Could somebody outline the differences between siRNA and miRNA?

siRNA and miRNA are proteomics tools used to study various aspects of gene expression. Depending on the source, you may find information stating that siRNA and miRNA are the same thing...or not. The two are formed in the same manner, from longer RNA precursors, and are processed in the cytoplasm by an enzyme called Dicer before becoming part of the protein complex called RISC.

The process of RNA interference (RNAi) can be moderated by either siRNA or miRNA, but there are subtle differences between the two. Both are processed inside the cell by the enzyme called Dicer and incorporated into a complex called RISC. siRNA, however, is considered exogenous double-stranded RNA that is taken up by cells, or enters via vectors like viruses, while miRNA is single stranded and comes from endogenous (made inside the cell) non-coding RNA, found within the introns of larger RNA molecules.

Another difference is that, in animals, siRNA typically binds perfectly to its mRNA target, a perfect match to the sequence, whereas miRNA can inhibit translation of many different mRNA sequences because its pairing is imperfect. In plants, miRNA tends to have a more perfectly complimentary sequence which induces mRNA cleavage as opposed to just repression of translation.

Both siRNA and miRNA can play a role in epigenetics through a process called RNA-induced transcriptional silencing (RITS). Likewise, both are important targets for therapeutic use, because of the roles they play in the controlling gene expression.

Small interfering RNA (siRNA), sometimes known as short interfering RNA or silencing RNA, are a class of 20-25 nucleotide-long double-stranded RNA molecules that play a variety of roles in biology. Most notably, siRNA is involved in the RNA interference (RNAi) pathway where the siRNA interferes with the expression of a specific gene. In addition to their role in the RNAi pathway, siRNAs also act in RNAi-related pathways, e.g. as an antiviral mechanism or in shaping the chromatin structure of a genome; the complexity of these pathways is only now being elucidated. These siRNAs are derived from processing of the trigger dsRNA by an enzyme known as Dicer in an RNase III-like fashion. The mechanism involves the recruitment of siRNA duplex products into a multi-protein siRNA complex, known as the RISC (RNA-Induced Silencing Complex). The RISC is then thought to be guided to the target mRNA of interest where the siRNA duplex interacts in a sequence-specific manner to mediate catalytic cleavage.

MicroRNAs (miRNA) are single-stranded RNA molecules of about 21-23 nucleotides in length thought to regulate the expression of other genes. miRNAs are encoded by genes that are transcribed from DNA but not translated into protein (non-coding RNA); instead they are processed from primary transcripts known as pri-miRNA to short stem-loop structures called pre-miRNA and finally to functional miRNA. Mature miRNA molecules are partially complementary to one or more messenger RNA (mRNA) molecules, and they function by targeting mRNAs for translational repression, degradation, or both. These molecules are emerging as important modulators in cellular pathways such as growth and proliferation, apoptosis, and developmental timing.

MicroRNAs were first described in 1993 by Lee, et al. (Lee RC, Feinbaum RL, Ambros V (1993) The C. elegans heterochronic gene lin-4 encodes small RNAs with antisense complementarity to lin-14. Cell 75: 843-854.) To date, thousands of miRNAs have been identified in organisms from viruses to primates through cloning and sequencing, or computational prediction based on strong conservation of miRNA sequence motifs. miRNA data are managed and annotated by the miRBase Sequences Database, hosted by the Sanger Institute as a publicly accessible repository.

Hi, miRNA and siRNA are classified on the basis of their origin.

(1) miRNA generated from an endogenous hairpin transcript and regulate the expression of another gene (trans regulator). it prevent the expression of protein.

(2) siRNA generated from exogenous dsRNA molecules (outside of a cell),virus or expression of overlapping gene to form a dsRNA. siRNA regulate the expression of sequence from which itself originate. It work by degrading the complementary DNA sequence.

• siRNA is 20-25 nucleotide-long double-stranded RNA molecules
• miRNA are single-stranded RNA molecules of about 21-23 nucleotides in length thought to regulate the expression of other genes
• So siRNA is double stranded; miRNA is single stranded

Essentially the difference is partially to do with the source of the RNA interference (RNAi), and partly down to the structure of the final molecules. Sort interfering RNA (siRNA) is exogenous (originates from outside of the cell), whereas micro RNA (miRNA) is endogenous (originates from within the cell.) Both siRNA and miRNA are non-coding and are primarily targeted at gene repression.

As miRNA are endogenous they are produced through protein/RNA interaction within the cell expression them.

They can be told apart as siRNA molecules exist in a duplex, whereas miRNA are single stranded. They can be further told apart through sequencing, miRNA tend to be less specific than siRNA being only partially complimentary to the RNA being knocked down.

siRNA is a synthetic molecule, consisting of double-stranded RNA with short single-stranded ends, which is transfected into cells and binds to RISC. siRNA is perfectly complementary to a particular target mRNA and cleaves that target. siRNA also suppresses translation of many off-target mRNAs that have partial sequence complementarity. The siRNA molecules may be indistinguishable from the double-stranded RNA molecules that are produced by the RNAi pathway, where Dicer cuts small (~21 base) chunks of dsRNA from a long piece of double-stranded RNA.

miRNA is a natural molecule, also consisting of double-stranded RNA with short single-stranded ends. Primary miRNA is transcribed from DNA and folds into a hairpin. The Drosha enzyme cuts the hairpin from the rest of the transcript, forming pre-miRNA. The Dicer enzyme cuts away the loop, forming the mature double-stranded miRNA. The double strand loads onto a complex which includes the Argonaute protein and Argonaute cleaves one strand of the dsRNA, incorporating the uncleaved single strand into the mature complex. This complex inhibits translation of partially-complementary mRNA.

The behavior of the two classes, siRNA and miRNA, is the same. Each can cleave perfectly complementary mRNA targets and decrease the expression of partially complementary targets. However, their origin is different, with siRNA an endogenous synthetic molecule and miRNA an endogenous genome-encoded molecule.

I hope more folks follow up on this, proposing alternative definitions; I am very interested in how these definitions are developing over time and I would be disappointed to have the last word here.

maybe try learing about it and answering then!

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