Ans. is 'c' i.e., Gene Silencing RNA o There is a class of genes that encode for gene-silencing RNAs, i.e. RNAs that do not encode proteins but instead inhibit gene expression (unlike mRNAs) which encode for proteinso Two important gene-silencing RNAs are :- (i) Micro RNAs, and (ii) small interfering RNAs (siRNAs). i) Micro RNAs (mRNAs)# miRNA is transcribed by RNA polymerase //. The transcription of miRNA gene produces primary miRNA transcripts, which is processed in nucleus by a RNAase called Dorsha. Dorsha acts with a nuclear double stranded RNA binding protein called DGCR 8 and converts primary miRNA transcript into pre-miRNA. Pre- miRNA is transported to cytosol by a transport protein (exportin 5). Dicer (cytoplasmic enzyme) does the additional cutting of pre-miRNA to produce mi-RNA duplex, i.e. double stranded miRNA. This double strand unwinds and one strand of this duplex is incorporated into multiprotein complex called RNA induced silencing complex (RCIS), which help in gene regulation.# Base-pairing between the miRNA strand and its target mRNA (messenger RNA) directs RISC to either cause mRNA cleavage or repress its translation. In this way, the gene from which the target was derived is silenced (at a post-transcription level).ii) Small interfering RNAs (siRNAs)# siRNAs work in similar manner as miRNA. Unlike miRNA, siRNA precursors are introduced by investigators into cells.# siRNA are becoming powerful tools for studying gene function and may in the future be used therapeutically to silence specific genes, such as oncogenes, whose products are involved in neoplastic transformation