What are the differences between miRNA and siRNA?

Although both miRNA (micro RNA) and siRNA (small interfering RNA) are small RNA molecules involved in RNA interference and work through similar mechanisms, there are some differences between these two molecules.

• Origin: The siRNA is an exogenous double-stranded RNA uptaken by cells, while miRNA is single-stranded and comes from endogenous non-coding RNA. Besides, the siRNA is present in lower animals and plants, but not found in mammals; whereas miRNAs are present in all the animal and plant.
• Structure: The siRNA is a 21-23 nucleotide long RNA duplex with a dinucleotide 3’ overhang, whereas the miRNA is a 19-25 nucleotide RNA hairpin which forms duplex by binding with each other.
• Target: The siRNA is highly specific with only one mRNA target, while miRNA can inhibit translation of multiple mRNA targets because of its imperfection in pairing.
• Purpose: The siRNA is primarily to provide viral defense and genome stability while the miRNA functions as endogenous gene expression regulator.