All DNA sequences coding for proteins begin with the same codon, ATG, which codes for the amino acid methionine. This codon is also known as the start codon. In Fig. 15, the ATG sequence on the negative strand demarcates the beginning of a coding sequence.ll DNA sequences coding for proteins begin with the same codon, ATG, which codes for the amino acid methionine. This codon is also known as the start codon. In Fig. 15, the ATG sequence on the negative strand demarcates the beginning of a coding sequence.