The triplet code is a degenerate one with many more codons than the number of amino acid types coded. An explanation for this degeneracy is provided by the ‘wobble hypothesis’ proposed by Crick (1966). Since there are 61 codons specifying amino acids, the cell should contain 61 different tRNA molecules, each with a different anticodon. Actually, however, the number of tRNA molecule types discovered is much Jess than 61. This implies that the anticodons of some tRNAs read more than one codon on mRNA. According to the wobble hypothesis only the first two positions of a triplet codon on mRNA have a precise pairing with the bases of the tRNA anticodon. The pairing of the third position bases of the codon may be ambiguous, and varies according to the nucleotide present in this position. Thus a single tRNA type is able to recognize two or more codons differing only in the third base. The anticodon UCG of serine tRNA recognizes two codons, AGC and AGU. The bonding between UCG and AGC follows the usual Watson-Crick pairing pattern. In UCGAGU pairing, however, hydrogen bonding takes place between G and U. This is a departure from the usual Watson-Crick pairing mechanism where G pairs with C and A with U. Such interaction between the third bases is referred to as ‘wobble pairing’. The degeneracy of the code is not random. Mostly, the different co dons for a particular amino acid have the same first two letters (leucine, serine and arginine are exceptions). Thus the first two letters of all the four codons for valine are GU and for alanine GC.