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