Multiple choice Which of the following amino acids is incorporated into protein by codon namely UGA? Cysteine Cystine Selenocysteine Alanine Reveal answer Fill a bubble to check yourself C Correct answer Explanation UGA is normally a stop codon, but in the presence of a SECIS element and specialized translation machinery, it codes for selenocysteine, the 21st proteinogenic amino acid. Selenocysteine contains selenium instead of sulfur.