RNA: The Transcribed Sequence 49
target genes’ mRNAs. Conversely, one mRNA may be targeted by multiple
miRNAs.
3.4.4.2 siRNA
While being similar in size and using basically the same system for gene
silencing (Figure 3.4), siRNA differs from miRNA in a number of aspects. On
origin, siRNA is usually exogenously introduced, such as from viral inva-
sion or articial injection. But they can also be generated endogenously, e.g.,
from repeat- sequence- generated transcripts (such as those from telomeres
or transposons), or RNAs synthesized from convergent transcription (in
which both strands of a DNA sequence are transcribed from the two opposite
orientations with corresponding promoters), or other naturally occurring
sense- antisense transcript pairs ...