Gene Cloning

288 Gene Cloning
Cleaved by DICER
siRNAs
(~22 bp long)
RISC
Associate with RISC
RISC
Separated to single-stranded RNA
RISC
Anneals with
complementary mRNA
Cleavage of mRNA by RISC
Figure 10.4 Steps in RNA interference (RNAi). Double-stranded RNA
becomes associated with the DICER enzyme complex, which cleaves the RNA
into short (about 22 base pairs) sequences. These in turn associate with the
RISC complex. The RNA is separated to give bound single-stranded RNA, which
anneals with the RISC complex to the complementary mRNA molecules. RISC
then cleaves these mRNAs, which hence prevents their translation into protein.
very effectively as the RISC complexes are catalysts that can cleave many
mRNA molecules.
Fortunately for scientists, three of the organisms where this method
works well are three of their favorite experimental model organisms: the
fruit fly Drosophila melanogaster, the nematode Caenorhabditis elegans,
and the thale cress Arabidopsis thaliana (see Box 2.3 for a discussion of
model organisms and their uses). This has made genome-wide screens of
gene function in these organisms plausible, along the same lines as the
genome-wide gene knockout study on baker’s yeast described above. The
first organism to be studied in this way was C. elegans, which is a relatively
simple multi-cellular organism and a very popular model for studying
many aspects of cell biology and development. This work was helped by a