It is seen that most of the amino acids (except methionine and tryptophan) can be directed to their specific places in
the peptide chain by more than one codon. The multiple system of coding is known as degenerate system. An
explanation for this degeneracy is provided by the "wobble hypothesis proposed by Crick (1966) with the help of the structure of anticodon of RNA.
Only 20 amino acids are involved in protein synthesis, while there are 64 codons. Hence more than 1 codon can code for an amino acid. If there are alternate codes, they act as a protection against mutation in a specificbase. Hence degeneracy of codes is must. Regarding RNA present in cytoplasm, since they have to bind themselves to
a specific codon, there should have been as many tRNAS as there are codons. But the number of tRNA is only as many as there are amino acids. This means, the anticodons of tRNAs must be able to 'read' more than one codon of mRNA. According to Crick's hypothesis only the first two bases of codon have a precise pairing with the bases of the anticodon of fRNA, while the pairing between the third bases of codon and anticodon may wobble (non-specific) The pairing in the third base is ambiguous. He postulated
that hydrogen bonding at the third position of the codon- anticodon
interaction is less constrained and need not adhere as specifically to the established base-pairing rules.The wobble hypothesis thus proposes a more flexible set of base-pairing rules. This relaxed base-pairing requirement, or "wobble", allows the anticodon of a single form of RNA to pair with more than one triplet in mRNA. Consistent with the wobble hypothesis and the degeneracy of the code, U at the first position (the 5' end) of the tRNA anticodon may pair with A or G at the third position (the 3' end) of the mRNA codon and G may likewise pair with U or C. Inosine, one of the modified bases found in tRNA, may pair with C, U or A. Applying these wobble rules, a minimum of about 30 different tRNA species is necessary toaccommodate the 61 triplets specifying an amino acid. Currently it is estimated that there are 30-40 tRNA species present in bacteria and upto 50 tRNA species in animal and plant cells.
