The functions of proteins are the essence of life itself. They make up more than 50% of the dry mass of animals. Proteins are organic compounds that contain nitrogen in addition to carbon, hydrogen and oxygen. The subunits of proteins are called amino acids.
Proteins are long, unbranched chains of amino acids that fold up into complex shapes because of attractive and repulsive forces between the R groups of different kinds of amino acids. Having certain amino acids in certain positions is crucial to the proteins overall shape and consequently to its function. Clearly, it is critical for every cell to have a process that guarantees accurate ordering of amino acids in every protein that it needs to carry out its life activities. For example, the change of just one amino acid alters the shape of hemoglobin (in our blood) enough to create the condition of sickle cell anemia. Though proteins themselves do not mutate, a mutation in the genetic material of an organism is expressed as a change in the order of amino acids of a protein.