Ribonucleic Acid (RNA)

Ribonucleic acid (RNA) is the other nucleic acid found in living cells. Unlike DNA, it is not usually the hereditary material — though in certain organisms, such as the tobacco mosaic virus, RNA does serve as the hereditary material. Like DNA, RNA is built as a polynucleotide chain, but with two key chemical differences: its sugar is ribose rather than deoxyribose, and it normally exists as a single strand rather than a double helix (though a few viruses, such as reovirus and wound tumour virus, do have double-stranded RNA). In place of DNA's thymine, RNA uses the base uracil. Because RNA is single-stranded, it does not show the purine = pyrimidine equality that Chargaff found in double-stranded DNA. An RNA strand is often folded back on itself at various points, which adds to its structural stability; most RNA chains begin with either an adenine or a guanine nucleotide.

Cells contain three principal kinds of RNA, each with a distinct role in protein synthesis:

• Messenger RNA (mRNA), also called template RNA, is a linear, single-stranded molecule that carries the genetic information specifying the order in which amino acids must be assembled into a polypeptide chain. It makes up only about 3% of total cellular RNA, but its molecular weight can run into the millions, and its length is directly related to the length of the message it encodes — the stretch of mRNA coding for one complete polypeptide is called a cistron. Synthesis of an mRNA molecule begins at the 5' end of the DNA template strand and proceeds to the 3' end.
• Ribosomal RNA (rRNA) makes up 50–60% of the mass of a ribosome and accounts for the great bulk — 80–90% — of all cellular RNA; it is synthesised in the nucleus (its discovery is credited to Kurland, 1960) and is extensively coiled through intrachain (within-strand) base pairing. …