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Proteins and DNA are polymers of amino acids and nucleotides, respectively. These and other kinds of macromolecules carry out the biological processes that make life possible. For example, some macromolecules store genetic information that is passed down to future generations. Some are involved in decoding genetic information. Yet other macromolecules carry out metabolism, breaking down molecules to obtain energy and using that energy to build other molecules. In addition to macromolecules, small molecules play a central role in living systems. Although there is no discrete size cut-off that distinguishes “small” molecules from macromolecules, most small molecules that are relevant to life contain fewer than about a hundred atoms. Unlike macromolecules, which are polymers of repeating units, small molecules are diverse in their structures and typically lack repeating units. Their diversity implies that small molecules are synthesized in the cell by a more diverse collection of chemical reactions than those used to make macromolecules.