CoachA non-coding RNA (ncRNA) is a transcript that functions as RNA rather than as a template for protein. Beyond the ribosomal and transfer RNAs of translation and the small nuclear RNAs (snRNAs) of the spliceosome, a eukaryotic cell contains: microRNAs (miRNAs), nucleotides, cut from hairpins in the cell’s own transcripts and guiding the repression of partly complementary messengers; small interfering RNAs (siRNAs), the same length, cut from long double-stranded RNA and guiding the cleavage of perfectly matching targets; Piwi-interacting RNAs (piRNAs), nucleotides, active in the germ line against transposons; small nucleolar RNAs that guide the modification of rRNA; and long non-coding RNAs (lncRNAs), transcripts longer than with no open reading frame of consequence, of which Xist of Chapter 1 is the best understood. Protein-coding exons are about of the human genome; a large fraction of the rest is transcribed at some time in some cell, but how much of that transcription is functional is an open question.
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Example 2.6 (A four-fold repression)
A messenger with a natural half-life of (), transcribed at molecules per minute, sits at copies per cell and takes to move half-way to a new level after its promoter changes. A microRNA that triples its decay rate () brings it to copies and cuts the half-time to . The same numbers, read backwards, show why miRNA knockouts often have mild phenotypes: a four-fold change in a messenger that is itself buffered downstream can leave the organism nearly normal, and the effect shows under stress or in timing.