Watson and Crick

• Watson and Crick proposed the model of DNA.
• Adenine and Guanine are purines with two rings, and Cytosine and Thymine (Uracil in RNA) are pyrimidines with one ring. Adenine pairs with Thymine/Uracil forming 2 hydrogen bonds, and Cytosine pairs with Guanine forming 3 hydrogen bonds.