Small interfering RNA (siRNA) are double stranded RNA molecules that are 20-25 base pairs in length and silence gene expression through RNA interference. Andrew Fire and Craig Mello discovered RNA interference in 1998 through experiments injecting double stranded RNA into nematode worms, finding it caused gene silencing. The mechanism involves dicer cleaving double stranded RNA into siRNAs which are then loaded into an RISC complex to bind and cleave target mRNAs, preventing protein synthesis from that gene. SiRNA has significance in protecting against viruses, regulating development, and suppressing transcription, and applications in research to determine protein function and potential clinical uses like cancer treatment.

1. Small interfering
RNA (Si RNA)
SUBMITTED TO- MR SAMRAT CHAUHAN
PREPARED BY- RISHABH CHALOTRA
M. PHARMACY (PHARMACOLOGY)
(M.M. COLLEGE OF PHARMACY)
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2. Contents
 Introduction
 History
 RNA interference
 Biosynthesis of Si RNA
 RNA interference Mechanism
 Significance of Si RNA
 Applications
 Conclusion
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3. Introduction
 Also known to be short interfering RNA or Silencing
RNA.
 These are a class of Double stranded RNA molecules.
 20-25 base pairs in length.
 Similar to miRNA. (Micro RNA)
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4. History
 The Nobel Prize in Physiology or
Medicine 2006 was awarded jointly
to Andrew Z. Fire and Craig C.
Mello "for their discovery of RNA
interference - gene silencing by
double-stranded RNA.“
 In 1998, the American scientists
Andrew Fire and Craig Mello
published their discovery of a
mechanism that can degrade
mRNA from a specific gene.
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5.  In 1990, plant biologist were trying to intensify the colour of the petals of Petunias,
by introducing the formation of red pigment in the flower, but instead it turned it into
white, the phenomenon was unable to explain until Fire and Mello made the
discovery.
 Fire & Mello were investigating how gene expression is regulated in Nematode
worm Caenorhabditis elegans. They inject mRNA molecule encoding protein led to
no changes in the behaviour of the worm, the genetic code of mRNA is described
as Sense sequence, and injecting antisense RNA, which can pair with mRNA , also
had no effect. But when injected sense and antisense RNA together, they observe
Unusual muscle contractions.
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6.  When sense and antisense RNA molecules meet, they bind to
each other and form double-stranded RNA.
 After a series of experiments, Fire and Mello deduced that
double-stranded RNA can silence genes.
 In every experiment, injection of double-stranded RNA carrying a
genetic code led to silencing of the gene containing that particular
code. The protein encoded by that gene was no longer formed.
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7. RNA Interference (RNAI)
 Biological process in which RNA molecules inhibit gene
expression, typically by causing the destruction of specific mRNA
molecules.
 Phenomenon in which Double stranded RNA suppresses
expression of a target protein by stimulating the specific
degradation of the target mRNA.
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8. Biosynthesis of Si RNA
 siRNA can be synthesized by using dsRNA obtained
exogenously or endogenously.
 Endogenous: By transcription of both strands of DNA to form
dsRNA.
 Exogenous: By the means of microinjecting dsRNA into the cell
or by viral infections to the cell
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9. RNA interference Mechanism
 Double stranded RNA (dsRNA) binds to the protein or emzyme
Dicer..
 It cleaves dsRNA into smaller fragments.
 One of the RNA strand is loaded into a RISC complex.
 It links the complex to the mRNA strand by base pairs.
 mRNA is cleaved and destroyed. No protein can be
synthesized.
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10. Significance of Si RNA
 RNAi protects against viral infection.
 RNAi secures genome stability by keeping mobile elements silent.
 RNAi-like mechanisms repress protein synthesis and regulate the
development of organisms.
 RNAi-like mechanisms keep chromatin condensed and suppress
transcription.
 RNAi offers a new experimental tool to repress genes specifically.
 RNAi might be a useful approach in future gene therapy.
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11. Applications
 Basic research
- Determining protein function
- Easier than a knockout and may be used for partial knockdowns.
 Clinical research
- Cancer, hypercholesterolemia, infections, developmental defects
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12. Conclusion
 RNAi is an important part of the cellular machinery that provides
viral immunity and mechanism for the control of gene expression.
 siRNA triggers function in the RNAi mechanism result in gene
suppression.
 Potential therapeutic application include viruses and cancer.
 Technical challenges: Stability, Targeting, Off-target, effects,
Immune response, Delivery.
 The field is relatively young and much remains to be discovered.
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