Silencing gene expression after transcription
RNA interference (RNAi) is a mechanism that inhibits gene expression after an RNA molecule has been transcribed. It acts in the cytoplasm and prevents translation of mRNA from selected target genes.
RNAi occurs in eukaryotes and some prokaryotes. Its effect is sequence-specific, so RNA molecules can direct the silencing machinery towards mRNA with a particular complementary base sequence.
If the target mRNA is destroyed, ribosomes can no longer translate that molecule. The corresponding protein is therefore not produced from that mRNA.
Why RNAi is specific
The specificity of RNA interference depends on complementary base pairing. A short RNA strand can bind only to an RNA sequence containing complementary bases.
This means that the RNAi mechanism can distinguish one target mRNA from other mRNA molecules present in the same cytoplasm.
target mRNA identified → mRNA degraded → translation prevented → target protein not produced
Exam Tip: RNA interference acts after transcription. If the question asks how RNAi affects gene expression, state that the target mRNA is prevented from being translated rather than saying that transcription itself is stopped.