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Thursday, September 30, 2010


Messsenger RNA (mRNA)of sequence of four different bases of nucleotides.A triplet of three bases is called a codon that encodes a specific amino acid.The sequence of these triplets specifies a specific sequence of amino acids in the polypeptide synthesized from it ,which is known as genetic code.In other words, the genetic code is the correspondence of codons (code words)to definite amino acids.Since there are four different bases,the number of sequence of three of them is 4^3 or 64.The genetic code exhibits the following features.

The code is universal,that is all prokaryotic and eukaryotic organisms use same codons to specify each amino acid.
More than one codon can code for same amino acid.Therefore genetic code is said to be degenerative.
Genetic code is nonoverlapping,that is adjacent codons do not overlap.
Out of 64 possible triplet codons,one is start codon(AUG)and three are nonsense or stop codon(UAA,UAG,UGA).

Sense codons and non-sense codons

The codon which specify a particular amino acid is known as sense codon.
Out of 64 codons,61 are sense codon.
Sense codon can be read by t-RNA.
Sense codon(especially AUG)is responsible for initiation of translation and elongation of amino acid sequence.

The codon which do not correspond to any amino acid is known as non sense codon.
The three non sense codons are UAA,UAG and UGA.
Non sense codon cannot be read by t-RNA.
Non sense codons are responsible for termination of translation.


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