Nucleotide

These are the structural units of nucleic acids, consisting of a sugar (ribose for RNA and deoxyribose for DNA), a phosphate group and a nitrogenous base. Four different nucleotides exist for DNA: adenine (A), guanine (G), thymine (T) and cytosine (C). They have the particular feature that they combine two by two in complementary pairs: adenine with thymine, guanine with cytosine. Uracil (U) is a nucleotide which is only found in RNA, replacing thymine, which is only found in DNA. Uracil matches up with adenine.