Micro RNAs are short ( 18-24 nt ) ribonucleic acid molecules performing regulatory functions at the posttranscriptional level . Micro RNAs are involved in the regulation of key developmental processes in eukaryotes . The maturation process of the micro RNAs in plants involves several proteins: DCL1 , HYL1 , Serrat , HEN1 , DDL , HASTY . Biogenesis micro RNA molecules in plants and animals differ in detail. Very little is known about the nucleotide sequences of micro RNA precursors and on the structure of the genes encoding a micro RNA molecules in plants.

Alternative splicing is a process in which a single pre-mRNA is formed of more than one isoform mRNA. Type nascent mRNA isoform is the result of a fairly complex regulatory mechanisms that allow to obtain , for example, tissue- specific splicing pattern or changes in cellular splicing profile at various stages of development. This is possible thanks to the cooperation of a number of elements , among which are: the nucleotide sequence and secondary structure of pre- mRNA splicing factors and additional factors proteinaceous or nonproteinaceous .

The 2006 Nobel Prize in Physiology or Medicine was awarded to two American scientists – Andrew Fire and Craig Mello for the fundamental discoveries in gene expression regulation in eukaryotes. They have shown that the presence of double-stranded RNA in the cell induces specific gene silencing. This pheno- menon was called RNA interference – RNAi. Double-stranded RNA is digested in the cell to short effector RNAs, called siRNAs, which are directly responsible for the selective degradation of target mRNA. The process takes place in the cytoplasm.