The wild-type DNA 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

   sense strand                    5'- CTA TAG ATT - 3' 
                        3'- GAT ATC TAA - 5'
   produces the mRNA               5'- CUA UAG 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 tRNA_tyr(UAC) , which recognizes the mRNA codon 'UAC' and inserts 'tyr', is

   tDNA gene                 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

  tDNA gene                  3'- ATC -5'
                             5'- TAG -3'
  which produces the tRNA    3'- AUC -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 substitution (glntyr); translation of the rest of the mRNA  will proceed normally, in the correct reading frame. The mutation in the tDNA gene is said to 'suppress' the phenotypic effect of the first mutation in a protein-coding gene, and the supressor tRNA may allow enough of the mRNA to be translated to permit a 'standard' phenotype. Note that other 'stop' codons will be read incorrectly as 'tyr': however, most will be read by 'standard' tRNAs and the protein products will be terminated normally.