U c c c a a a g g g u u u u g c c c c u a g a g ta g

U C C C A A A G G G U U U U G C C C C U A G A G
TA G
AT C
A U G A U U U C C A A A G G G U U U C C C U A G
DNA
(Revertant allele-
double mutant)
DNA
(Mutant allele)
mRNA
mRNA
Protein
Suppressor
mutation
(Single
base-pair
deletion)
Met Ile Ser Gln Arg Val Leu
A
T
Inserted base pair alters reading frame.
C
G
Deletion base pair restores original reading frame.
Altered amino acid sequence
Mutant
phenotype
Wild-type
phenotype
Transcription
Transcription
Translation
Translation
AT G
TA C
A T T
T A A
G T A
C A T
C C C
G G G
A A A
T T T
G G G
C C C
T T T
A A A
C C C
G G G
TA G
AT C
A U G A U U G U A C C C A A A G G G U U U C C C U A G
DNA
(Gene)
mRNA
Protein
Three base-pair insertions.
Met Ile Val Pro Lys Gly Pro (term) Phe
Original wild-type amino
acid sequence
Altered sequence
with one amino
acid added
A
T
G
C
A
T
Wild-type
phenotype
Transcription
Translation

MBG 2040 33
directed the synthesis of a mixture of 3 homopolymers
The Genetic Code
-
initiation and termination codons”
-
initiation codon: AUG
-
termination codons: UAA , UAG , UGA
-
degeneracy: partial degeneracy (pruine to pyrimidine at 3’ changes amino acids) or
complete degeneracy (any of the bases at 3’ will not change amino acids)
-
ordered
-
nearly universal (exceptions: mitochondria and some protozoa)
Key Points
-
each 20 amino acid is specified by one or more nucleotide triplets in mRNA
-
of the 64 possible triplets, given the 4 bases in mRNA, 61 specify amino acids and 3
signal chain termination
-
the code is non-overlapping, with each nucleotide part of a single codon, degenerate, with
most amino acids specified by two or four codons, and ordered, with similar amino acids
specified by related codons
-
the genetic code is nearly universal with minor exceptions, the 64 triplets have the same
meaning in all organisms
The Wobble Hypothesis
-
codons in mRNA molecules are recognized by aminoacyl-tRNAs during translation
Codon-tRNA Interactions
-
translation of a sequence of nucleotides in mRNA into the correct sequence of amino acids
in the polypeptide products requires the accurate recognition of codons by aminoacyl-
tRNAs
-
because of degeneracy of the genetic code, either several different tRNAs must recognize
the different codons specifying a given amino acid or the anticodon of a given tRNA must
be able to base-pair with several different codons
-
both of the phenomena occur
-
several tRNAs exist for certain amino acids, and some tRNAs recognize more than
one codon
Wobble Hypothesis
-
hydrogen bonding between the bases in the anticodons of tRNAs
and the codons of mRNAs follows strict base-pairing rules only
for the first 2 bases of the codon
-
the base pairing involving the third base of the codon is less
stringent allowing “wobble” at this site
-
based on molecular distances and steric (3D structure)
considerations, Crick proposed that wobble would allow several
types, but not all types, of base-pairing at the third codon base
during the codon-anticodon interaction
TABLE 12.2
Base-Pairing between the 5 ′ Base of the
Anticodons of tRNAs and the 3
′
Base
of Codons of mRNAs According to the
Wobble Hypothesis
Base in Anticodon Base in Codon
G U or C
C G
A U
U A or G
I A, U, or C

MBG 2040
Base-Pairing with Inosine at the Wobble Position
-