Final answer:
The genetic code uses triplets of nucleotides to code for amino acids because with four types of nucleotides, triplets allow for 64 possible combinations, sufficient to code for all 20 amino acids with redundancy. Doublets would only allow for 16 combinations, which is not enough to represent all amino acids.
Step-by-step explanation:
The genetic code is a biological system that allows genetic information to be translated into the proteins necessary for life. With the four different nucleotides available (adenine, cytosine, guanine, and uracil in RNA), a codon size of three nucleotides is required to provide enough different combinations to code for all 20 common amino acids. If only doublets were used, there would be only 16 possible combinations (4²), which is insufficient. However, using triplets, there are 64 possible combinations (4³), which is more than enough to encode every amino acid, with some redundancy to spare. This redundancy is referred to as degeneracy, which means several codons can code for the same amino acid, enhancing the robustness of the genetic code against mutations.
Three special codons do not code for amino acids; instead, they function as stop codons, signaling the end of protein synthesis. Additionally, the codon AUG serves a dual function: it codes for the amino acid methionine and also acts as the start codon, initiating the translation process.