3. Wobble hypothesis
• Crick (1996) proposed the 'wobble hypothesis' to
explain the degeneracy of the genetic code.
• Except for tryptophan and methionine, more than
one codons direct the synthesis of one amino acid.
• There are 61codons that synthesis amino acids,
therefore, there must be 61 tRNAs each having
different anticodons. But the total number of
tRNAs is less than 61.
4. • This may be explained that the anticodons of
some tRNA read more than one codon.
• In addition identity of the third codon seems
to be unimportant. For example CGU, CGC,
CGA and CGG all code for arginine.
• It appears that CG specifies arginine and the
third letter is not important. Conventionally
the codons are written from 5'end to 3'end.
5. • Therefore the first and second bases specify amino
acids in some cases.
• According to the wobble hypothesis only the first and
second bases of the triple codon 5‘→3' mRNA pair
with the bases of the anticodon of tRNA, i.e., A with U
or G with C.
• The pairing of the third base varies according to the
base at this position for example G may pair with C
the convention pairing [A =U , G=C] is known as
wotson - Crick pairing and the second abnormal
pairing is called wobble pairing.
6. • This was observed from the discovery that the
anticodon of Yeast alanine tRNA contains the
nucleoside inosine in the first position[ 5'→3'] that
paired with the third base of the codon [5'→3'].
• Inosine was also found at the first position in
other tRNA. e.g. isoleucine and serine.
• The purine inosine is a wobble nucleotide and is
similar to guanine which normally pairs with A,
U,G, and C.
7.
8. • For example a glycine tRNA with anticodon 5'TCC
3' will glycine codons GGU, GGA, and GGG.
• The U at the wobble position will be able to pair
with an adenine or a guanine.
• Due to the wobble base pairing one tRNA
becomes able to recognise more than one codons
for an individual amino acids.
9.
10. • By direct sequence of several tRNA molecules the
wobble hypothesis is confirmed which explain the
pattern of redundancy in genetic code in some
anticodon [e.g. the anticodons containing U,I, and
G in the first position in 5'→3' direction.]
• Generally Watson-Crick pairing occurs between
AGC and GCU.however in AGU and GCU pairing
hydrogen bonds are formed between G and U.
• Such abnormal pairing called wobble pairing.
11.
12. • Three types of wobble pairing have
been proposed.
1. U in the wobble position of the tRNA anticodon
pairs with A or G of codon.
2. G pairs with U or C .
3. I pairs with A ,U,or C.
13. New Genetic Codes
1. Genetic code in Mitochondria
• Earlier we described that the genetic code is
universal.
• It was also believed that the genetic code does not
undergo any kind of evolution and therefore
should be static.
• However during the last few years it was shown
that variations in genetic code are found in
Mitochondria particularly studied in Yeast and
mammals.
14. • In the case of Yeast Mitochondria UGA codes for
tryptophan, although in the nuclear genes UGA is
a termination codon.
• Similar the codons beginning with CU represent
The instead of Leu and the AUA codon represent
Met instead of Ile.
• It has also been shown that although a new
genetic code may exist in Mitochondria, but
Mitochondria in all organism may not have the
same genetic code.
15. • For instance, UGA in mitochondria does not
always code for tryptophan.
• In the gene coding for maize cytochrome oxidase
subunit II , UGA codon is not present and
tryptophan is coded by codon CGG codes for
methionine in mitochondria of mammals,
xenopus, Yeast and Drosophila, but not in
Neurospora and Aspergillus.
16. • It has also been shown that in mitochondria a
number of single tRNA species with U in the
wobble position read all four codons in a family.
• It is also shown that there are only 22 tRNAs in
mitochondria as against 55 tRNAs in universal
code and these are adequate for reading 60
codons the remaining four being termination
codons.
17. • 2 . Genetic Code in Ciliate Protozoa
• In 1986 a different genetic code was shown to be
present in Ciliate protozoa ( Mycoplasma
capricolum).
• In this genetic code codons UAA and UAG specify
glutamune instead of stop signals.
• In future, more such cases may be discovered
showing diversity in the genetic code.
