实际上DNA在转录时是一个“圆”而不是一条”线“。有时候，教材上简化了的示意图便于多数人理解，但也给一些人的心里埋下了疑问的种子。
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In the series of diagrams to your left you can see a model that shows these transcription factors at work.
Let’s start with diagram 1: Letter “a” is a region of DNA that acts as a kind of switch that determines whether or not transcription will occur. These regions are called enhancers or regulatory switches. Note that these enhancers can be some distance away from the protein-coding region of the gene.
The enhancer consists of one or more control elements (indicated by “c.”). The transcription factors that bind with these control elements (shown at “b”) are often referred to as activators (because they activate genes).
Now look at diagram 2. With these activators in place, the enhancer can bind with general transcription factors (at “h”) and mediator proteins(at “g”). If you look carefully at “g” and “h” you can see how their shape was drawn to be complementary to the activators that are bound to the enhancer. On another region of the DNA, a DNA bending protein (at “f”) bends the DNA in a way that causes the enhancer to approach the promoter.
In diagram 3, all of this comes together to form a transcription initiation complex (j). This allows RNA polymerase (i) to bind with the promoter, enabling transcription (“k”).
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