Capping is a three-step process that utilizes the enzymes RNA triphosphatase, guanylyltransferase, and methyltransferase.  Through a series of three steps, the cap is added to the first nucleotides 5 hydroxyl group of the growing mRNA strand while transcription is still occurring.  First, RNA 5 triphosphatase hydrolyzes the 5 triphosphate group to make diphosphate-RNA. Then, the addition of GMP by guanylyltransferase produces the guanosine cap. Last, RNA methyltransferase transfers a methyl group to the guanosine cap to yield 7-methylguanosine cap that is attached to the 5' end of the transcript. These three enzymes, collectively called the capping enzymes, are only able to catalyze their respective reactions when attached to RNA polymerase II, an enzyme necessary for the transcription of DNA into pre-mRNA.  When this complex of RNA polymerase II and the capping enzymes is achieved, the capping enzymes are able to add the cap to the mRNA while it is produced by RNA polymerase II.