The genetic code is the set of rules used by cells to translate information into living cells (DNA or mRNA sequences of nucleotide triplets or codons) into proteins. Translation is accomplished by the ribosome, which links amino acids to an order specified by messenger RNA (mRNA), using transfer RNA molecules (tRNA) to transport amino acids and to read three mRNAs at the nucleotides at a time. The genetic code is very similar to all the organisms and can be expressed in a simple table with 64 entries. The code defines how the codons specify which protein synthesis is next. With a few exceptions, a single amino acid specifies a nucleic acid sequence in three nucleotides with a codon. The vast majority of genes are coded with a single scheme (see table of RNA codons). This scheme is often called the canonical or standard genetic code, or simply the genetic code, although there are variant codes (as in human mitochondria). While the "genetic code" determines what a protein in the amino acid sequence is, other genomic regions are known when and where these proteins are produced.
Translation starts with a chain-initiation codon or start codon. The start codon alone is not sufficient to begin the process. Nearby sequences such as the Shine-Dalgarno sequence in E. coli and initiation factors are also required to start translation. The most common start codon is AUG, which is read as methionine or, in bacteria, as formylmethionine. Alternative start codons depending on the organism include "GUG" or "UUG"; these codons normally represent valine and leucine, respectively, but as start codons they are translated as methionine or formylmethionine.