RNA, ribonucleotide acid, is built up of a phosphate and nitrogenous base, a ribose sugar, and a phosphate. The bases used are adenine (A), cytosine (C), guanine (G) and uracil (U).
The chemical structure of RNA 
There are four major groups of RNA: messenger RNA (mRNA), ribosomal RNA (rRNA), transfer RNA (tRNA) and small, regulatory RNAs (sRNA). mRNA is transcribed from DNA by the enzyme RNA polymerase, and is then used as a template in translation. rRNAs are a major component of the ribosome, the translation machinery. They are divided into the 50S large subunit (23S and 5S) and small 30S (16S) in prokaryotes. The rRNAs decode the mRNA and interact with tRNAs. The tRNAs are attached to specific amino acids and carry them (with the help of elongation factor Tu) to the ribosome during translation. The sRNAs form a quite recently discovered group of regulatory RNAs that are thought to be of great importance especially during stress, when they bind specifically to their targets and as a consequence effect the expression of genes, either at the level of transcription or translation.