DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are nucleic acids. They are molecules comprised of monomers (structural unit of a polymer) known as nucleotides. These molecules may be relatively small (as in the case of certain kinds of RNA) or quite large (a single DNA strand may have millions of monomer units). Individual nucleotides and their derivatives are important in living organisms. For example, ATP, the molecule that transfers energy in cells, is built from a nucleotide, as are a number of other molecules crucial to metabolism.