nucleotide

A nucleotide is a single bead on the long string that is DNA. Just as a sentence is built from letters and a necklace from beads, a DNA strand is built by linking millions of nucleotides end to end. Get the building block, and you understand most of what DNA is made of.

Each nucleotide has three parts: a five-carbon sugar (deoxyribose in DNA), one or more phosphate groups, and a nitrogenous base. The sugar and phosphate are the same in every nucleotide of DNA and form the structural backbone; the base is the part that varies and carries information. There are four bases in DNA — adenine, thymine, guanine, and cytosine — so there are four kinds of DNA nucleotide.

When nucleotides join into a strand, the phosphate of one links to the sugar of the next, forming a chain with a clear direction. The bases stick out to the side, where they can pair with the bases of a partner strand. This is how nucleotides become both the storage and the read-out medium for genetic information.

RNA is also built from nucleotides, but with the sugar ribose instead of deoxyribose and the base uracil in place of thymine. So nucleotide is the general term; whether it builds DNA or RNA depends on which sugar and bases are used.