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.
In the DNA, base complements are A-T (adenine - thymine) and G-C (guanine - cytosine). In the RNA, base complements are A-U (adenine - uracil) and G-C (guanine - cytosine). Hence, only DNA contains thymine, while in RNA uracil is present instead of thymine. Comments.