Genetic code is the instructions in a gene that tell the cell how to make a specific protein. A, T, G, and C are the "letters" of the DNA code; they stand for the chemicals adenine, thymine, guanine, and cytosine, respectively, that make up the nucleotide bases of DNA. Each gene's code combines the four chemicals in various ways to spell out 3-letter "words" that specify which amino acid is needed at every step in making a protein. All living things use the same genetic code system, however, there exist some viruses that contain only RNA, which has the nucleotide base of uracil ("U") instead of thymine ("T"). While there are 64 possible three nucleotide combinations (codons), only 20 amino acids are normally used. Most of amino acids are encoded by multiple codons.
Three codons do not encode any amino acid but instead serve to mark the end of the protein. However, in some very specific proteins two of these codons encode the two very rare amino acids (selenocysteine and pyrrolysine). They are not present in majority of the proteins and require additional regulatory sequences to be included.
Genetic code is the same for human, animals, plants and all other organisms. It may be differences and exceptions in some small and very specific genomes, but these are minor and rather rare. Scientists currently do not have explanation why the genetic code is universal when it would be 1.5 × 1084 possible genetic codes to choose from. It is also not clear why exactly these 20 amino acids have been selected as the protein building blocks. Much more different amino acids occur in the living world.
The "word" combinations in more complex genetic strings if written out could literally fill a space as large as an encyclopedia.
- (2010) Life from an RNA World: The Ancestor Within. Cambridge: Harvard University Press, 163. ISBN 0-674-05075-4.