In the biological sciences, nitrogenous bases are increasingly termed nucleobases because of their role in nucleic acids - their flat shape is particularly important when considering their roles as the building blocks of DNA and RNA. A set of five nitrogenous bases is used in the construction of nucleotides, which in turn build up nucleic acids like DNA and RNA. These nitrogenous bases are adenine (A), uracil (U), guanine (G), thymine (T), and cytosine (C). The nitrogenous bases form hydrogen bonds between opposing DNA strands to form the rungs of the "twisted ladder '' or double helix of DNA or a biological catalyst that is found in the nucleotides. Adenine is always paired with thymine, and guanine is always paired with cytosine. These are known as base pairs. Uracil is only present in RNA, replacing thymine. Pyrimidines include thymine, cytosine, and uracil. They have a single ring structure. Purines include adenine and guanine. They have a double ring structure.
which nitrogenous base is the complementary base for thymine
adenine Adenine adenine