Download RNA interference was popularized by work in C

RNA interference was popularized by work
in C.elegans. When long double-stranded
RNAs were injected into a worm’s gonad, a
standard way of introducing transgenes into
worms, they blocked the expression of
endogenous genes in the sequence specific
manner.
In eukaryotes, most protein coding genes
are transcribed by RNA polymerase II,
which generates pre-mRNAs that then
process to form mature mRNAs. These
mRNAs are then transported from the
nucleus to the cytoplasm, where they are
translated.
RNAi can regulate endogenous gene
expression. RNAi can be set in motion by
genomically encoded short regulatory RNAs
known as microRNAs. In algae, worms and
flies, RNAi can be activated by endogenous
transposition. In plants and cultured insect
cells, RNAi also has a role in antiviral
defense, in which viral double-stranded
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RNAs are targeted for destruction by the
RNAi machinery.
When long double-stranded RNAs enter a
cell, they are recognized and cleaved by
Dicer, which is a member of the RNaseIII
family of double-stranded RNA specific
endonucleases. Cleavage by Dicer creates
short double-stranded RNAs that are
characterized by two-nucleotide long 3’
overhangs. These are called small
interfering or siRNAs. siRNAs can form a
ribonucleoprotein complex called RISC, or
RNAi Silencing Complex. This complex
includes “Slicer”, an Argonaute protein
with a RNAse H-like domain called
PIWI.( Argonaute
Is the signature component of RISC)
RISC first mediates
the unwinding of siRNA duplex. A
single-stranded siRNA that is coupled to
RISC. Then binds to target mRNA in the
sequence specific manner.
The binding mediates target mRNA
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cleavage by slicer. The site of cleavage falls
in the middle of the region of siRNA
complementarity. The cleaved mRNA can
be recognized by the cell as being aberrant
and then destroyed. This prevents
translation from occurring, silencing the
expression of the gene from which the
mRNA was transcribed.
In plants, the aberrant RNA resulted from
RISC mediated cleavage can also serve as a
template for RNA dependent RNA
polymerase, or RDRP. This process relies on
unprimed RNA synthesis, in which the
aberrant RNA is used as a template. The
resulting double-strand RNA IS a substrate
for DICER activity, which generate more
siRNAs.
In
some
organisms
with
endogenous RNAi mechanisms, for example,
fungi, plants worms and mammals, RNAi
also involve another amplification step. In
this step, single-stranded siRNAs not
associated with RISC bind to their target
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RNAs in the sequence specific way, and
serve as a primer for RDRP to polymerize
the antisense RNA strand, such specificity is
intrinsically sensitive to natural sequence
variation. The double-stranded RNA
molecule that is created serves as a
substrate for DICER, which cuts it into
siRNAs. In turn this can either unwind and
prime RNA dependent RNA polymerization,
or together with RISC, mediate the cleavage
of target mRNAs. This amplification,
coupled with RNAi spreading between cells,
is thought to underlie germline transmission
of RNAi in worms. RNAi spreading has also
been described in plants, but not in
mammals.
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