The Genetic Code

James Richard Fromm


The relationship between the order of bases of DNA and the order of amino acids specified by them is known as the genetic code. The genetic code is given in the Table below. As far as is known, all forms of life follow this coding system from the simplest of viruses through man.

The code for each amino acid is a triplet of bases, read always in the same direction (5'-3'), and this triplet is known as a codon. There are no punctuation marks or separators indicating the beginning or end of a codon. Since there are only four different bases in DNA and a codon is made up of three bases in sequence, statistically there can be only 64 codons. Of the 64 possible codons, three (UAA, UAG, UGA, which are known as ochre, amber, and umber respectively) are believed to be used as signals which end the synthesis of a protein chain; the remaining 61 codons each code for amino acids.


Table: The Genetic Code
Codon Acid Codon Acid Codon Acid Codon Acid
UUU Phe UCU Ser UAU Tyr UGU Cys
UUC Phe UCC Ser UAC Tyr UGC Cys
UUA Leu UCA Ser UAA Ochre UGA (Umber)
UUG Leu UCG Ser UAG Amber UGG Trp
               
CUU Leu CCU Pro CAU His CGU Arg
CUC Leu CCC Pro CAC His CGC Arg
CUA Leu CCA Pro CAA Gln CGA Arg
CUG Leu CCG Pro CAG Gln CGG Arg
               
AUU Ile ACU Thr AAU Asn AGU Ser
AUC Ile ACC Thr AAC Asn AGC Ser
AUA Ile ACA Thr AAA Lys AGA Arg
AUG Met ACG Thr AAG Lys AGG Arg
               
GUU Val GCU Ala GAU Asp GGU Gly
GUC Val GCC Ala GAC Asp GGC Gly
CUA Val GCA Ala GAA Glu GGA Gly
GUG Val GCG Ala GAG Glu GGG Gly

Since several different codons code for the same amino acid, it has been suggested that the modern three-base genetic code developed from a primordial two-base codon system. Such a system could code for only 15 amino acids plus an end symbol, since only 16 different codons are possible in a two-base system. A plausible primordial code is given in the Table below.

Mutation and the Genetic Code

Errors in transcription of the genetic code can occur, but as long as the DNA chain is intact the redundancy of information inherent in the double helix structure acts to preserve the information. Should one chain break (as may be induced by chemical agents or ionizing radiation) the complementary chain is available as a template for resynthesis of the damaged chain. This resynthesis is carried out by appropriate enzymes.


Table: Possible Primordial Genetic Code
Codon Acid Codon Acid Codon Acid Codon Acid
               
UU Phe UC Ser UA End UG Cys
CU Leu CC Pro CA His CG Arg
AU Ile AC Thr AA Lys AG Ser
GU Val GC Ala GA Asp* GG Gly

Notes to Table: * indicates "or Glu". The codons UU, UC, CU, CC, CG, AU, GC, and GG are preserved in the modern three-base genetic code; alteration of the third base does not change the amino acid. Amino acids found in the modern genetic code but not in this one are Met, Tyr, Glu, Asn, Asp*, and Trp.


Mutation of the DNA code can occur in several ways: insertion of one or more bases; deletion of one or more bases; alteration of one base into another (missense mutations); or frame-shift mutations, in which the sequences of bases remains the same and the reading frame shifts along the sequence. The effects of these vary. Deletion or addition of a base is not common, and the usual effect is to produce nonsense coding from that point on; the erroneous base is usually replaced or excised by the appropriate enzymes.

Missense mutations either yield a missense triplet codon, in which case one amino acid in the protein is replaced with another, or a different codon giving the same amino acid, in which case there is no change in the resultant protein.


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Copyright 1997 James R. Fromm