Molecular Genetics

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• As with all polymerization processes (replication, transcription, translation), transcription can be divided into 3 parts: intitiation, elongation and termination. Transcription events that require signalling are initiation and termination. Once synthesis of an RNA has been inititiated, elongation takes place until a termination signal is reached.
• Proper inititiation of transcription requires:
  • a mechanism to ensure initiation of every RNA at the same site on the DNA; and
  • a mechanism to ensure that the gene is transcribed at a frequency consonant with the cell's demand for the gene product.
• The sequences that signal start site selection are sometimes regarded as the promoter. The frequency of transcription can be influenced by the promoter in procaryotic systems. Additional signal elements, called operators, play a role in regulating frequency in some bacterial transcription units. In eucaryotic systems, additional signal elements are the principal way in which transcription frequency is adjusted. These elements are parts of two kinds of regions: upstream regulatory regions and enhancer-silencer regions.
• To recognize signals that specify the correct initiation site, it is important to find where the 5' end of the RNA is relative to the DNA.

This is page 22 of Molecular Genetics by Ulrich Melcher, © 1997