relatively complex tertiary structure had been revealed was transfer RNA. Then came the structures of
ribozymes and those of numerous oligonucleotide-sized fragments, offering a glimpse at the repertoire of
RNA’s tertiary structural motifs, and a flurry of protein–RNA complexes, and – at last – atomic reso-
lution structures of ribosomal subunits and whole ribosomes. Much remains to be discovered in terms of
the mechanism of translation,83,84but the availability of ribosome structures and oligonucleotide fragments
mimicking portions thereof has reinvigorated the interest in RNA as a drug target.85,86On the functional
side, in vitroselection and the emergence of a flurry of the so-called aptamers, RNA molecules with the
capacity to recognise and tightly bind small and large molecules have given a boost to the RNA-world
hypothesis, and have subsequently led to the identification of natural control elements in messenger
RNAs, ‘ribsoswitches’ (Section 5.7.2) that regulate gene expression.87,88And as if this were not enough,
the advent of RNA interference (RNAi) has further underscored the importance of RNA in the mediation
and control of biological information transfer. Perhaps it does not come as a surprise then that well over
one third of the human genes appear to be conserved miRNA targets.^89 New functions come with their
structural underpinnings and the structural biology of RNA-mediated gene silencing has already yielded
first insights into novel RNA–protein interactions.^90 A little over 50 years after Watson and Crick’s model
of the DNA double helix there is no end in sight in the quest for the structural analysis of DNA and RNA.
DNA and RNA Structure 71
Figure 2.46 Electron micrograph of a histone-depleted chromosome showing that the DNA is attached to the scaffold
in loops
(Adapted from J.K. Paulson and U.K. LaemmliCell, 1997, 12 817–828. © (1977), with permission from
Elsevier)