Understanding the post transitional mechanism is a challenging task.
The processing of both type of small RNA are almost same and the length as reports so far are 21 to 23 nucleotides for siRNA and 18-25 nt for microRNA. The basic difference is on their mechanism of action; siRNA in general have perfectly complementarity but it not so much perfect for miRNA to target its transcripts. So, one miRNA is able to target a number of different transcripts. another basic difference is siRNAs in general shut down gene expression at a post-transcriptional level through mRNA degradation, while miRNAs silence their target genes mainly through translational repression.
One of the easiest way to distinguish siRNA and miRNA is the synthesis pathway. miRNAs are transcribed under the promoters as usual mRNAs, whereas siRNAs are not.
Are they structurally similar? what about their length?
I read that they form by same mechanism. among them which is small that is siRNA & which is larger than that of siRNA is miRNA, is it true?
The processing of both type of small RNA are almost same and the length as reports so far are 21 to 23 nucleotides for siRNA and 18-25 nt for microRNA. The basic difference is on their mechanism of action; siRNA in general have perfectly complementarity but it not so much perfect for miRNA to target its transcripts. So, one miRNA is able to target a number of different transcripts. another basic difference is siRNAs in general shut down gene expression at a post-transcriptional level through mRNA degradation, while miRNAs silence their target genes mainly through translational repression.
Just to add a few more informations. The mechanism of action of both siRNAs and mIRNAs is mediated by Dicer and RISC complex. Dicer will cleave either pre-miRNAs or dsRNA to generate the final active molecules. As already pointed out when the complementarity between the acting RNA and the target is complete, we have a mRNA degradation (siRNA) induced by Ago2 that harbors endonuclease activity. In the case of miRNAs the complementarity is only partial with the target and we got translational repression mediated by RISC. Interestingly in plants miRNA are different from a animals, showing almost perfect matching with their targets and in this case they act as siRNAs.
miRNAs are derived from gene units. The origin of endogenous siRNAs is more polemic. In fact some siRNAs appeared to be generated from Dicer action over big dsRNAs generated from repeated regions in the genome.
Hope it helps..
This post is helpful as i was also confused by this problem. Thanks!
@Shankar Suman you mentioned that miRNAs silence their target genes mainly through translational repression. however, it may not be true for mammalian microRNAs, predominantly acting to decrease target mRNA levels as described by a Nature paper. here is the link
http://www.nature.com/nature/journal/v466/n7308/full/nature09267.html
Hi Girish,
Briefly, it could be understood that siRNA is a dounble-stranded RNA molucule that could be chemically synthesized and introduced to the host cells as exogenous RNA molecules. miRNA, however, is produced endogenously by the cell itself. I hope the link below could help.
I also wonder what the differences between miRNA and shRNA are. So I hope anyone could help me clarify these 2 terms. Thank you in advance.
http://principlesofmolecularvirology.blogspot.jp/2013/11/the-great-rna-confusion-rnai-mirna.html
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