Used_in The definition of C++ [ c++.syntax.html ]. Used_in The definition of Java [ java.syntax.html ] Notation This uses my XBNF Extended BNF Notation where | indicates or, (...) indicates priority. For more information see [ intro_ebnf.html ].
To look at it in another way: Thymine(uracil in RNA) and cytosine are pyrimidines. Pyrimidines are single-ringed organic base that will only attach to purines, or adenine and guanine by hydrogen bond. Because cytosine and thymine are both pyrimidines, it cannot form a hydrogen bond with each other.
Guanine, the other purine, always pairs with the pyrimidine cytosine. These couplings are known as base pairs. Adenine is of equal importance in RNA. However, in RNA, adenine pairs with a base called uracil as thymine is not present.It is the particular sequencing of the base pairs within strands of DNA that determine the unique genetic code of each living organism.he sides of the ladder are composed of a sugar-phosphate combination. The rungs of the ladder are the nitrogenous bases adenine, thymine, guanine, and cytosine. Each individual rung consists of two bases bonded together by hydrogen bonds. Because of their size and structure, purines always pair with pyrimidines.