Final answer:
Each nucleotide within the genetic code does not by itself encode an amino acid. Instead, a set of three nucleotides, known as a codon, allows for 64 different combinations, providing the possibility of coding for all 20 amino acids found in proteins. The genetic code's redundancy ensures that multiple codons can specify the same amino acid, reducing the impact of mutations.
Step-by-step explanation:
The reason why each nucleotide coding for one of the twenty amino acids does not limit us to only four amino acids involves the concept of the genetic code and its organization into codons. Each codon, made up of three nucleotides, allows for 64 (43) different combinations, which is more than enough to encode all 20 amino acids found in proteins. This three-nucleotide system ensures that the code is extensive enough to direct the synthesis of the primary structure of protein molecules. Moreover, since there are more nucleotide triplets than amino acids, the genetic code is redundant, meaning several codons can code for the same amino acid, which helps reduce the negative impact of random mutations on protein function.
For example, ACU, ACC, ACA, and ACG all code for the amino acid threonine, exemplifying this redundancy. Additionally, the redundancy ensures that similar amino acids have similar codons, which can mitigate the effects of mutations. The codon AUG is a special case as it signals the start of protein synthesis and also encodes for the amino acid methionine.