Watson and Crick DNA Model

• DNA molecule consists of two parallel chains of polynucleotides forming a double helical structure.
• Both the strands are spirally coiled.
• The two strands are anti-parallel.
• Both the base pair have a sugar-phosphate chain on the outside and a purine and pyrimidine base on the inside of the helix.
• Both the strands are joined together by weak hydrogen bonds.
• The distance between two strands is 20 A⁰
• There are 10 base pairs on the complete turn ( 34 A⁰). The distance between the two base pairs is 3.4 A⁰.
• A purine base is attached to its specific base. i.e. A=T and G ≡ C

• A DNA molecule contains 28% adenine, 24% Guanine, 20% cytosine, and 28% thymine.
• Nucleotides in a helix are joined together by phosphodiester bonds.