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.