Amino acid is the organic compound that contains amine and carboxyl as their functional groups. All amino acids have a side-chain, also called the R group, which is specific to each amino acid. The amino acids are mainly composed of carbon, hydrogen, oxygen, and nitrogen. Though above 500 amino acids have been identified so far, there are 20 basic amino acids, which are essential for humans.
The triplet codons code for respective amino acids. For example, if UUU is coding for phenylalanine, there are other three triplet codes starting with U coding for this amino acid. Any change in this first base, will cause mutation. For example change from U to C will change phenylalanine to leucine.
Similarly, second base is particularly for an amino acid. Both first base and second base together direct the transcription and code for an amino acid in most cases, while the third base is less particular. For example UUU and UUC code for phenylalanine.
First and second base is more particular than the third base. Hence, if there is a random triplet codon XYZ coding for an amino acid, mutation in third base pair Z, is least likely to result in a change in an amino acid.