The abbreviation for ribonucleic acid, one of the two types of NUCLEIC ACID that exist in nature. It is present in both the cytoplasm and nucleus of the CELLS of the body, but principally in the former. With DNA it is an essential component of the genetic code. It exists in three categories known, respectively, as ribosomal (r), transfer (t), and messenger (m) RNA. Genetic information resides in the linear sequence of nucleotides (see NUCLEIC ACID) in DNA and is transcribed into messenger RNA before protein is synthesised. In the language of the computer, the genetic code consists of 64 three-letter code-words, or codons. The code in DNA is comparable to a tape which contains information written linearly in the form of these codons, each of which is the code for one of the 20 AMINO ACIDS from which proteins are made. The genetic information encoded in DNA is used to programme the manufacture of proteins (see PROTEIN) in two stages.

In the first, the information is transcribed from DNA on to a molecule of mRNA. In the second, the messenger RNA-intermediary transports the information to the protein-manufacturing centres of the cell where the information is translated from the linear sequence of codons in the RNA into a linear sequence of amino acids which are concurrently converted into protein. (See also GENES.)