RESUMO
Generally gene silencing is applied to reduction of expression of gene or RNA sequence that is expressed. This phenomenal has been discovered among plants for the first time and have been named "post transcriptional gene silencing". This phenomenai has been named quelling in fungus and RNA interference in animals. Totally gene silencing is considered at two different levels, transcriptional gene silencing and post transcriptional gene silencing. Transcriptional gene silencing is performed by mutilation of promoter of gene that in this process. One group of methyl is joined to 5' of cytosine base by methyl transferase enzyme that this action is led to stop of transcription. General important and common factor, collaborate in gene silencing that include MRNA, double-strand RNA and analyzing of protein complex. Post transcriptional gene silencing begins with creation of double-strand RNA, then, this RNA is divided to smaller fragments about 21-26 nucleotides that is called small RNA interference by DICER system. Then, this small RNA interference in a special complex which is called RISC, that is able to distinguish and analyze of complementary mRNA. Gene silencing is transferred to adjacent cells by plasmodesmata and is spread to all of plants by vessel. Small interfering RNA under takes the role of silencing signals