(ribonucleic acid) A single-stranded nucleotide polymer, each nucleotide being constructed from phosphoric acid, ribose, and an organic base. The base may be adenine, or guanine (the purine bases), or cytosine or uracil (the pyrimidine bases). The polynucleotides are held together by phosphodiester bonds between the phosphate group of one nucleotide and the sugar of an adjacent nucleotide. The RNA molecule may be linear, as in messenger RNA, or fold back on itself to form a three-dimensional clover-leaf-shaped molecule, as in transfer RNA. RNA is the principal agent for the transcription (copying) and translation (conversion) of the genetic code during protein synthesis. Various types of RNA are associated with these processes, namely *messenger RNA
, *transfer RNA
, and *ribosomal RNA
. Viruses are exceptional and may not conform to the above generalizations. In many plant viruses, such as tobacco mosaic virus, the genetic material is RNA not *DNA
. In some viruses, such as ø6 bacteriophage, the RNA is double stranded, like DNA.