Biology Reference
In-Depth Information
double-stranded RNA (ds RNA) is introduced into the organism or produced naturally.
Dicer, an RNase III enzyme, cleaves the ds RNA into fragments called small interfering
RNAs (siRNAs) that are 21-23 nt in length.
The siRNAs unwind and the antisense strand binds to the RNA-induced silencing complex
(RISC), which includes Argonaute proteins. Argonaute cleaves and eliminates the sense
strand of the siRNA duplex, leading to an active RISC.
The complex of small interfering RNAs (siRNAs) and RISC couples to the target
homologous mRNA.
The mRNA is cleaved and is unable to be translated.
Figure 2.8
Simplified outline of the steps in RNAi. RNAi is an evolutionarily conserved response to
the invasion of mobile elements (transposable elements or viruses). It also allows loss-of-function
phenotypes to be studied when no mutants are available. In some organisms, feeding or injection
of ds RNA into the body can induce the RNAi response systemically. Some insects seem to lack the
ability to respond to RNAi in a systemic manner.
Drosophila melanogaster
, for example, does not show
a robust RNAi response.
quinquefasciatus
and found there were
>
60 conserved and seven novel miRNAs,
some of which are implicated in viral infections in these mosquito vectors.
2.15 Gene Regulation in Eukaryotes
At any one time, only
≈
15% of all the genes in an insect cell are turned on
(
Harshman and James 1998
). Thus, insect development, behavior, and repro-
duction are determined by the expression of different genes at different times
in different tissues. Research on the control of gene expression in
Drosophila
