RNA secondary structure
If DNA is a tidy double-stranded archive, RNA is a single loose thread — and a single thread does something a tied pair cannot: it folds back on itself. RNA's habit of folding into intricate shapes is what lets it do jobs far beyond carrying a message.
Most RNA is single-stranded, so its bases have no partner strand to pair with. Instead, the strand loops back and pairs with itself wherever its own sequence is complementary, forming short double-stranded stems capped by unpaired loops. The simplest of these is a hairpin or stem-loop — a length of strand that folds in half and zips up. These base-paired regions are the secondary structure; further folding of stems and loops against one another gives the compact, three-dimensional tertiary structure. RNA pairs by the same rules as DNA, with A pairing to U instead of T, plus a tolerated weaker G-U 'wobble' pair.
This folding matters because shape is function. A transfer RNA folds into a cloverleaf-then-L shape that lets it read the genetic code; a ribosomal RNA folds into the working heart of the ribosome; and some folded RNAs, called ribozymes, are genuine catalysts that speed up reactions — overturning the old idea that only proteins can be enzymes. In molecular biology RNA is not just a passive copy of DNA; folded into shape, it is a worker in its own right.
Transfer RNA folds first into a flat cloverleaf of three hairpins, then twists into a compact L-shape; at one tip sits its anticodon, which reads the messenger RNA, while the opposite tip carries the matching amino acid.
One thread, folded — and suddenly RNA can read code and catalyse.
Single-stranded does not mean shapeless. The biggest misconception is to picture RNA as a floppy line; in the cell most functional RNAs are tightly and specifically folded, and that folded shape, not the bare sequence, is what does the work.