FIGURE 11.13 Experimental Induction of RNA Interference

RNA interference occurs when both the sense and antisense RNA of a gene are present and form dsRNA. Two constructs are shown that direct the synthesis of a dsRNA molecule. The first construct (A) has a sense region and an antisense region that base pair. A spacer separates the sense and antisense regions and forms a loop at the end of the hairpin. The double promoter construct (B) has a promoter that directs the transcription of the sense strand, and another promoter for the antisense strand. The two resulting RNA molecules are complementary and form a dsRNA molecule.

by certain plant RNA viruses. This again suggests that the natural role of PTGS/RNAi is in protection against virus infection.

Like RNAi, PTGS requires the formation of dsRNA. Indeed, PTGS may be induced in plants by administration of small segments of pre-made dsRNA, just as RNAi is induced in animals. However, the main difference between PTGS and RNAi is that dsRNA may accumulate in plants after transcription of an introduced transgene. The mechanism for this is not fully understood. In some cases transcription of both strands of the introduced DNA constructs by opposing promoters may lead to the formation of RNA duplexes. In other cases, base pairing may occur internally between regions that are complementary in sequence. Nonetheless, PTGS may also be induced by single transgenes that highly transcribed. It is thought that aberrant mRNAs may sometimes be made and that these act as templates for an RNA-depend-ent RNA polymerase (RdRP). This then generates the dsRNA.

Micro RNA—A Class of Small Regulatory RNA

Micro RNA (miRNA) molecules are short RNA molecules that share several properties in common with siRNA. However, microRNA molecules regulate gene expression by blocking translation of mRNA, not by promoting degradation of the mRNA. They block translation by binding to mRNA, often in the 3' untranslated region, less commonly in the coding region.

micro RNA (miRNA) Small regulatory RNA molecules of eukaryotic cells

Precursor to miRNA lllllllllllllllllllllllll


Dicer cleaves precursor


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