Ribonucleic acid (RNA) is an unbranched polymer of purine (adenine, gua- nine) and pyrimidine (cytosine, uracil) nuc1eotides joined by phosphodiester
bonds.
The RNA polymer is bulkier than that of DNA (which lacks the 2'OH on
the ribose). RNA is usually single-stranded, and it tends to fonn tertiary struc-
tures of high complexity, including hairpin loops, internal loops, bulges, and
pseudo-knots (in which self complementary sequences align to form short,
antiparallel double helical strands), and triple stranded structures (1-3). This
complexity gives rise to functional molecules of much greater diversity than
DNA, supporting the provocative concept that RNA evolved in the prebiotic
era (4). Intact RNA is difficult to isolate because, as a long polymer, it is
prone to mechanical or chemical degradation, and because of the existence
of RNase, which is widely distributed on laboratory surfaces and difficult
to destroy.