Transcription is the formation of an mRNA molecule from its complementary DNA template.

First the enzyme DNA helicase unwinds the double helix, and the hydrogen bonds between complementary bases break.  This occurs along the length of the gene that will be transcribed.

Next the template, or antisense, strand acts as a template and RNA nucleotides form new hydrogen bonds with the exposed DNA bases to form complementary pairs.

RNA polymerase catalyses the formation of covalent bonds between adjacent RNA nucleotides and once the RNA molecule is complete, the hydrogen bonds holding it to the DNA template break and the RNA is released.

This type of RNA is called messenger RNA (mRNA) and exits the nucleus via nuclear pores, travelling to ribosomes in the cytoplasm where it will be used in translation to produce a polypeptide chain.