Small RNAs are important regulators of gene expression

As you’ll see in Chapter 17, less than 5 percent of the genome in most plants and animals codes for proteins. Some of the genome encodes ribosomal RNA and transfer RNAs, but until recently biologists thought that the rest of the genome was not transcribed; some even called it “junk.” Recent investigations, however, have shown that some of these noncoding regions are transcribed. The RNAs produced from these regions are often very small and therefore difficult to detect. In both prokaryotes and eukaryotes, these tiny RNA molecules are called microRNA (miRNA).

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There are as many as 5,000 miRNAs, with about 1,000 of them encoded in the human genome. Each miRNA is about 22 bases long and usually has dozens of mRNA targets because the base pairing between the miRNA and the target mRNA doesn’t have to be perfect. MicroRNAs are transcribed as longer precursors that fold into double-stranded RNA molecules and are then processed through a series of steps into single-stranded miRNAs. A protein complex guides the miRNA to its target mRNA, where translation is inhibited (Figure 16.18A). The remarkable conservation of the miRNA gene-silencing mechanism indicates that it is evolutionarily ancient and biologically important.

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Figure 16.18 mRNA Inhibition by RNAs MicroRNAs and small interfering RNAs can inhibit translation by binding to target mRNAs.

In addition to miRNAs, there is a similarly acting class of molecules called small interfering RNAs (siRNAs). These often arise from viral infections, when two complementary strands of a viral genome are transcribed. Large double-stranded RNAs are formed, and as with miRNAs, these are converted into shorter single-stranded sequences; these bind to the target RNA and cause its degradation (Figure 16.18B). Small interfering RNAs are also derived from transposon sequences, which are widespread in eukaryotic genomes (see Key Concept 15.1). Therefore it is likely that gene silencing involving siRNAs evolved as a defense mechanism to prevent the translation of viral and transposon sequences. MicroRNAs and siRNAs are similar molecules that are processed by the same cellular enzymes. A major difference between them is that: