This process is called transcription. In the nucleus, an enzyme helicase is used to break the hydrogen bonds between the two strands of the double helix. Only one DNA strand is used the template, which we call the template strand. Free RNA nucleotides are attracted to the exposed bases according to the base pairing rule. RNA polymerase joins the RNA nucleotides together to form pre mRNA. The pre mRNA is then spliced to remove introns.