A versatile nucleic acid that combines with a protein to make ribosoomes, the site of protein assembly (ribosomal RNA); copies genetic information from DNA for transformation into proteins (messenger RNA), and incorporates animo acid combinations into developing proteins (transfer RNA). The RNA molecule is identical to DNA (from which it is made) except for the sugar ribose instead of deoxyribose and uracil for thymine. At one time RNA might have been the only form of life (the RNA World hypothesis): it can replicate without a cell nucleus or even any DNA.