Transfer RNA
Transfer RNA (tRNA) availbility is one of the reasons that a silent mutation might not be silent at all. For every codon there's a different tRNA molecule. So there's a tRNA specifically for the codon UCU and another specifically for the codon UCC (and so on for all the codons). Both of those tRNA molecules carry the amino-acid serine to the ribosome that is translating a mRNA molecule. However, if there's (for example) a thousand times less UCC tRNA than UCU tRNA, then the incorporation of serine happens a thousand times slower when a mutation causes the codon to change from UCU to UCC. If it takes longer for the amino-acids to reach the ribosome, translation takes longer. This results in a lower expression of a certain gene with that 'silent' mutation. Also, if the ribosome has to wait too long, it might terminate translation prematurely.
Read more about this topic: Silent Mutation
Famous quotes containing the word transfer:
“No sociologist ... should think himself too good, even in his old age, to make tens of thousands of quite trivial computations in his head and perhaps for months at a time. One cannot with impunity try to transfer this task entirely to mechanical assistants if one wishes to figure something, even though the final result is often small indeed.”
—Max Weber (1864–1920)