Two classes of small regulating RNA are found in a sponge and actinia, which are known to be only at the higher animals: microRNA for regulation of the work of own genes and RNA for the control over the mobile genetic elements (transposons). The findings have shown that the difficult control systems of the genome work by means of small RNA are characteristic for the entire animal kingdom. These systems could play an important role in the progressive evolution of animals.
The RNA-interference mechanism has appeared at the first eukaryotes, and its separate elements (in particular, proteins from the Argonaute family) have been developed even earlier - at prokaryote. The primary function of the RNA-interference is protection of a cell from genomic parasites - viruses and mobile genetic elements (transposons).
In its elementary form the RNA-interference mechanism works as follows. The special protein Dicer, having found out in a cell of two-chained molecule RNA, cuts it on small slices. Two-chained molecules RNA are unrepresentative for normal cells, but they are an obligatory stage of the life cycle of many (but not all) genomic parasites. The turned out small slices of the two-chained RNA "untwine" on separate chains. Then these one-chained fragments of the "enemy" RNA (which at this stage are called siRNA - small interference RNA) join the proteins of the Argonaute family. The turned out complex consisting of Argonaute and siRNA proteins, is called RISC (RNA-induced silencing complex). The RISC complex is capable to find DNA and RNA (including one-chained) in a cell of a molecule, containing the same sequence of nucleotides, as in a siRNA molecule. Having found such molecule, RISC joins it and either destroys it, or inactivate it (here are possible many variants). Thus, the short scraps of the alien two-chained RNA serve as a "sample", on which basis large-scale search and neutralization of the genomic parasite is conducted - not only of the two-chained, but also of the one-chained stage of its life cycle, including the stage of one-chained matrix RNA, which forms a basis for protein synthesis.
On the basis of the ancient mechanism of the RNA-interference, more specialized control systems of work of genes has been developed further by means of small interference RNA. At higher animals, such as mammals, insects and roundworms, two such systems are known based on two classes small interference RNA -MicroRNA and Piwi-interacting RNA. The main difference of these systems from the "usual" RNA-interference is that small interference RNA, participating in their work, are not made from alien two-chained RNA molecules, but they are initially coded in the genome of the organism-owner.
MicroRNA genes are found in the genomes of higher animal, and also in the higher plants and some seaweed. It is necessary to specify that these genes are not coded MicroRNA, but larger molecules-predecessors. The long molecule RNA of such gene is turned by itself into a double spiral because in it there are palindrome (mirror) sequences of nucleotides, which "stick together" with each other by a principle of complementation.