16. miRNA & siRNA


biological process in which RNA molecules inhibit gene expression, usually through destruction of specific mRNA molecules


micro RNA
evolutionarily conserved, small non-coding RNAs that function to regulate gene expression in eukaryotes via inhibition & degradation
partially complementary to target mRNAs


small interfering RNA
short, double stranded RNA molecules that selectively degrade mRNA molecules
form chromatin structures that function to inhibit gene expression
perfectly complementary to target mRNAs

miRNA synthesis

1. RNA pol II makes primary miRNA (forms stem loop)
2. pri-mRNA ---> pre-mRNA via DROSHA
3. pre-miRNA (in nucleus) --> pre-miRNA (in cytoplasm) via exportin5-RanGTP
4. pre-miRNA processed into a small miRNA duplex via DICER, assembles into RISC, inhibits

functional strand in P body

retained to provide specificity and guidance to the target mRNA

passenger strand in P body

removed & degraded


not made in cells (introduced externally)
double stranded, form RISC, perfectly complementary

methods of siRNA introduction to cells?

aptamer-conjugated siRNA
cholesterol-conjugated siRNA
liposomal formulated siRNA
antibody complex

miRNA: translation repression

miRNA guides the complex to target mRNA sequence, binds w/partial homology to mRNA, prevents translation

mRNAs bound by miRNA are stored where?

contain argonaute & Dcpla ***this is where translation suppression occurs

siRNA uses?

exogenous dsRNA pathway
has perfect homology, leads to direct cleavage/degradation
(miRNA uses endogenous)

miRNA & siRNA can both use?

transcriptional repression
both associate with RITS
causes change in chromatin structure (helps heterochromatin stabilize--methylation) so that transcription is suppressed

how can miRNAs act as oncogenes?

cause overexpression of growth cell genes, by inhibiting apoptosis & allowing proliferation of cancer cells & inhibiting genes that prevent apoptosis & inibiting genes that regulate cell proliferation

similarities between transcriptional factors & miRNAs

combinatorial & cooperative activity
network motifs