The genetic code uses three bases to encode one amino acid. Why can't the code use only two bases to encode each amino acid?

The genetic code is a set of rules by which information encoded in genetic material (DNA or RNA sequences) is translated into proteins by living cells. The basic unit of the genetic code is the codon, which is a sequence of three nucleotide bases. Each codon corresponds to a specific amino acid or a stop signal during protein synthesis. There are four different nucleotide bases in RNA (adenine [A], uracil [U], cytosine [C], and guanine [G]) and in DNA (adenine [A], thymine [T], cytosine [C], and guanine [G]).

If the genetic code used only two bases to encode each amino acid, there would be a total of 4^2, or 16, possible codons (since there are four types of bases and two positions to fill). However, there are 20 standard amino acids used by cells to build proteins. With only 16 codons, it would be impossible to encode all 20 amino acids, let alone the additional stop signals needed to indicate the end of a protein-coding sequence.

By using three bases per codon, the genetic code can generate 4^3, or 64, different codons. This allows for all 20 amino acids to be encoded, and it provides multiple codons for some amino acids, which is known as redundancy or degeneracy of the code. This redundancy can help protect against some types of mutations by allowing for synonymous substitutions that do not change the amino acid being encoded. Additionally, the extra codons are used as stop signals (also known as termination or nonsense codons) to mark the end of a protein-coding sequence.

In summary, the use of three bases per codon in the genetic code allows for the encoding of all the amino acids necessary for protein synthesis, as well as providing stop signals and a buffer against mutations through codon redundancy.