Transcription occurs in the nucleus of the cell. The process involves making a strand of messenger ribonucleic acid (RNA) using deoxyribonucleic acid (DNA) as a template. Transcription is the first step in making a protein.
Enzymes first unwind the DNA helix. DNA exists as a double strand, with one strand used for coding and the other strand acting as the template. The messenger RNA (mRNA) builds on the template strand. Because the template strand is complementary to the coding strand, and the mRNA is complementary to the template strand, the mRNA strand is a duplicate of the coding strand (except that RNA uses a different sugar molecule and has uracil instead of thymine for one of its bases). RNA polymerase, the enzyme that makes mRNA, adds 20 to 50 bases at a time. Once the mRNA strand is complete, it goes from the nucleus to the ribosomes for translation.