Final answer:
Activators turn on eukaryotic genes by binding to enhancers which are DNA regions that can be distant from the gene itself. This prompts the DNA to loop so that the enhancer is near the promoter, facilitating the binding of transcription initiation complex and starting transcription. Transcription factors and co-factors also play a critical role in this regulatory mechanism.
Step-by-step explanation:
An activator can help turn on eukaryotic genes by binding to regions of DNA known as enhancers. These enhancers may be located upstream, within the coding region, or downstream of a gene. The activators facilitate the binding of the transcription initiation complex to the promoter of a gene by causing the DNA to fold in a way that brings the enhancer closer to the promoter. This folding enables a protein-protein interaction necessary for RNA polymerase to initiate transcription. Eukaryotic gene expression is also regulated by transcription factors that bind to specific sequences, such as the TATA box, GC box, and CAT box in higher eukaryotes or TATA box and UAS in yeast.
Moreover, the transcription initiation complex may include co-factors attracted by the activators' binding to enhancers, which helps stabilize the DNA shape needed for RNA polymerase II to begin transcription efficiently. However, transcriptional repressors can also bind to these regions to prevent transcription in response to certain external stimuli, maintaining control over gene expression.