The Genetic Code
In 1968 the Nobel Prize in Medicine was awarded to Robert W. Holley, Har Gobind Khorana and Marshall W. Nirenberg for their interpretation of the Genetic Code (http://nobelprize.org/medicine/laureates/1968/). The Genetic Code represents translations of all 64 different codons into 20 different amino acids. Therefore it is no problem to translate a DNA/RNA sequence into a specific protein. But due to the degeneracy of the genetic code, several codons may code for only one specific amino acid. This can be seen in the table below. After the discovery of the genetic code it has been concluded that different organism (and organelles) have genetic codes which are different from the "standard genetic code". Moreover, the amino acid alphabet is no longer limited to 20 amino acids. The 21'st amino acid, selenocysteine, is encoded by an 'UGA' codon which is normally a stop codon. The discrimination of a selenocysteine over a stop codon is carried out by the translation machinery. Selenocysteines are very rare amino acids.
The table below shows the Standard Genetic Code which is the default translation table.
TTT F Phe | TCT S Ser | TAT Y Tyr | TGT C Cys |
TTC F Phe | TCC S Ser | TAC Y Tyr | TGC C Cys |
TTA L Leu | TCA S Ser | TAA * Ter | TGA * Ter |
TTG L Leu i | TCG S Ser | TAG * Ter | TGG W Trp |
CTT L Leu | CCT P Pro | CAT H His | CGT R Arg |
CTC L Leu | CCC P Pro | CAC H His | CGC R Arg |
CTA L Leu | CCA P Pro | CAA Q Gln | CGA R Arg |
CTG L Leu i | CCG P Pro | CAG Q Gln | CGG R Arg |
ATT I Ile | ACT T Thr | AAT N Asn | AGT S Ser |
ATC I Ile | ACC T Thr | AAC N Asn | AGC S Ser |
ATA I Ile | ACA T Thr | AAA K Lys | AGA R Arg |
ATG M Met i | ACG T Thr | AAG K Lys | AGG R Arg |
GTT V Val | GCT A Ala | GAT D Asp | GGT G Gly |
GTC V Val | GCC A Ala | GAC D Asp | GGC G Gly |
GTA V Val | GCA A Ala | GAA E Glu | GGA G Gly |
GTG V Val | GCG A Ala | GAG E Glu | GGG G Gly |