• Wobble in the Genetic Code • According to the genetic code, the cell would need tRNAs with 61 different anticodons to

complement the available 61 codons. However, due to the degeneracy of the genetic code, the third base is less discriminatory for the amino acid than the other two bases. This third position in the codon is referred to as the wobble position. At this position Us and Cs may be read by a G in the anticodon. Similarly, As and Gs may be read by a U or y (pseudouridine) in the anticodon.

The tRNAs also contain a series of unusual HYPERLINK "http://www.nobel.se/medicine/educational/dna/a/translation/trna_modification.html" modified nucleotides. One of these nucleotides is inosine (I). If the tRNA contains an I in the anticodon at the wobble position, this tRNA may read codons having As, Us or Cs in the third position.

• Nucleotideat 5' end of anticodon

• Nucleotideat 3' end of anticodon

• G• >• U or C

• The genetic code is degenerate. That is, many amino acids are encoded by more than one codon. For example, CCU, CCA, CCC, and CCG all encode proline. In some of these cases different tRNAs recognize the different codons, but certain tRNAs recognize several different codons.

• Crick suggested that the base at the 5' end of the anticodon does not have as strict base-pairing requirements as the other two base pases, allowing it to form hydrogen bonds with several bases at the 3' end of the codon [Crick, 1966].

• This is especially true when the base at the 5' end of the anticodon is inosine (abbreviated I), it is particularly "wobbly".

• (Recall that tRNAs contain a number of unusual bases that are not normally found in DNA or other RNAs.) Unlike the standard base pairing rules, the base pairing rules for this "wobble position are shown below:

• The hereditary substance consists of so called DNA molecules. These are long sequences of small molecules, tied together.

• There are four kinds of these molecules. • The sequence of these molecules constitutes the genetic code which instructs what proteins should be synthesised by the cell. Some proteins act like signal substances, controlling the activities, others are enzymes, catalysing various cellular processes and still others become the material out of which the cell is built. And all this variety of substances is governed entirely by the activity in the DNA and the sequence of the DNA code. Only a small proportion of the whole DNA of a

• cell is active.