In cells, genes are segments of DNA that code for proteins; not sugars or complex carbohydrates, or lipids, but proteins. The code in the sequence of nucleotides in a gene specifies the sequence of nucleotides in messenger RNA which in turn specifies the sequence of amino acids in a protein. The sequence is variable because of mutations which can change the sequence of amino acids in the protein coded for by that gene. Therefore, in evolution, a change in a single code for a single amino acid may have an effect on individual survival. Proteins carry out many functions in a cell such as:.