CoachRNA is usually single-stranded, and folds back on itself wherever short complementary stretches allow, into hairpins, loops and bulges — a secondary structure — that then packs into a shape. Three classes carry the flow of genetic information (Chapter 19): messenger RNA (mRNA), a copy of a gene that the ribosome reads; transfer RNA (tRNA), some nucleotides folded into a cloverleaf, carrying an amino acid at its end and an anticodon that pairs with the message; ribosomal RNA (rRNA), the structural and catalytic core of the ribosome, four fifths of the RNA of a cell. Many other RNAs regulate, splice, and guide (the Year 3 volume).
