Here is an illustration from a recent PLoS Biology paper:
Two complexes:
1- miRNA. Imperfect base pairing between the small RNA and the target. This complex sorts the RNA to p-bodies (processing bodies) where other proteins join in. The mRNA is either destroyed or stored.
2- siRNA. Perfect base pairing. The mRNA is destroyed and that's it.
A picture is worth a thousand words.
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microPrimer: the biogenesis and function of microRNA
Tingting Du and Phillip D. Zamore
i think the major distinction is whether or not it started out as a pri-miRNA trancribed from an endogenous locus. if it was processed by Drosha it's a miRNA even if it has perfect target complementarity and causes cleavage.
I guess there are two ways to define them. miRNA are derived from endogenous small hairpin RNAs (shRNAs), siRNA from exogenous transcripts or are chopped from single strand endogenous transcripts. You can also define these two classes functionally - siRNA resulting in the formation of a siRISC or RITS complex, miRNA resulting in the miRISC complex. The siRISC and miRISC having two distinct fates (see fig). In some ways the functional definition is more useful.
Here is a question. How many miRNAs are thought to have 100% complementary to their targets? Or I guess the better question is how many targets have 100% complement to their miRNA? (I'm not up on that latest figures)
My impression from my readings was also that the distinction is based on the biogenesis, and not the function. As for figures about perfect matches of miRNA targets, I don't know them either, but as far as I remember it is ALWAYS the case in plants (and yet they are called miRNAs, and not siRNAs).
And what about piRNAs? We will soon run out of short lowercase combinations for the amazing RNA bestiary. Recently I was trying to name a class of small RNAs in a theory I was working on, and each time I came to some nice idea of the form "xxRNA", Google showed me that someone had already used it.
i don't know the figures either, but its an interesting question so i might spend some time this afternoon looking..
re: names of new RNA-types.. if someone could get the clout to alter the nomenclature to things that are less clumsy to pronounce i think we would be better off.. casually, in discussions i have referred to them as "S.I.'s" and what not, but that's gonna get clumsy again for "r.a.s.i. and t.a.s.is".. it'd be nice if people could take a sec to try to shorten their descriptive abbreviation.. maybe pronounce those like "raisy and taisy"..
I beleive the structure is a big player. miRNA contain the hairpin or stem loop, whereas siRNAs are short double stranded RNAs with a 2 nt 3'overhang on either end, with a 5'p and 3'oh group