miRNA | siRNA | |
Structure | Single stranded, 19–25 nucleotides long | Double stranded, 21–22 nucleotides long |
Origin | Endogenous to every cell Transcribed from genomic DNA | Chemically synthesised molecules that are transfected into mammalian cells Occur naturally in viruses, plants and lower animals |
Complementarity to target mRNA | Not perfect, therefore a single miRNA may target several hundred mRNAs | 100% perfect complementarity, therefore siRNAs knock down transcription of specific genes Only very minor off-target exceptions |
Biogenesis | Expressed by genes whose purpose is to make miRNAs | Regulate the same genes that express them |
Action | Inhibit mRNA translation mRNA destabilisation | Cleave mRNA |
Function | Regulate gene expression (mRNA) | Gene silencing proto-immune system in some organisms |
Adapted from [10] by permission from Macmillan Publishers Ltd: Nature Biotechnology, copyright 2007.