A chemical found in the nucleus and cytoplasm of cells; it plays an important role in protein synthesis and other chemical activities of the cell. The structure of RNA is similar to that of DNA. There are several classes of RNA molecules, including messenger RNA, transfer RNA, ribosomal RNA, and other small RNAs, each serving a different purpose.

Ribonucleic acid is made of nucleotides and is very similar chemically to DNA. The sugar of RNA is slightly different than the one found in DNA; this difference accounts for the names of the two molecules (deoxyribose versus ribose). One of the bases in DNA, thymidine, is substituted by a different base, uridine (U) in RNA.

Unlike the large double-stranded DNA, RNA molecules have only one strand and are generally smaller. RNA can be thought of as an intermediate in the decoding of DNA into protein; genes in DNA are copied into an RNA molecule through a process called transcription and the RNA is then decoded (translated) into amino acids that make a protein.

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.