RNA molecules are involved primarily in protein translation. Standard wisdom of the era was that RNAs generally are long molecules that come in three major types: messenger (m) RNAs that are transcribed from housekeeping genes and encode polypeptides; and transfer (t) RNA molecules and ribosomal (r) RNAs that also are involved in the protein translation process on ribosomes.
regulatory loci; microRNAs
RNA molecules are involved primarily in protein translation. Standard wisdom of the era was that RNAs generally are long molecules that come in three major types: messenger (m) RNAs that are transcribed from housekeeping genes and encode polypeptides; and transfer (t) RNA molecules and ribosomal (r) RNAs that also are involved in the protein translation process on ribosomes (but see also Chapter 55).
In 1993, Rosalind Lee and her colleagues discovered a class of short (each about 20 nucleotides in length) non-coding RNA molecules that proved to be regulators of eukaryotic gene expression at the transcriptional or post-transcriptional level. Known as micro (mi) RNAs, these little molecules function by base-pairing with complementary mRNA sequences from structural genes and thereby usually silencing the latter’s functional expression. They have proved to be both abundant and widespread in many eukaryotic species, with the human genome (for example) housing more than 1000 miRNAs probably targeted at huge numbers of structural genes. Many miRNA genes lie in the introns of functional genes (see Chapter 49), but others reside either in exons or in intergenic regions. In effect, miRNAs probably exert their regulatory influence by turning an imprecise number of mRNA transcripts into a more precise number of protein molecules. Recently, another category of abundant non-coding RNAs – known as long non-coding RNAs or lncRNAs – has also been implicated in regulating cellular processes, perhaps by binding to the 3D structures of chromosomes. Evidence suggests that more than 10,000 lncRNAs, with roles remaining unknown, may reside in the human genome.
To the extent that gene regulation is important in evolution and ontogeny (which now seems insuperable), the discovery of regulatory miRNAs must rank rather high on the list of conceptual breakthroughs in molecular evo-devo during the past two decades. The mode of operation of these molecules is broadly reminiscent of some earlier scenarios for how structural genes might be coordinately modulated by batteries of regulatory loci (see Chapter 41). Furthermore, some authors (e.g., Peterson et al., 2009) speculate that miRNAs have played key roles in shaping metazoan evolution.
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