Mrna stability and micrornas mrna stability: lifetimes of eukaryotic mrna vary, poly(a) tail length may play a role. Poly(a) tail reduced as mrna resides in cytoplasm. Certain destabilizing proteins in the 3" utr may affect the rate of poly(a) tail shortening. Micrornas: mirnas act by binding to site in the 3"utr of their target mrnas, studies suggest that mirnas carry out translational-level control by inducing the degradation of target mrna. Potential mechanisms by which mirnas might decrease gene expression at translational level. Tried to make flowers more purple with extra gene copy - instead white. Both rnas were being made but being degraded: c. elegans (mello and fire nobel prize) Injected worms with sense, antisense and double stranded rna complementary to transcript they wanted to knock-down. Rnai: dsrna taken up by the cells selectively degraded mrnas having the same sequence, may have evolved as a genetic immune system . Becomes associated with pre-risc a protein complex containing an.