Question

21. Discuss the 'wobble' hypothesis as it pertains to the genetic code? (500 words)

Answer 1

As per the standard (Watson- crick ) base pairing principle the cell must possess 64 different t RNA molecules for recognition of 64 possible codon sequences on mRNA. However the fact is that cells usually possess only approximately 45 different tRNA molecules which are able to recognize and pair with all 64 different codons. To explain this phenomenon , Francis Crick in 1966, proposed the Wobble concept. The word wobble literally means “to move unsteadily”. The wobble hypothesis states that the base at the 5′ end of the anticodon in addition to standard base pairing potential also shows non standard base pairing with any of the several base pairs located at the 3′ end of the codon. So, first base of the anticodon and third base of the codon is the wobble position e.g. U at 5′ end of anticodon can pair up with A or G at 3′ end of codon. Similarly I (ionosine) can pair with U, C or A. The wobble rules only permit the recognition of maximum 3 different codons by t RNA that too only when I occupies the wobble position on tRNA. Conclusively Wobble hypothesis explains the presence of synonymous tRNA molecules (i.e One tRNA molecule can pair with more than one codon).

Possible pairing combinations with wobble concept

Base in anticodon                               Base in codon

A                                                         U

G                                                         U or C

C                                                         G

U                                                         A or G

I                                                           A, U or C