SMALL RNAs
Small RNAs (non-coding RNAs, ncRNAs) are single-stranded RNAs, which are 21 to 26 nucleotides long. They participate in the process of RNA interference. Two types of small RNAs are known. They are: siRNAs and miRNAs. They differ in their origin and functioning.
MiRNAs are formed from an extended double-stranded RNA (dsRNA). The latter comes out as the result of actions of RNA-dependent RNA-polymerases, or bi-directional transcription of genes or transposones. SiRNAs usually start RNA interference for accomplishing of degradation mRNAs and/or modification of chromatin.
MicroRNAs are formed as the result of two-stage ageing process, where two enzymes of ribonuclease-III type from primary transcripts participate. These enzymes contain partially complementary inverted repeats inside them. They form double-stranded hairpin structures. MicroRNAs cause translational repression, and sometimes even degradation of mRNAs.