Biology Reference
In-Depth Information
in the nematode
C. elegans
. The Nobel Prize in Physiology or Medicine was
awarded to Andrew Fire and Craig Mello in 2006 for this discovery.
The integration of transposons or viruses results in the production of ds
RNA, which activates sequence-specific degradation of homologous single-
stranded mRNA or viral genomic RNA. This prevents expression or replication of
the foreign genetic material. The ds RNA is used as the guide in this sequence-
specific RNA degradation process. Often transgenes inserted into genomes will
be silenced together with the endogenous gene from which the transgene was
derived (
Pal-Bhadra et al. 2002
). RNAi can be used by molecular biologists to
learn about gene function for which no phenotype is known (
Mohr et al. 2010
),
which is useful because
≈
30% of putative genes identified in insect genomes are
so-called orphan genes, genes without a known homolog. Determining what
these orphans do is a high priority if we are to understand their role in the biol-
ogy of insects. Even for the genes that appear to be homologs to
Drosophila
genes, additional analysis often reveals different functions and pathways.
RNAi is divided into cell-autonomous and non-cell-autonomous RNAi
(
Huvenne and Smagghe (2010)
. In
cell-autonomous RNAi
, the process is limited
to the cell in which the ds RNA is present and the whole process occurs within
individual cells. The mechanism of cell-autonomous RNAi involves the processing
of long ds RNAs into short interfering RNA (siRNA) duplexes of 21-25 nucleotides
by a ribonuclease III enzyme, called Dicer. The siRNAs are then incorporated in
the
R
NA-
I
nduced
S
ilencing
C
omplex (RISC). After the siRNA is discarded the RISC
binds to a homolog mRNA, cutting it so that translation cannot occur (
Box 9.1
).
Non-cell-autonomous RNAi
can take place in tissues or cells different from the
location of the application or production of the dsRNA. Non-cell-autonomous
RNAi is further categorized as
environmental RNAi
and
systemic RNAi
.
Environmental RNAi involves situations in which ds RNA is taken up by a cell
from the environment by a single cell. Systemic RNAi takes place in multicellu-
lar organisms and involves the transport of a silencing signal from one cell to
another or from one tissue to another. Systemic RNAi can follow environmental
RNAi in multicellular organisms.
The production of ds RNA to purposefully interfere with translation of pro-
tein-coding genes can be achieved and allows geneticists to determine gene
function. Genome-wide RNAi screening is possible using tissue-culture cells
or live
Drosophila
. The
Drosophila
RNAi Screening Center at Harvard Medical
School provides services and information on the extensive studies conducted
using
Drosophila
to study genes relevant to cell viability, metabolism, cell
death, transcription, translation, RNA biology, response to pathogen infec-
tion, neurobiology, neurodegenerative disease, signal transduction, import/
