A single-stranded nucleic acid found in the nucleus and cytoplasm of a cell. It is a polymer of the sugar ribose, phosphate, purine and pyrimidine bases. RNA is very similar to DNA, but substitutes the nucleotide, uracil, for thymine. It acts as a "middle-man", converting genetic information from DNA to proteins. There are three types of RNA: mrna (messenger RNA), which contains the specific sequence of nucleotides necessary to dictate amino acid sequence in proteins; trna (transfer RNA), which serves as the "adaptor" to position the appropriate amino acid next to a growing polypeptide chain during protein synthesis; and rrna (ribosomal RNA), which is the RNA component of ribosomes. In some viruses, RNA is the genetic material.