Start codon

ATG and AUG denote sequences of DNA and RNA respectively that are the start codon or initiation codon encoding the amino acid methionine (Met) in eukaryotes and a modified Met (fMet) in prokaryotes.

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The principle called Central dogma of molecular biology describes the process of translation of a gene to a protein. Basically specific sequences of DNA act as a template to synthesize mRNA in a process termed "transcription" in the nucleus. This mRNA is exported from the nucleus into the cytoplasm of the cell and acts as a template to synthesize protein in a process called "translation."

Three nucleotide bases form one amino acid in the genetic code. Usually the first three bases of the coding sequence(CDS) of mRNA to be translated into protein are AUG (or ATG in DNA). AUG encodes for methionine, and therefore the first amino acid of many proteins is methionine. The start codon is almost always preceded by an untranslated region 5' UTR.

Very rarely in higher organisms (eukaryotes) non AUG start codons are used.

In addition to AUG, alternative start codons, mainly GUG and UUG are used in prokaryotes. For example E. coli uses 77% ATG (AUG), 14% GTG (GUG), 8% TTG (UUG) and a few others.^{[citation needed]}

Well known coding regions that do not have ATG initiation codons are those of lacI (GTG)[1] and lacA (TTG)[2]in the E. coli lac operon.