Given the mRNA sequence shown below, if translation were to start at the first base, what would the tRNA anticodon be for the last amino acid translated in the chain ?
5’----GCACCGGCCUGACUAUAA—3’
1.3’GCG 5’
2.3’CGC 5’
3.3’GAU 5’
4.5’CGG 3’
5.3’CGG 5’
SOLUTION
The correct tRNA sequence is 3’ CGG 5’. You must know that UGA, UAA, UAG are stop codons in the mRNA sequence. The codon immediately before the stop codon is the last one translated.
mRNA is read 5’ to 3’, so the complementary tRNA sequence (anticodon) is 3’ to 5’. Using these two rules, the first stop codon in the mRNA sequence above is 5’ UGA 3’. The mRNA sequence immediately before that is 5’ GCC 3’. Therefore the tRNA sequence should be 3’ CGG 5’.
3’----CGUGGCCGGxxxxxxxxxx—5’
5’----GCACCGGCCUGACUAUAA—3’
Illustration A shows the steps in translation of mRNA to tRNA.