Only one of the unmound DNA strands acts as a template for the RNA synthesis.
RNA polymerase can only add nucleotids to the 3' end of the strand so like DNA, RNA must be synthesized in the 5' to 3' direction.
Free ribonucleotides triphosphates from the cytoplasm are paired up with their commplementary base on the exposed DNA template.
RNA polymerase joins the ribonucleoside triphosphates to form an mRNA strand.
As RNA polymerase advances, the process continues.
The DNA that has been transcribed, re-winds to form a double helix.