The full name of RNA is Ribonucleic Acid. It is the genetic material of certain viruses (RNA viruses) and, in cellular organisms, the molecule that directs the middle steps of protein production. In RNA viruses, the RNA directs two processes -- protein synthesis (production of the virus's protein coat) and replication (the process by which RNA copies itself). In cellular organisms deoxyribonucleic acid (DNA) carries the information that determines protein structure but relies upon RNA to transfer this crucial information during protein synthesis.
RNA consists of a chain of chemical compounds called nucleotides, made up of a sugar molecule called ribose and one of four different nitrogen-containing compounds called bases (adenine, guanine, uracil, and cytosine). These components are joined together as in a DNA molecule, but RNA differs chemically from DNA in that the RNA sugar molecule contains an oxygen atom not found in DNA, and RNA contains the base uracil in the place of the base thymine in DNA.
In cellular organisms RNA is a single-stranded polynucleotide chain (a strand of nucleotides linked together). Some RNA viruses have double-stranded RNA (RNA molecules consisting of two polynucleotide chains). For these viruses, the process of RNA replication in a host cell follows the same pattern as that of DNA replication. Each newly formed RNA molecule contains a polynucleotide chain from an earlier RNA molecule. The base of each RNA nucleotide in the chain pairs with a corresponding base in another RNA nucleotide.