Transcription, RNA Processing and Translation Exercise D: The Genetic Code 37 Triplet code hypothesis Once it was determined that DNA’s main function was to code for proteins, the next question was how. The language of DNA is based on four nucleotides (A, T, C, and G), while proteins are composed of 20 amino acids. Thus, it is mathematically impossible for one nucleotide to code for one amino acid, as there are more than four amino acids. A+T+C+G=41 = 4 < 20 Therefore, amino acids must be coded by some combination of D-nucleotides. What if DNA was coded by a combination of two neighboring nucleotides? AA+AT+AC+AG + TA + TT + TC + TG + CA + CT + CC + CG + GA +GT + GC + GG = 4.16(20 This mechanism also results in fewer than 20 amino acids. Therefore, it was hypothesized that the simplest possible code would have to be based on a triplet combination of neighboring D- nucleotides. 15. Calculate how many possible amino acids could result from a triplet code The code is redundant As you see from your calculation, there are far more possibilities of amino acids provided by a triplet code, than the number of amino acids (20) we see in nature. Therefore, it is said that the code is redundant, meaning that amino acids can be coded by more than one triplet code. For example, the triplet codes of CCU and CCC of mRNA code for the same amino acid, proline (Fig. 10). In fact, all amino acids are coded by more than one triplet code except for methionine and tryptophan. The code is unambiguous Further investigations indicated that a specific triplet code always coded for the same amino acid. In other words, the code is unambiguous. For example, the triplet code of AUG in mRNA always codes for methionine (Fig. 10). The code is universal Amazingly, the code works exactly the same for all living organisms, from bacteria to plants and animals! While there are very few exceptions to this, the consistency of the code across widely variable organisms hints that we all stem from a single common ancestor. 16. What property of the code is used to suggest that all living organisms stem from a single common ancestor? www.thebiologyprim