Short for ribonucleic acid. The nucleic acid that is used in key metabolic processes for all steps of protein synthesis in all living cells and carries the genetic information of many viruses. Unlike double-stranded DNA, RNA consists of a single strand of nucleotides, and it occurs in a variety of lengths and shapes. RNA also differs from DNA in having the pyrimidine base uracil instead of thymine and in having ribose instead of deoxyribose in its sugar-phosphate backbone. In eukaryotes, RNA is produced in the cell nucleus. ◇ Messenger RNA is RNA that carries genetic information from the cell nucleus to the structures in the cytoplasm (known as ribosomes) where protein synthesis takes place. ◇ Ribosomal RNA is the main structural component of the ribosome. ◇ Transfer RNA is RNA that delivers the amino acids necessary for protein synthesis to the ribosomes. Compare DNA.