Name _______________ ID _____________
3.
A nonsense mutation is a mutation that converts a sense codon into a nonsense codon (a stop codon) leading to premature termination of translation. A nonsense suppressor mutation is a mutation in a tRNA gene that converts the anticodon into a sequence that is complementary to a stop codon. Such a mutation can sometimes allow the translational machinery to read through nonsense codons, nullifying the effect of the nonsense mutation. a.
Draw out the entire two-step process of a hypothetical nonsense mutation and then a complementary nonsense suppressor mutation. Annotate your diagram showing the outcome of each step. Possible structure of answer. Two sequential mutations, first in mRNA (codon: UGG →
UGA) then in tRNA
Trp
(anticodon: ACC → ACU). The first mutation would cause a premature stop codon, leading to a truncated peptide sequence. The second mutation would complement this first mutation, allowing for Trp to potentially be added instead of translation being aborted. This would suppress the original nonsense mutation (hence, a nonsense suppressor). b.
Recall that class II aaRS often recognize their cognate tRNAs through features in the acceptor stem, while class I aaRS recognize their cognate tRNAs, in part, through interactions with the anticodon. Which type of tRNA is most likely to give rise to a nonsense suppressor, one that is charged by a type I aaRS or one that is charged by a type II aaRS? Explain your answer. A nonsense suppression mutation requires a complementary change in the anticodon (as shown above). Given that class I aaRS recognize the anticodon, any change in its sequence would make them not recognize their tRNA targets, leading to an uncharged tRNA th
at wouldn’t suppress the nonsense mutation. However, a class II aaRS recognizes the acceptor stem, meaning that the anticodons of their target tRNAs can be changed to create a nonsense suppressor. 4.
In what respect is editing by Ile-RS similar to editing by DNA polymerase I? How are these editing mechanisms dissimilar? Both have secondary active sites for editing that only perform catalysis in the event of a mistake. However, gatekeeping for the Pol I editing site relies on duplex destabilization/denaturation (movement into the site) whereas gatekeeping for the Ile-
RS is sterically constrained by the shape of the active site itself. In essence, Pol I regulates movement into the active site (but doesn't regulate the active site) and Ile-RS regulates active site activity but not movement into said active site.
