nucleotide
/ NOO-klee-oh-tide /
If DNA is the instruction book of life, nucleotides are its letters — the individual units that, strung in order, spell out every gene. They are the monomers of the nucleic acids, DNA and RNA, and the same basic part also turns up as the cell's energy currency.
Each nucleotide is built from three pieces snapped together: a sugar, a phosphate group, and a nitrogen-containing base. The phosphate and sugar are the same boring backbone in every nucleotide; the base is the part that varies and carries the information. There are four common bases in DNA — adenine, thymine, guanine, cytosine, abbreviated A, T, G, C — and RNA swaps thymine for a close cousin, uracil (U). Linking nucleotides through their sugar and phosphate forms the long backbone of a nucleic-acid strand, with the bases sticking out to spell a sequence.
Nucleotides matter because sequence is information. The order of the four bases along a DNA strand is a code, and that code carries the instructions for building every protein in you. The same molecular toolkit does double duty in energy: ATP, the cell's main energy carrier, is essentially a nucleotide (an adenine base, a sugar, and a chain of phosphates) used not to store information but to store and release usable energy.
Spelling out a stretch of DNA as a string like A-T-G-C-C-A is reading off its nucleotides one base at a time, exactly the way a sequencing machine reports a genome.
A genome printed as letters is just its nucleotides read off one base at a time.
The base, the sugar, and the phosphate are three different parts of one nucleotide, not three separate things; people often say base when they mean the whole nucleotide because only the base carries the information.