What is the different between microRNA and siRNA? - (Feb/10/2005 )

Dear all
I would like to ask the difference between microRNA and siRNA?

Thx all

siRNAs are chemically synthesised molecules that are transfected into mammallian cells and are 21 nt of double stranded RNA. They match perfectly 21 nt of sequence of their target gene. By enducing the RNAi machinery they knockdown their target RNA.

miRNAs are endogenous to every cell. They are processed out of long RNA transcripts to produce mature miRNAs. Processing also involves components of the RNAi machinery. Once processed they seem to bind to their target mRNAs, however, the complementary is not 100%, with a couple of base mismatches. THis seems to modulate the translation of the mRNA rather than knocking down RNA levels.

Yeah, that's mostly it. You can check:

Expanding roles for miRNAs and siRNAs in cell regulation (Curr Op Cell Biol Volume 16:127-133, 1004 )

MiRNAs and siRNAs can inhibit mRNA expression by similar mechanisms (PNAS 100:9779-9784, 2003)

hi

miRNA are non coding genes present in various organisms. They are transcribed by the DNA polymerase II (yontaee lee et al, Emboj, 2004 vol 23 4051-4060).
The natural RNAi machinery not only keeps the mobile transposable elements from disrupting the integrity of genomes, as was suggested by analyses in lower plants, A. thaliana, C. elegans, D. melanogaster, and animals , but also participates in organism development. Genetic defects in C. elegans RNAi genes ego1 and dicer cause known, specific developmental errors.
Similarly, the Argonaute family of genes of A. thaliana (especially the ZWILLE proteins) is also responsible for plant architecture and meristem development, and the Dicer homologue of A. thaliana, CAF1, is required for embryo development . Thus, genetic evidence illustrates the role of the RNAi machinery as a controller of development-related genes.
miRNA are long molecules that get an hairpin structure naturally. They are subsequently processed by Drosha (yontaee lee et al, nature 2003 vol 405, 415-9) and get into the risc complex.

As said by expresson_help, siRNA are most of time, short double strand DNA molecules (about 21-25nt) that perfectly match with their targeted sequence. But it seems that only 14 contigous matches are required, and one or two mismatch are accepted without loosing specificity. But siRNA can also be trascribed in vivo or in cellulo, by coupling an siRNA cassette under control of a pol III promoter (U6 in most of case, but also H1, Valine and methionine derivated) or polII promoter (CMV and recently U1promoter (denti et al))

If you want an excellent overview of these phenomenon, i recomend you the review of Agrawal et al., in Microbiol. Mol. Biol. Rev., vol 67 2003, pages 657-685.

Thx all of your help
Thank you