sequence 5'- CTA
CAG ATT - 3'
3'- GAT GTC TAA - 5'
produces the mRNA 5'- CUA CAG AUU - 3'
which codes for the polypeptide - leu-gln-iso - [etc]
A 1st position mutation (CT) in the second triplet gives a 'mutant' DNA sequence
5'- CTA AG
ATT - 3'
3'- GAT TC TAA - 5'
produces the mRNA 5'- CUA AG AUU - 3'
which codes for the polypeptide - leu- *
because UAG is a 'stop' codon (the so-called amber stop). Chain growth in the polypeptide terminates prematurely.
The portion of the transfer RNA gene (tDNA) that codes for the anticodon loop of tRNAtyr(UAC) , which recognizes the mRNA codon 'UAC' and inserts 'tyr', is
3'- ATG -5'
5'- TAC -3'
which produces the tRNA 3'- AUG -5' anticodon loop
which reads the mRNA 5'- UAC -3'
as - tyr -
A 3rd-position mutation (CG) on the 5'3' strand in this region produces a 'mutant' tDNA gene
5'- TA -3'
which produces the tRNA 3'- AU -5' anticodon loop
which reads the mRNA 5'- UAG -3'
as - tyr -
Then, the mutated tDNA gene produces a tRNA molecule that will read the the mutated gln codon UAG in part 1 as a tyr codon, instead of as a 'stop' codon.
The polypeptide will
have an amino acid
of the rest of the mRNA will proceed normally, in the
correct reading frame. The mutation in the tDNA gene 'suppresses'
the phenotypic effect of the mutation in
a protein-coding gene, because the supressor
tRNA allows enough of the mRNA to be
to permit a 'standard' phenotype. Note that other 'stop' codons
be read incorrectly as 'tyr': however, most will be read by
tRNAs and the protein products
will be terminated normally.