Watson And Crick Model Of DNA

A model is a visual image of an object or idea which simplifies the object or idea. James Watson and Francis Crick assembled the molecular model and published their two-page article on their molecular model of DNA in the journal “Nature” in April 1953. Few milestones in the history of biology have had as broad an impact as their double helix. Watson and Crick were awarded the Nobel Prize in 1962 for their model of DNA.

A rope ladder model for the double helix

The Important Points of This Ladder Model of DNA Are:

1. There are two polynucleotide strands running in opposite directions and winding out each other in a form of double helix.

2. The double helix looks like a ladder.

3. The sugar and phosphate part of the nucleotide makes the upright part of the ladder.

4. The nitrogen bases of the nucleotide make up the rungs of the ladder.

5. A double-ring base purine must always be paired with a single ringed base pyrimidines on the opposite strand. Individual structures of the bases form the pairing more specifically. Each base has chemical side groups that can best form hydrogen bonds with one appropriate partner.

DNA Model

6. The matching of the bases are specific. Adenine makes the pair with Thymine and Guanine with Cytosine.

7. The base pairs are held together by the hydrogen bond. There are three hydrogen bonds between Guanine and Cytosine and two hydrogen bonds between Adenine and Thymine.

8. The ratio of A:T and C:G is equal but the ratio of AT: CG is different.

9. The helix is 2nm in diameter and makes a full spiral turn every 3.4 nm i.e. after every ten base pairs. The distance between two base pair is 0.34 nm.

10. There are two grooves a major groove and a minor groove.

11. The helix rotates about its axis 3.60° longitudinally with each base pair or rung, the spiral winds through a full circle i.e. 360° in the length of helix occupied by ten base pairs.

12. There is no restriction on the sequence of nucleotides along the length of a DNA strand. The sequence can vary in countless ways. The sequencers specific for different species, organisms and even individuals. 

Structure of DNA