complex compound of high molecular weight that functions in cellular protein synthesis and replaces DNA (deoxyribonucleic acid) as a carrier of genetic codes in some viruses. RNA consists of ribose nucleotides in strands of varying lengths. The structure varies from helical to uncoiled strands. One type, transfer RNA (tRNA), sometimes called soluble, or activator, RNA, contains fewer than 100 nucleotide units; other types contain thousands of units. The nitrogenous bases in RNA are adenine, guanine, cytosine, and uracil.

There are three main types of RNA: messenger RNA (mRNA), transfer RNA, and ribosomal RNA (rRNA). In protein formation, mRNA carries codes from the DNA in the nucleus to the sites of protein synthesis in the cytoplasm (the ribosomes (ribosome)). Ribosomes are composed of rRNA and protein; they can “read” the code carried by the mRNA. A sequence of three nitrogenous bases in mRNA specifies incorporation of an amino acid; tRNA brings the amino acids to the ribosomes, where they are linked into proteins.

In addition to the main types of RNA, a number of other small RNA molecules and ribonucleoproteins (molecules composed of RNA and protein) occur in the cell. The RNA portion of at least one cellular ribonucleoprotein has been shown to act as a biological catalyst, a function previously ascribed only to proteins. R.W. Holley (Holley, Robert William) described the structure of an RNA molecule in 1965.