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.