RNA Interference (AQA A Level Biology)

• RNA interference (RNAi) is a form of post-transcriptional modification which occurs in the cytoplasm
• RNAi is sequence-specific silencing of gene expression and therefore can be very precise in silencing certain genes

Small interfering RNA

• In eukaryotes and some prokaryotes, translation of the mRNA produced from target genes can be inhibited by RNA interference (RNAi)
• Small, double-stranded RNA molecules called small interfering RNA (siRNA) bind to mRNA that has been transcribed from target genes (the genes to be ‘silenced’) as their base sequence is complementary
• Each siRNA is attached to a protein complex which is able to breakdown the mRNA that has been transcribed from target genes (the genes to be ‘silenced’)
  • Therefore, the mRNA is unable to be translated into proteins

The RNA interference pathway

• Double stranded RNA (dsRNA) is produced by RNA-dependent RNA polymerases (RDRs)
• dsRNA is hydrolysed into smaller fragments, roughly 23 nucleotides long, called small interfering RNAs (siRNAs)
• In the cytoplasm, siRNAs bind to protein complexes which use energy from ATP to separate the two strands of the siRNA
  • This exposes the nucleotide bases so they are able to pair with bases from an mRNA molecule

• Once the target mRNA leaves the nucleus and enters the cytoplasm,  single-stranded siRNA binds to the target mRNA through complementary base pairing
• The mRNA molecule is cut into fragments by the enzyme/protein complex associated with the siRNA
  • Cut mRNA cannot be translated and therefore will not produce proteins

• After the target mRNA has been cut up into fragments, the fragments are broken down into RNA nucleotides by enzymes

The broken-down mRNA fragments can not be translated to produce functional proteins

Therapeutic Application

• siRNAs created against viral genetic material will signal for their degradation and stop the virus from using the host's cellular machinery to replicate itself
• siRNAs can be used in cancer treatment by targeting oncogenes that have been expressed or upregulated
  • This reduces the number of proteins produced that can lead to cancer or that maintain cancerous growth