In molecular biology, complementarity describes the relationship between two structures which follow the lock-and-key principle. Complementarity is the base principle of RNA and DNA. Each DNA base has a complementary partner: cytosine binds with guanine (C-G), and adenine binds with thymine (A-T) (or uracil in the case of RNA, A-U). Two DNA strands are complementary if their bases match up when the strands are aligned in opposite directions.