Answer: The nucleotide sequence of the mRNA molecule is read in groups of three nucleotides. The three coding letters in mRNA are A, U, and G, where A - Adenine, U - Uracil, and G - Guanine.
Explained: The three-letter nature of codons means that the four nucleotides found in mRNA — A, U, G, and C — can produce a total of 64 different combinations. Of these 64 codons, 61 represent amino acids, and the remaining three represent stop signals, which trigger the end of protein synthesis.