Double helix DNA model by Watson and Crick

Watson and Crick model (Key Facts)

• It is called the B-DNA and the most common form of DNA proposed by J.D Watson, F.H.C Crick
• Laboratory: Cavendish laboratory
• Other contributors: Maurice Wilkins and Rosalind Franklin of Kings College, London provided the x-ray crystallographic data.
• Phobeus Levene proposed the Tetra nucleotide theory and Linus Pauling's triple helix model. Both these even though wrong provided many clues regarding the DNA structure
• Erwin Chargaff of Columbia University proposed the complementarity of base pairing. 

Summary of Watson and Crick model

1) The DNA molecule is composed of two chains of nucleotides

2) The two chains spiral around the central axis to form a pair of right handed helices (helical nature revealed in the pattern of spots produced by x-ray diffraction)

3) The sugar phosphate backbone is located on the outside of the molecule with two sets of bases projecting towards the centre. The phosphate group gives the molecule a negative charge.

4) The 2 chains are held together by hydrogen bonds between each base of one chain and associated base of other chain.

5) The bases are stacked one on top of the other. Hydrophobic interactions and Vander walls forces between the stacked bases provide stability for the entire DNA molecule.

6) The two strands run in opposite direction or anti- parallel in nature, if one chain is aligned in the 5’-3’ direction, the other in 3’-5’ direction.

7) The helix has two external grooves, a deep wide one called a major groove and a shallow one called the minor groove. These are the regions that interact with protein molecules.

8) Base complementarity of polynucleotide chains: pairing always occurs between adenine and thymine by 2 hydrogen  bonds and between guanine and cytosine by 3 hydrogen bonds.

Dimensions:

9) The distance between adjacent nucleotides or inter-nucleotide distance is 10 A0. The double helix makes a complete turn in every 10 residues. Therefore each turn is 34 Angstrom.

10) The width of the double helix is 20 Angstrom.

11) The base pairs are rotated 36o with respect to each adjacent pair.

Notes; H-bond is very weak and can be easily broken, thus allowing the DNA strands to separate during replication. The strength of H-bonds are additive.

The importance of Watson and Crick model

A genetic material should have the following functions:

1. Storage of genetic information
2. Self duplication and inheritance (DNA replication)
3. Expression of genetic message (protein expression)

This model could explain the first two functions of a genetic material.

Related post: Chargaff’s Experiment and Why is Chargaff's Rule Important?