What are the four nitrogen bases of DNA and how do they pair?

The four bases of RNA are adenine guanine cytosine and uracil. (Uracil is only found in RNA) The four bases of DNA are adenine, guanine, cytosine and thyamine (thyamine replaces uracil in DNA). The bases in uracil are denoted A, G, C and U. When RNA pairs with DNA, A of the DNA always pairs with U of the RNA, T of the DNA always pairs with A of the RNA, and C and G always pair with each other. If you look at a drawing of each molecule, it is easy to see that each pair of complementary bases are perfectly matched for each other. The bases pair with each other through hydrogen bonding. This is a very strong type of bond that occurs between a hydrogen and an electronegative atom (such as F, O or N). The bases are cyclic structures of carbons and hydrogens, and they have some N-H groups and carboxylic groups. So the bases of RNA pair with their complementary bases in DNA by hydrogen bonding between the N-H groups on one base with the oxygen of the carboxylic group on the complementary base.