named enzyme Dicer'

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named enzyme Dicer.
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Imagine you could identify the role of a gene in
a disease by switching it off easily, in the
space of just a day, and in almost any organism.
Imagine you could then take this tool and treat
certain diseases, such as cancer or AIDS, by
switching off the causative genes. That's the
promise that RNAi offers. RNAi RNA interference
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RNA Coding RNA mRNA Non-coding RNAs
Transcriptional RNAs rRNA

tRNA
Small RNAs siRNA

miRNA
snoRNA (spliceosome)

smRNA (rRNA process)
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Micro RNA (miRNA) and short interfering RNA
(siRNA) use a similar mechanism to direct gene
silencing.
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(No Transcript)
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.RNAs of just 22 nucleotides in length, called
small interfering RNAs (siRNAs), are snipped from
longer dsRNA chains by an enzyme called Dicer.
The antisense strand of the siRNA is used by an
RNA-induced silencing complex (RISC) to guide
messenger RNA (mRNA) cleavage, so promoting mRNA
degradation.
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The human genome contains around 200 miRNA
genes. They represent 1 of the genome How
significant are they? Their sequence is very
conserved Expression is tissue specific They have
recognition sites on many mRNAs Why were they
not discovered before? Efforts were made on
coding RNAs
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miRNA control development in higher animals
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Example Ciliated protozoan, Tetrahymena
Thermophila. Unicellular organism, has two
nuclei. One nucleus loses 15 of his DNA during
development, through the action of small
RNAs. Other example Human. Fragile X mental
retardation is associated with a protein that
regulates a number of mRNAs, through miRNAs and
siRNAs. Other miRNAs are implicated in spinal
muscular atrophy and Chronic lymphoid leukemia
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RNAi is believed to have evolved as a defense
mechanism against RNA viruses. It is now a
powerful tool for silencing genes
experimentally, by introducing double-stranded
RNA coding for the gene researchers want to
switch off.
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Therapeutic possibilitiesTaking the DNA sequence
of a gene and designing dsRNA that can
specifically and effectively silence a
disease-related gene is analogous to monoclonal
antibody production.
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HIV. Several groups have shown that siRNAs can
inhibit HIV replication effectively in culture.
HIV infection can also be blocked by targeting
either viral genes (for example, gag, rev, tat
and env) or human genes (for example, CD4, the
principal receptor for HIV) that are involved in
the HIV life cycle. This is promising, as
antiviral therapies that can attack multiple
viral and cellular targets could circumvent
genetic resistance of HIV. But these results
have been achieved by transfecting the siRNAs
into cells, and getting the siRNAs to function in
vivo is likely to be a more difficult task.