DNA

1. 1

2. Topic : Watson and Crick Model of DNA
Presented to :
Presented by :
Roll No : 146

3.  Early Models of DNA
 Important Scientists in the “Race for the Double Helix”
 X-ray diffraction
 X-Ray Diffraction and the Structure of DNA
 Building model
 DNA’s Structure
 DNA Bases
 The Final model
Contents
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4. Watson and Crick(1953)- Double
helix model of DNA

5. 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.

6. Important Scientists in the
“Race for the Double Helix”
 James Watson & Francis Crick (1953)
 Created the first accurate model of DNA
 Won the Nobel Prize

7. X-ray diffraction
 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

8. X-Ray Diffraction and the Structure of DNA
 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 x-ray diffraction studies, and a final piece of the
puzzle was fitted. In 1953 they created a visual model of DNA

9. 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

10. 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)

11. Building model
 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.

12. DNA’s Structure
 DNA is a double helix – this makes DNA look like a “twisted ladder”.
 Each molecule of DNA is composed of a “backbone” on the outside
and bases that point towards the middle of the molecule.
 The monomer for DNA is the nucleotide

13. DNA Bases
 Purines – double-ringed
molecule
 Ex. Adenine and Guanine
 Pyrimidines – single-ringed
molecule
 Ex. Cytosine and Thymine

14. Complementary Base-Pairing Rules
 DNA is built following the complementary base-pairing rules.
 These state that:
 A always pairs with T
 T always pairs with A
 C always pairs with G
 G always pairs with C
 In RNA, adenine (A) pairs with uracil (U). We will look at this in a later
lesson.

15. Hydrogen bond
 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.

16. Hydrogen bond

17. The Final model
 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

19.  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
The Final model

20.  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
The Final model