Final answer:
Three nucleotides are required to form a codon, which allows for 64 possible combinations, more than enough to encode the 20 amino acids found in proteins.
Step-by-step explanation:
To answer the question, three nucleotides are needed to form a codon. The three-letter structure of a codon is necessary because there are four types of nucleotides in RNA (adenine (A), cytosine (C), guanine (G), and uracil (U)). If codons were formed with less than three nucleotides, there wouldn't be enough combinations to specify all 20 amino acids that proteins are made of. With a three-nucleotide code, however, there are 64 possible combinations (4³), which is more than enough to account for all 20 amino acids. Of these 64 codons, 61 codons specify amino acids, and the remaining three function as stop codons signaling the termination of protein synthesis.
It's interesting to note that because some amino acids are specified by more than one codon, this coding system is known as being 'degenerate'. The redundancy of this system ensures accuracy and flexibility in the genetic code.