Nitrogenous bases, along with a phosphate group and a 5-carbon sugar, constitute nucleotides, which are the monomers for nucleic acids. Namely, DNA (dioxyribonucleic acid) and RNA (ribonucleic acid). The nitrogenous bases can be classified as either purine or pyrimidine bases. A DNA molecule contains 4 types of nitrogenous bases: adenine (denoted as A), guanine (denoted as G), cytosine (denoted as C) and thymine (denoted as T). Out of these, A and G are purine bases, while C and T are pyrimidine bases.
These nitrogenous bases pair up with a complementary nitrogenous base on the complementary strand. Adenine pairs up with thymine and cytosine pairs up with guanine in the case of a DNA molecule.
In the case of an RNA molecule, uracil (denoted as U) is present instead of thymine. In an RNA molecule, A pairs with U and C pairs with G.
This complementary base pairing is due to the hydrogen bonds between the pairing bases.
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