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A Ribonucleic acid (RNA) is a molecule found in all cells, comprised of many individual units of nucleic acid. It differs from DNA (deoxyribonucleic acid) in that it only contains a single, not double strand, and substitutes uracil for thymine. The sugar backbone of RNA is also composed of ribose (DNA contains deoxyribose). RNAs fall into several different categories, depending on function.

The primary role of mRNA in is to transfer a copy of the information coded in DNA to a ribosome to be expressed as a protein, using a form of RNA known as messenger RNA (mRNA). mRNA in eukaryotes undergoes spicing after transcription from DNA to remove introns or to use alternative splicing to create a different gene product. Alternatively-spliced transcripts can also express the same protein, but at different levels, due to the removal or addition of regulatory sequences.

The main role of tRNA, or transfer RNA is to carry an amino acid to the mRNA (while the mRNA is in the ribosome) during translation of the mRNA into a protein.

The main role of rRNA or ribosomal RNA is to form the ribosome. RNA that has catalytic properties (such as the ribosome) is referred to as a ribozyme.

Purification of RNA is problematic as enzymes that degrade RNA (RNases) are ubiquitous. Successful RNA purification depends on degradation of the DNA template used via DNase and avoidance of RNase using sterile technique.

The polio virus is an example of an organism which contains only RNA to carry its genetic information. Some protists such as Paramecium carry similar RNA genes in structures called micronuclei. These are involved in mating and replicate independently from the main cell nucleus.