2. • 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.
• Nucleotide
at 5' end of
anticodon
• Nucleotide
at 3' end of
anticodon
• G
• >
• U or C
3. • 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:
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23. • 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.
