According to Watson and Crick model of DNA :
1. The Double Helix consists of two polynucleotide chains, which are coiled like a rope in helical (spiral) fashion.
2. The two strands of polynucleotides are run in the opposite direction and are antiparallel.
3. The backbone is formed by the sugar-phosphate-sugar chain.
4. The nitrogen bases lie perpendicular to this
backbone but face inside.
5. A always pairs with T and G pairs with C. A and T are joined by two hydrogen bonds while the G and C pair have three bonds.
6. A base pair represents each step of ascent of the spiral. At each step the strand turns 36.
7. There are ten base pairs in a complete turn which has ten steps.
8. The pitch is 34Å. the rise per base pair is 3.4Å.
The Watson-Crick model of DNA.