The Genetic Code


The Genetic Code

The relationship between the codons of nucleic acids, and the amino acids for which they code, is embodied in the Genetic Code, (which is NOT universal since slight variations on it are found in protists, in mitochondria and in chloroplasts). The 64 possible triplets of bases in a codon, and the amino acid coded for are shown in this table :

 

First               Second Position             Third 
Position   ------------------------------------ Position
  |            U(T)    C       A       G         |
  
  U(T)        Phe     Ser     Tyr     Cys        U(T)
              Phe     Ser     Tyr     Cys        C
              Leu     Ser     STOP    STOP       A
              Leu     Ser     STOP    Trp        G

  C           Leu     Pro     His     Arg        U(T)
              Leu     Pro     His     Arg        C
              Leu     Pro     Gln     Arg        A
              Leu     Pro     Gln     Arg        G

  A           Ile     Thr     Asn     Ser        U(T)
              Ile     Thr     Asn     Ser        C
              Ile     Thr     Lys     Arg        A
              Met     Thr     Lys     Arg        G

  G           Val     Ala     Asp     Gly        U(T)
              Val     Ala     Asp     Gly        C
              Val     Ala     Glu     Gly        A
              Val     Ala     Glu     Gly        G



Some characteristics of the genetic code:

 

NB. Different organisms exhibit different statistical preferences of triplet codon usage, as well as using the amino acids in widely varying proportions. See Of URFs and ORFs' by Russell Doolittle, University Science Books (1986) ISBN 0-935702-54-7. Click here to see the codon usage table for the filarial parasite Brugia malayi

The Mitochondrial Genetic Code

Human mitochondrial DNA encodes only 22 tRNA species and these are the only tRNAs used for the translation of mitochondrial mRNAs. This is accomplished by an extreme form of wobble in which U of the anticodon in tRNA can pair with any of the four bases in the third codon position of the mRNA, allowing four codons to be recognized by a single tRNA. In addition some codons specify different amino acids in mitochondria than in the universal code.


Differences between the Universal and Mitochondrial Genetic Codes

 Codon

 Universal code

 Human mitochondrial code

 UGA

 Stop

 Trp

 AGA

 Arg

 Stop

 AGG

 Arg

 Stop

 AUA

 Ile

 Met


Tables with codon usage (Table 10, p 95 Li and Grauer)
Acknowledgment: Modified from: Li and Graur, 1991, Fundamentals of Molecular Evolution , Sinauer Publ.


Last updated: 8 August 1997.
created by :Fred Opperdoes