Start and stop codons are important because they tell the cell machinery where to begin and end translation, the process of making a protein. The start codon also sets up the reading frame of the DNA strand, indicating that each triplet after that point codes for a specific amino acid.
Start and stop codons are found both on the original DNA strand in the nucleus of the cell and on the messenger RNA strand that serves as the protein template. The mRNA that corresponds to a specific gene on the DNA strand is synthesized in the nucleus using the antisense strand of DNA as a guide to the order of codons. This mRNA strand then travels to a ribosome in the cell nucleus, where protein assembly takes place.
In most organisms, the only start codon is ATG, a triplet made up of the DNA bases adenine, guanine and thymine. ATG also codes for the amino acid methionine when found in the middle of a gene. In the mRNA template, ATG is replaced by AUG because the base uracil always appears in place of thymine in RNA.
Stop codons come in three different forms: TGA, TAG and TAA. In RNA, these three codons appear as UGA, UAG and UAA. Unlike the start codon, none of the stop codons code for an amino acid.