RNA : Noncoding

Abstract

Abstract After decades of being either underestimated as a simple messenger in the expression of proteins from deoxyribonucleic acid (DNA), or revered as the molecule from which all life potentially originated, ribonucleic acid (RNA) is at last enjoying a well‐deserved golden age as modern techniques allow researchers to uncover the myriad of subtle roles that this molecule plays in the cells of today. Noncoding RNAs, by definition, do not become translated into protein. This broad category includes cellular RNAs that range in size from 20 to 20,000 nucleotides, can contain chemical modifications, adopt a wide variety of secondary and tertiary structures, associate with proteins and other RNA molecules, and have physiological roles, in all organisms, that run the gamut from catalysis to regulating gene expression. As this article is intended to complement the other RNA‐related sections of this volume, a brief introduction to all types of noncoding RNA will be followed by a comprehensive discussion of three important categories: riboswitches, small RNAs in bacteria, and large RNAs in mammals. Riboswitches are cis ‐acting RNA elements that modulate gene expression in a highly specialized manner by means of an aptamer domain, an RNA sequence that tightly and selectively binds a given metabolite. Small RNAs are 70–200 nucleotide molecules in bacteria that regulate gene expression, primarily at a posttranscriptional level. In mammalian cells, large RNAs of up to 19,000 nucleotides are essential to X‐chromosome inactivation and genomic imprinting.