∞ generated and posted on 2016.12.20 ∞
Entity defining the beginning of a reading frame.
The start codon, usually AUG, denotes the first amino acid in a polypeptide that is "added" by ribosomes (with added in quotes because subsequent amino acids more properly are added to it rather than the other way around).
The base sequence that specifies this start is AUG, and the amino acid added is methionine (Met or M) or a chemical variation on methionine (fMet), though there are a substantial number of exceptions where codons other than AUG are employed.
Figure legend: Start codon base paired with associated charged tRNA. Note the base pairing between the start codon, or any codon, and the corresponding anticodon along with the antiparallel nature of this arrangement. That is, together the codon-anticodon interaction represents a very small double helix. Note too how the tRNA-associated amino acid, here methionine, is some distance from the codon-anticodon, and that the interaction depicted takes place within the context of a ribosome.
In DNA, the corresponding base sequence to the standard AUG start codon is ATG.