A gene is a nucleotide sequence on a strand of DNA. The "one gene - one polypeptide" hypothesis states that a single gene provides the code for a specific enzyme, protein or polypeptide. This means the gene provides the instructions to build a polypeptide but it does not build it directly; it needs RNA as a bridge. The four nucleotides adenine, thymine/uracil, guanine and cytosine are the monomer bases for DNA and RNA (nucleotides only fit to a complementary nucleotide; A-T/U and C-G; this is called the base-pairing rule). In polypeptides and proteins the monomer bases are the 20 amino acids. Since the information is encoded in two different chemical languages, transcription and translation are used to get from one language to the other. Transcription is the synthesis of RNA under the direction of DNA. Because both DNA and RNA use the same language, the transcription phase is just the copying of information from one molecule to the other. In order for the copy to be made, the DNA must unwind and separate. An enzyme pries apart the two strands of DNA and attaches the RNA nucleotides to their complementary base-pairs along the DNA template. This enzyme is called the RNA polymerase. The RNA polymerase scans the DNA until it finds a start sequence. The start sequence of three specific nucleotides is called the promoter. In Eukaryotic cells (a cell with a membrane closed nucleus and membrane closed organelles), transcription factors can turn the transcription process on and off as needed. The RNA polymerase then works its way up the DNA moving from the 5" end to the 3" end. As it moves only, the RNA polymerase continues to unwind the double helix DNA exposing 10-20 bases at a time. The enzyme adds nucleotides to the 3" end of the growing RNA molecule. Transcription continues until the RNA polymerase transcribes a DNA sequence called a terminator. This is a signal for the enzyme to stop copying the template.