Firstly, the enzyme, DNA Helicase breaks the Hydrogen bonds between a specific region of the DNA molecule and unwinds the duplex DNA molecule at the targeted gene sequence site.
Then, RNA Polymerase binds at the 3` site of the gene sequence of the sense/coding strand(only one strand) that acts as the template for mRNA synthesis. RNA Polymerase then matches free nucleotides by complementary base-pairing(A-U & C=G), working in the 5`to3` direction. Hydrogen bonds then form between the complementary bases, holding the nucleotides in place. Each free adjacent nucleotide is joined at 3` ends by Condensation reactions with the other to form Phosphodiester bonds.
Once the mRNA molecule is formed, it is separated from the synthesis complex and leaves the nucleus through pores in the nuclear envelope to moves towards ribosomes for Translation.
Lastly, the DNA double helix rewinds by forming Hydrogen Bonds.