The ultimate determinant of the range of targets regulated by a particular miRNA depends on complementarity between mature miRNAs and mRNA targets.
Depending on the complementarity of the miRNA to its target, one miRNA may bind several regions on the same mRNA or on different mRNAs (Filipowicz et al, 2008).
Depending on the complementarity of the miRNA with the 3'-untranslated region of the target mRNA, the RISC complex will mediate either translational repression/activation or degradation of the target mRNA.
Depending on their complementarity with target mRNAs, miRNAs can either block their translation or hasten their degradation.
In addition, each microRNA can target up to hundreds of different mRNAs, the efficiency depending on the complementarity between microRNA and mRNA (Baek et al., 2008).
A miRNA is usually about 18 27 nucleotides (nt) in length and is capable of either directly degrading its messenger RNA (mRNA) target or suppressing the protein translation of its targeted mRNA, depending on the complementarity between the miRNA and its target.
On the other hand, siRNAs primarily trigger mRNA degradation, whereas miRNAs can induce either mRNA degradation or suppression of protein synthesis, or both, depending on the sequence complementarity to their targeted gene transcripts.
In mammalian cells, miRNAs regulate the expression of target genes by inducing mRNA degradation or translational repression, depending on the sequence complementarity between the small RNA and the target mRNA [31].
Protein-ligand interactions depend on the complementarities of protein and ligand properties.
MicroRNAs (miRs) (www.mirbase.org) belong to a family of small (19 to 25 nucleotides in length) noncoding RNAs that target specific sequences of mRNAs thereby regulating gene expression [ 25, 26], with the induction of translational repression or mRNA degradation, depending on the degree of complementarities between miRs and the target sequences [ 27, 28].
