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In this module:

The Genetic Code
Transfer RNA


All RNA is synthesized from DNA in a process called transcription . The mechanism is similar to the process for the formation of DNA (click to open a new window that demonstrates how DNA and RNA are synthesized, then close the other window when you are done to return to this module).

The DNA template contains several regions important to transcription. The first are two specific promoter sites that tell the transcription enzymes where to bind to DNA and start making RNA. In bacteria, these promoter regions are 10 and 35 base pairs on the 5' side (upstream) of the starting site, while in eukaryotes they are 25 and 75 base pairs upstream.

Second, at the end of a DNA sequence that is to be transcribed there is a termination signal that tells the enzymes to stop making RNA.

Finally, enhancer and repressor regions around the DNA to be transcribed can increase or decrease the likelihood of transcription. These regions are often hundreds or thousands of base pairs away from the DNA region that will be transcribed, but the DNA is flexible, so the enhancers and repressors wind up close to the transcription start site. Proteins carrying signals from outside the cell often bind to these regions. In this way the cell can start or stop synthesizing other proteins that will allow it to respond to changing conditions in the environment.