The terms are not very tightly defined; there is disagreement about their usage. I think this is a useful rule-of-thumb distinction: siRNA is exogenous (introduced by researchers), miRNA is endogenous (produced naturally by the cells). In general, an siRNA is completely complementary to its intended target while an miRNA is partially complementary to the mRNAs it regulates. A sequence associated with RISC which is completely complementary to an mRNA usually induces cleavage of the mRNA by the argonaute protein in RISC, while the translation of a partially-complementary target can by altered (usually suppressed) by RISC. A complementary miRNA might, in addition to cleaving its target, also alter translation of other, partially-complementary mRNAs; this leads to the widespread of-target gene regulation typically seen with siRNA. While endogenous miRNAs also regulate expression of many mRNAs, their long history ensures that this regulation has been tuned by selection pressure and the cell is well-adapted to the complex regulation of the miRNA. An siRNA, on the other hand, when introduced to a cell may have catastrophic effects because unexpected off-target interactions have not been experienced by the cell over evolutionary time.
__________________ Jon D. Moulton, Ph.D.
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Last edited by Jon Moulton; 11-05-2012 at 04:10 PM.
Reason: Tense change of verb