RNA is a polynucleotide.The pentose sugar is ribose and the nucleotides are joined by phosphodiester bonds.RNA contains four nitrogen-containing bases, adenine, cytosine, guanine and uracil.There are three types of RNA: mRNA, rRNA and tRNA. All are involved in protein synthesis.Ribosomal RNA (rRNA) is synthesised in the nucleolus and is a component of ribosomes.Messenger RNA (mRNA) is a copy of a length of DNA called a gene. The mRNA then moves from the nucleus into the cytoplasm to determine the order in which amino-acids are joined together to make proteins.Transfer RNA (tRNA) holds the amino acids in place on a ribosome so a peptide bond can be formed.