(ribose nucleic acid; RNA). Generic term for a group of natural polymers consisting of long chains of alternating phosphate and D-ribose units, with the bases adenine, guanine, cytosine, and uracil bonded to the 1-position of the ribose. Ribonucleic acid is universally present in living cells and has a functional genetic specificity due to the sequence of bases along the polyribonucleotide chain.Four types are recognized:(1) Messenger RNA, synthesized in the living cell by the action of an enzyme that carries out the polymerization of ribonucleotides on a DNA template region that carries the information for the primary sequence of amino acids in a structural protein. It is a ribonucleotide copy of the deoxynucleotide sequences in the primary genetic material.(2) Ribosomal RNA, that exists as a part of a functional unit within living cells called the ribosome, a particle containing protein and ribosomal RNA in roughly 1:2 parts by weight having a particle weight, of about three million. See Ribosome.
Messenger RNA combines with ribosomes to form polysomes containing several ribosome units, usually 5 (e.g., during hemoglobin synthesis), complexed to the messenger RNA molecule. This aggregate structure is the active template for protein biosynthesis.(3) Transfer RNA, the smallest and best characterized RNA class. Its molecules contain only 80 nucleotides per chain. Within the class of transfer RNA molecules there are probably at least 20 separate kinds, correspondingly related to each of the 20 amino acids naturally occurring in proteins. Transfer RNA must have at least two kinds of specificity: (a) It must recognize (or be recognized by) the proper amino acid–activating enzyme so that the proper amino acid will be transferred to its free 2′- or 3′-OH group. (b) It must recognize the proper triplet on the messenger RNA–ribosome aggregate. Having these properties, the transfer RNA accepts or forms an intermediate transfer RNA–amino acid that finds its way to the polysome, complexes at a triplet coding for the activated amino acid, and allows transfer of the amino acid into peptide linkage.(4) Viral RNA, isolated from a number of plant, animal, and bacterial viruses, may be considered as a polycistronic messenger RNA. It has been shown to have molecular weights of one or two million. Generally speaking, there is one molecule of RNA per infective virus particle. The RNA of an RNA virus can be separated from its protein component and is also infective, bringing about the formation of complete virus.From article by F. J. Bollum in Encyclopedia of Biochemistry.See Deoxyribonucleic Acid; RNA.
