Question

i. What phenomenon in genetic coding does “wobble” refer to? Give a brief description.

ii. Explain the benefit of using thymine in DNA instead of uracil. Thymine has greater resistance to photochemical mutation, making the genetic message more stable.

Answer 1

(i) The 'wobble' refers to the particular position in a genetic codon. The wobble position is the third nucleotide in a triplet codon. There is a set of base pairing rules at the third nucleotide of the genetic codon, which enables a small number of tRNA molecules to identify multiple codons. This was explained by Francis Crick in 1966 through the "Wobble hypothesis". It states that 'the third base in a triplet codon of mRNA can undergo non - Watson - Crick base pairing with the first base of a tRNA anticodon'. Therefore one tRNA molecule can recognize and attach to more than one genetic codon. The Wobble base pairing is necessary in RNA secondary structure and for the proper translation of the genetic codon.

tRNA anticodon loop Anticodon Wobble position 5 3 A G GE U C C/U io Codon 3 mRNA
  
Leu Leu 3 5' 3 5' Identical leucine tRNAS 20 .. G ........ 0 G ........ GAG Normal mRNA CUC pairing 5'... 3' G A G Wobble CU U pairing ..3 5'... © 2010 Pearson Education, Inc.

(ii) In DNA thymine is more suitable than uracil in many aspects. In DNA, cytosine undergoes chemical degradation and produces uracil. This conversion happens by the deamination of cytosine. This deamination is potentially mutagenic. In this case the normal base thymine detects and repairs this mutations more efficiently. But in RNA uracil is present instead of thymine. This is because uracil is energetically less expensive to produce than thymine. Thus uracil is more suitable for RNA, where quantity is given more importance than the lifespan. Whereas, thymine is used in DNA to enhance the fidelity (accuracy) of the genetic information.