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by: Stephen Jones
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RNA interference (RNAi) is the process of mRNA degradation
that is induced by double-stranded RNA in a sequence-specific manner. RNAi has
been observed in all eukaryotes, from yeast to mammals. The RNAi pathway is
thought to be an ancient mechanism for protecting the host and its genome
against viruses and rogue genetic elements that use double-stranded RNA (dsRNA)
in their life cycles. They have also been shown to play a role not only in mRNA
and dsRNA stability/degradation, but also in regulation of translation,
transcription, chromatin structure, and genome integrity. In plants and
animals, RNA silencing has been adapted to play a critical role in regulation
of cell growth and differentiation using a class of small RNAs. In the RNA
interference process, the dsRNAs get processed into 20-25 nucleotide (nt) small
RNAs by an RNase III-like enzyme called Dicer. Then, the small RNAs assemble
into endoribonuclease-containing complexes known as RNA-induced silencing
complexes (RISCs), unwinding in the process. The small RNA strands subsequently
guide the RISCs to complementary RNA molecules, where they cleave and destroy
the cognate RNA (effecter step). Cleavage of cognate RNA takes place near the
middle of the region bound by the siRNA strand. The small RNAs that provide
target specificity to the silencing machinery includes short interfering RNAs
(siRNAs), repeat-associated siRNAs (rasiRNAs), and microRNAs (miRNAs) and is
distinguished by their origin. siRNAs are processed from dsRNA precursors made
up of two distinct strands of perfectly base-paired RNA, while miRNAs originate
from a single, long transcript that forms imperfectly base-paired hairpin structures.
siRNAs were originally identified as intermediates in the RNAi pathway after
induction by exogenous dsRNA; however, endogenous sources of siRNAs have now
been recognized. The endogenous siRNAs are derived from repetitive sequences
within the genome, and are termed repeat-associated siRNAs, or rasiRNAs. miRNAs
were discovered through their critical roles in development and cellular
regulation, and represent a large class of evolutionarily conserved RNAs.
miRNAs have always been recognized as being of endogenous origin. RNA
interference has emerged as a natural mechanism for silencing gene expression
over the past decade. This ancient cellular antiviral response can be harnessed
to allow specific inhibition of the function of any chosen target genes, including
those involved in causing diseases such as cancer, AIDS, and hepatitis. It is
already proving to be an invaluable research tool, allowing much more rapid
characterization of the function of known genes. More importantly, the
technology considerably bolsters functional genomics to aid in the
identification of novel genes involved in disease processes. Last but not the
least the technology can be harnessed as a novel therapeutic agent and is
suitable for combating viral diseases, cancers and inflammatory diseases. Imgenex (
IMGENEX India Pvt Ltd. the only
biotech company in Orissa and one of its kinds in