Redundancy of the genetic code?

Question

One particular codon codes only for one amino acid, but an amino acid can be coded for by several different codons. Now according to the genetic code, the codon `UUU` codes for the amino acid phenylalanine and `UUA` codes for leucine. But, according to the Wobble Hypothesis, the base on the third position of the codon and that on the anticodon need not be complementary (which helps explain why there are very few types of tRNA molecules, inspite of there being 61 codons). If this hypothesis is true, then we could have a phenylalanine placed in a position which was meant to be for leucine, and vice versa (since the codons coding for them differ only in their third base). The same holds true for pairs like aspartic acid & glutamic acid and serine & arginine. So how does translation of a particular mRNA molecule result in the right polypeptide sequence?

Answer 1

There are still rules when it comes to Wobble base pairing (ie you can't just have any non-canonical base pair occur in the wobble position). If that were the case, then you are right that there would be loads of incorrect amino acids found in proteins. (If you google "wobble base pairing rules" you will find a table showing known wobble base pairs).

Consider the modified base inosine which can bind A/U/C on the 3' end of a codon when present in the wobble position of a tRNA. There are only a handful of known human tRNAs that use inosine wobble pairing and they still only recognize codons for a single amino acid. You would never expect to find inosine wobble pairing in tRNAs for amino acids that are encoded by fewer than 3 codons because this would lead to frequent insertion of incorrect amino acids.

That said, it is interesting to ponder the effects wobble pairing has had on protein evolution in different organisms.

Answer 2

Even though there are more than 1 nucleotide sequences to make a particular amino acid, each specific 3 letter sequence only codes for 1 specific amino acid.