GENETIC CODE

 Genetic Code


The genetic code is non overlapping 3 nucleotide or bases code for 1 amino acids & one later is read only once.

Non overlapping codon CAT GAT – specified 2 amino acid.

#The code is non-anbiguous : The genetic code has 64 codons. Out of this 61 codons code for 20 different amino acids . However , non of the codons code for more than one amino acids . Each codon codes only for one amino acid. This clearly indicates that genetic code is non-ambigous. The code is
redundant or degeneracy of genetic code. In most of the cases several codons code for the same amino acid. Only 2 amino acids ( Tryptophan & methionine (AUG) are coded by one codon each.



This multiple system of coding is known as degenerate or redundant code system. Such system provides a protection to the organism against many armful mutations, because if one base of a codon is mutated, there are other codons which will code for the same amino acid & there will code for the same amino acid & there will be no alteration in the polypeptide chain.



#
Code has polarity :- The code has a definite direction for reading a message which is referred to as polarity . Reading a codon in opposite direction will specify for another amino acid duie to alteration in the best sequence in the code. The message in mRNA is read in 5’-3’ direction. Thus the polarity of genetic code is 5’ehd to 3’ end.

Degeneracy of Genetic code & wobble hypotheses :-

chart..


There is a considerable degeneracy in the code of 20 amino acids, only 2 amino acids i.e, methionine (AUG) & Triptophan ( UGG) are encoded by a single codon each of the remaining amino acids (phe, thr, His, Glu, Lys, Asn, Asp, Gln & Cys) are encoded by 2 codons each one i.e isolusin by three codons, 5 amino acids (Val,Pro,Thr,Ala & Gly) by 4 codons each & three amino acids ( Leu, Ser & Arg) by 6 codons each. This multiple system of coding is known as degenate or redundant
code system such system provides a protection to the organism against many harmful mutations because if one base of a codon is mutated , there are other codons which be no alteration in polypeptide chain. To explain the possible cause of degenency of codons , 1966 , Francis Grick proposed the wobble hypotheses. According to this hypothesis only the first two bases of codon have a precise pairing with the bases of the Anticodon of tRNA , while the pairing between the third bases of codon & anticodon may wobble (to sway or move unstable ) The phenomenon permits single DNA to recognised more than one codon . Therefore, although there are 61 codons amino acid, the no. of tRNA is for less ( around 40) which is due to wobbling.
The wobble hypothesis states that the base at 5’ end of the anticodon is not specially confined as the other two bases allowing it to form hydrogen bonds with any of several bases located at 3’ end of codon.





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