Proteins are the molecules that do the majority of the work in a cell. They make up most of the cell’s structure, facilitate chemical reactions, carry molecular signals in and out of the cell, turn genes on and off and much more. A cell’s genes provide the recipe for assembling the correct building blocks – called amino acids – to make specific proteins (see DNA 101).

A short sequence of amino acids is called a peptide; a chain of many amino acids is a polypeptide. But a protein is more than just the sum of its parts – how it folds, the shape it takes and its interactions with other proteins also influence its identity and function. Those factors are determined not only by the amino acid sequence, but also by other chemical modifications made to the protein as it’s being built, or even later. Protein modifications are named for the type of chemical group added. For example, a phosphate group is added in a process called phosphorylation, which can alter a protein’s function.

Once assembled, the best-known proteins are those that perform specialized functions, such as enzymes, antibodies, toxins or hormones.