The 4 major bases that make up DNA would be Adenine, Thymine, Guanine and Cytosine. The pyrimidines which are Cytosine and Thymine are structurally similar to each other, as they both only contain a single Carbon containing ring. While Purines, which are Adenine and Guanine, contain 2 chemically bonded carbon containing rings with each other.
As per the base pairing in DNA. Adeline always pairs with Thymine with 2 hydrogen bonds, while Guanine pairs with cytosine with 3 hydrogen bonds.
The base pair that is not found in RNA would be Thymine, instead another pyrimidine, which is uracil is used instead for processes dealing with RNA molecules.
