Nucleic acid involved in the process of
translating DNA, the genetic material into
proteins. It is usually single-stranded,
unlike the double-stranded DNA, and consists
of a large number of nucleotides strung
together, each of which comprises the sugar
ribose, a phosphate group, and one of four
bases (uracil, cytosine, adenine, or
guanine). RNA is copied from DNA by the
formation of base pairs, with uracil taking
the place of thymine. Although RNA is
normally associated only with the process of
protein synthesis, it makes up the hereditary
material itself in some viruses, such as
retroviruses. RNA occurs in three major
forms, each with a different function in the
synthesis of protein molecules. Messenger RNA
(mRNA) acts as the template for protein
synthesis. Each codon (a set of three bases)
on the RNA molecule is matched up with the
corresponding amino acid, in accordance with
the genetic code. This process (translation)
takes place in the ribosomes, which are made
up of proteins and ribosomal RNA (rRNA).
Transfer RNA (tRNA) is responsible for
combining with specific amino acids, and then
matching up a special `anticodon' sequence of
its own with a codon on the mRNA. This is how
the genetic code is translated.