any of a group of molecules that, when linked together, form the building blocks of DNA or RNA: composed of a phosphate group, the bases adenine, cytosine, guanine, and thymine, and a pentose sugar, in RNA the thymine base being replaced by uracil.
The molecules that form the basic modular structure of the double helix of the DNA molecule. A nucleotide consists of three molecules — a sugar, a phosphate group, and a molecule called a base. If the double helix is a twisted ladder, the sugar and phosphates form the sides of the ladder and pairs of bases form the rungs. There are four different bases, usually abbreviated A, C, G, and T for adenine, cytosine, guanine, and thymine). The order of bases in DNA determines the genetic code.