1. A nucleic acid, found mostly in the cytoplasm of cells, that is important in the synthesis of proteins. The amount of RNA varies from cell to cell. RNA, like the structurally similar DNA, is a chain made up of subunits called nucleotides. In protein synthesis, messenger RNA (mRNA) replicates the DNA code for a protein and moves to sites in the cell called ribosomes. There, transfer RNA (tRNA) assembles amino acids to form the protein specified by the messenger RNA. Most forms of RNA (including messenger and transfer RNA) consist of a single nucleotide strand, but a few forms of viral RNA that function as carriers of genetic information (instead of DNA) are double-stranded. 2. A nucleic acid associated with the control of chemical activities inside a cell. One type of RNA transfers information from the cell's DNA to the protein-forming system of a cell outside the nucleus. Some viruses (e.g., HIV) carry RNA instead of the more usual genetic material DNA. See also Cytoplasm; DNA; Retrovirus.
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