The process of synthesizing a polymer of RNA using DNA as a template is called transcription . Transcription proceeds in a single direction by the addition of one nucleotide at a time on to the 3' end. Either strand may be copied at any one time, but, since the two DNA strands are oriented oppositely, the direction of transcription depends on whether the top or bottom strand is being transcribed. Reference to the animation above may help clarify this point. The peculiar looking figure in green is supposed to represent the enzyme, RNA polymerase.
Transcription goes on until the end of the gene is reached.
At this juncture, the enzyme responds to a signal on the DNA and stops adding on new nucleotides. The newly formed RNA is released. In fact, genes may be transcribed virtually unlimited times, producing large numbers of transcripts of a specific gene in a given cell. Equally important is the fact that at any period in a cell's life, many genes are not being transcribed. Imagine that you have a large, old, and valuable cookbook that's been in the family for generations.