1). Occurs in three major forms -- as messenger RNA, transfer RNA, and ribosomal RNA. Some RNA molecules are the genetic molecules of viruses (e.g., tobacco mosaic virus and HIV) and viroids. Some RNA molecules may have enzymatic activity (see ribozymes). 2). Any of various nucleic acids that yield ribose as one product of hydrolysis, that are found in the cytoplasm and sometimes in the nucleus. 3). Required for the synthesis of proteins in all living systems It consists of a linear array of the nucleotides cytidine-, guanosine-, adenosine-, and uridine-s'-monophosphates.