CONTROL OF GENE EXPRESSION [EUKARYOTES]

=Control of Gene Expression in Eukaryotes:= In eukaryotes transcription is triggered by a signal. This signal can be as simple as a change in temperature or pressure. However, more often a chemical signal either diffuses into the nucleus or binds to a receptor on the cell surface triggering a chain reaction that eventually reaches the nucleus, ‘switching on’ a particular gene. [image:http://i.imgur.com/jWNRgsA.png] Eukaryote genes are much larger (due to Introns) and therefore Eukaryote RNA polymerases are much faster. However, this also means that they need a little extra help finding & binding to the promoter sequence. Once the signal reaches the nucleus, it causes special proteins known as transcription factors to bind to the Promoter Sequence. These Transcription factors help to position the RNA polymerase on the Promoter. RNA polymerase can now transcribe the gene; the system is ‘switched on’. [image:http://i.imgur.com/7NzyFmD.png] However the signal also causes another class of transcription factors, called activators, to bind to ‘enhancer’ sequences further upstream. Hundreds of activators can bind to enhancer sequences depending on the strength of the signal (only three have been shown for simplicity). The activators that have bound to enhancer Sequences move to associate with the other transcription factors. This results in a DNA loop that can actually be seen under the microscope. The activators induce a slight shape change in the other transcription factors, helping them to more quickly position RNA polymerase. [image:http://i.imgur.com/Ei4dga9.png] Essentially eukaryotes can control not only whether a gene is switched on or off, but also how much it is expressed. The Transcription Factors act as the On / Off switch, positioning the RNA polymerase onto the promoter no matter how weak the signal. The activators control how much the gene is expressed; if the signal is strong, many activators bind to enhancers and help the other transcription factors. As a result RNA polymerase can bind more often and more mRNA copies are made. [image:http://i.imgur.com/HZwETWc.png]