Principles of Biochemistry

7.4.2 Genetic code

The nucleotide sequence of an mRNA molecule is the informational part of such a molecule. The unit of this base sequence in a given mRNA determines the amino acid sequence for the protein synthesized under that mRNA’s direction.

 

The letters A, G, T, and C correspond to the nucleotides found in DNA. They are organized into three-letter code words called codons, and the collection of these codons makes up the genetic code. The genetic Code is universal. A codon is a three-nucleotide sequence in an mRNA molecule that codes for a specific amino acid. The genetic code is the assignment of the 64 mRNA codons to specific amino acids.

 

One codon, AUG codes for the initiation of the protein synthesis and hence called initiation codon. Three combinations were termed termination codons-UAA, UGA and UAG. The remaining 60 codons code for 20 amino acids .

First Nucleotide	Second Nucleotide				Third Nucleotide
	U	C	A	G
U	Phe Phe Leu Leu	Ser Ser Ser Ser	Tyr Tyr Term Term	Cys Cys Term Trp	U C A G
C	Leu Leu Leu Leu	Pro Pro Pro Pro	His His Gln Gln	Arg Arg Arg Arg	U C A G
A	lle lle      lle Met	Thr Thr Thr Thr	Asn Asn Lys Lys	Ser Ser Arg Arg	U C A G
G	Val Val Val Val	Ala Ala Ala Ala	Asp Asp Glu Glu	Gly Gly Gly Gly	U C A G

 

Thus there must be “degeneracy” in the genetic code; i.e., multiple codons must code the same amino acid. Some amino acids are encoded by several codons; e.g. six different codons specify serine. Other amino acids such as methionine and tryptophan, have a single codon. In general, the third nucleotide in a codon is less important then the other two in determining the specific amino acid to be incorporated, and this accounts for most of the degeneracy of the code. However, for any specific codon, only a single amino acid is indicated; with rare exceptions, the genetic code is unambiguous i.e, given a specific codon, only a single amino acid is indicated.