We start with a double stranded DNA, an enzyme RNA polymerase and the four ribonucleoside triphosphates ATP, CTP, GTP and UTP. The RNA polymerase inspects the DNA by binding loosely to it for an instant, then releasing and moving on in a repeated fashion. There are certain sequences of DNA (along one strand) that the RNA will bind to more tightly, thereby ``recognising'' them. When it gets to such a promoter site it locally unwinds the double stranded DNA and starts to work on the strand. Ribonucleoside triphosphates are incorporated, with a DNA strand A being matched to a U in the mRNA, C to G, G to C and T to A, producing an mRNA chain. This process continues, with the RNA polymerase shifting along the DNA, until it reaches a terminator site --- a region of sequence that leads to the RNA polymerase being thrown off, ending transcription.