RNA
ribonucleic acid: a nucleic acid that directs the synthesis of protein molecules in living cells. There are three main types of RNA. Messenger RNA carries the genetic code from the cell nucleus to the cytoplasm. Ribosomal RNA is found in ribosomes, small particles where proteins are assembled from amino acids. Transfer RNA carries specific amino acids for protein synthesis. Each of the 20 amino acids has a corresponding transfer RNA molecule to place the amino acid in the proper sequence in protein assembly. RNA is similar to DNA in structure except that it consists of a single strand of nucleotides (compared with the double strands of DNA), the base uracil (U) occurs instead of thymine (T), and the sugar unit is ribose, rather than deoxyribose.