The genetic code is a set of rules defining how thefour-letter code of DNA is translated into the 20-letter code of amino acids,which are the building blocks of proteins. The genetic code is a set ofthree-letter combinations of nucleotides called codons, each of whichcorresponds to a specific amino acid or stop signal. The concept of codons wasfirst described by Francis Crick and his colleagues in 1961. During the sameyear, Marshall Nirenberg and Heinrich Matthaei performed experiments that begandeciphering the genetic code. They showed that the RNA sequence UUUspecifically coded for the amino acid phenylalanine. Following this discovery,Nirenberg, Philip Leder, and Gobind Khorana identified the rest of the geneticcode and fully described each three-letter codon and its corresponding aminoacid.
There are 64 possible permutations, orcombinations, of three-letter nucleotide sequences that can be made from thefour nucleotides. Of these 64 codons, 61 represent amino acids, and three arestop signals. Although each codon is specific for only one amino acid (or one stopsignal), the genetic code is described as degenerate, or redundant, because asingle amino acid may be coded for by more than one codon. It is also importantto note that the genetic code does not overlap, meaning that each nucleotide ispart of only one codon-a single nucleotide cannot be part of two adjacentcodons. Furthermore, the genetic code is nearly universal, with only rarevariations reported. For instance, mitochondria have an alternative geneticcode with slight variations.
