While various positive control components lie close to the gene they regulate additional can be situated long distances away (sometimes 10-50 kb) either upstream or downstream of the gene shown in the figure. A long-distance positive control series of this type is called as an enhancer if the transcription factor(s) which binds to it enhances the rate of transcription. An enhancer is classically 100-200 bp long and haves several sequence elements which act together to give the whole enhancer activity. When they were first found, enhancers were viewed as a distinct class of control component in which they:
? Over long distances can activate transcription;
? It can be located downstream or upstream of the gene being controlled;
? Are active in moreover orientation with respect to the gene.