Transcription, also known as RNA synthesis, is the process of transcribing DNA into RNA.  The process on consists of three stages initiation, elongation, and termination and is catalyzed by an enzyme called RNA polymerase.  The RNA polymerase attaches to the DNA at the promoter site and starts to unwind the DNA double helix.  A single strand of DNA is then used at a template, which tells the polymerase what to ribonucleic bases to add and when to start and stop.  As the polymerase moves along the DNA strand it selects a ribonucleic base and forms a phosphodiester bond between two bases.  This bond is formed when the 3’-hydroxyl group of the last nucleotide attacks the 5’-a phosphoryl group of the incoming nucleotide.