)
2) the length of the period (34A)
3) the rise of the helix (36 degrees)
Building model
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Therefore, knowing that DNA existed and
contained four bases, a ribose sugar and
phosphate. Inspired by Pauling's
successful attempts at building 3-D
models of proteins, Crick and Watson
believed this to be the correct way to
proceed.
Bases
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John Griffith, the mathematician
nephew of Fred Griffith, calculated the
attractive forces between 'like' bases.
Crick's idea was that since the bases were
flat, perhaps they coud be stacked on top
of one another, and attracted that way.
Griffith informed him that adenine
attracted thymine and guanine attracts
cytosine
Chargaff’s 1:1 Rule
A:T=G:C+1:1
When Crick compared the bases that Griffiths had told him
with Chargraff's data, he realized that complementary base
pairing could be the cause of the 1:1 rule.
Hydrogen bond
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Watson and Crick was the thought that
hydrogen bonding was too unstable to be
responsible for replication. Crick was also
assuming that both tautameric forms of
the bases existed in the same DNA
molecule, and that the proton could shift
from one position to another, thus altering
the sites for hydrogen bond formation.
The final model
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This structure has two helical chains each coiled
round
the same axis
usual chemical assumptions, namely, that each
chain consists of phosphate diester groups
joining ß-D-deoxyribofuranose residues with 3',5'
linkages
Both chains follow right- handed helices, but
owing to the dyad the sequences of the atoms in
the two chains run in opposite directions
The final model
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an angle of 36 degrees between adjacent
residues in the same chain, so that the
structure repeats after 10 residues on each
chain, that is, after 34 A. The distance of a
phosphorus atom from the fibre axis is 10 A.
As the phosphates are on the outside,
cations have easy access to them
The phosphates are negatively charged,
and attract cations. The phosphates, being
charged, are also hydrophilic
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For their outstanding work in discovering
the double helical structure of DNA,
Watson and Crick shared the 1962 Nobel
Prize for Physiology and Medicine with
Maurice Wilkins. Sadly, Rosalind Franklin,
whose work greatly contributed to this key
discovery