Final answer:
The genetic code is redundant, allowing multiple codons to specify the same amino acid, except for tryptophan and methionine which are each encoded by a single codon. This redundancy, called degeneracy, reduces the impact of mutations and includes a special role for the codon AUG as an initiation signal and for coding methionine within sequences.
Step-by-step explanation:
In the genetic code, some codons specify only one amino acid, while other amino acids are specified by multiple codons due to the redundancy in the genetic code. For instance, the amino acid glycine is specified by four codons: GGU, GGC, GGA, and GGG. However, two amino acids, tryptophan and methionine have unique codons: UGG for tryptophan, and AUG for methionine.
The AUG codon has a dual role of signaling the start of protein synthesis as the initiation codon and coding for methionine when it appears in the middle of mRNA sequences. This redundancy, also called degeneracy, helps to protect against the potential detrimental effects of mutations, particularly because codons that specify the same amino acid often only differ in their third nucleotide, a position known as the 'wobble' position.