The production of a protein takes place in two stages. First, one of the two strands of DNA is transcribed (or copied) into a single strand of complementary RNA termed messenger RNA (mRNA ). As you should now understand, the mRNA is complementary only to the template strand of DNA.
The process of transcription occurs in the nucleus of eukaryotic cells and requires that the two strands of DNA separate, or open up sufficiently enough so that mRNA can be produced. The enzyme RNA polymerase separates the DNA strands and joins the RNA nucleotides along the exposed DNA template strand. This process is initiated when certain proteins, known as transcription factors , bind to the starting point of a gene. This starting point is known as a "promoter" . The promoter is a sequence of DNA bases that signals the beginning of RNA synthesis. RNA polymerase II adds nucleotides to the 3' end of the elongating RNA molecule. The enzyme moves down the DNA strand, unwinding as it goes and allowing the DNA helix to reform after a sequence has been transcribed. This continues until a DNA sequence, known as a "terminator sequence" , signals the end of RNA synthesis. Transcription is broken down into three stages: initiation, elongation, and termination. At the end of transcription a molecule of mRNA has been produced that carries the instructions for synthesizing a protein.