The mRNA formed from the repeating tetranucleotide UUAC incorporates only three amino acids, but the use of UAUC incorporates four amino acids. Why? The mRNA formed from the repeating tetranucleotide UUAC incorporates only three amino acids, but the use of UAUC incorporates four amino acids. Why? The UAC triplet, produced from the first sequence, does not code any amino acid. The triplets UUA and CUU, produced from the first sequence, code the same amino acid. The second sequence is more random. The first sequence has a "double U" fragment.