The nitrogenous bases in DNA pair up in a specific way, following the base-pairing rules. The rules of base pairing are:

• Adenine (A) always pairs with thymine (T), forming two hydrogen bonds between them.
• Cytosine (C) always pairs with guanine (G), forming three hydrogen bonds between them.

These rules apply to both double-stranded DNA and double-stranded RNA, with the exception of A = T in DNA and A = U in RNA. The nitrogenous bases connect in groups of two, called complementary pairs, using hydrogen bonds. The five nitrogenous bases found in DNA and RNA are adenine (A), cytosine (C), guanine (G), thymine (T), and uracil (U) .