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Present three models to explain how an enhancer can act on a promoter hundreds of base pairs away.
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Enhancers are regulatory DNA sequences that can increase the likelihood of transcription of a particular gene, while promoters are regions of DNA that initiate transcription. Enhancers can be located far from the genes they regulate, sometimes hundreds of base Show more…
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When enhancers were initially found to influence transcription from many thousands of nucleotide pairs away from the promoters they control, two principal models were invoked to explain this action at a distance. In the "DNA looping" model, direct interactions between proteins bound at enhancers and promoters were proposed to stimulate transcription initiation. In the "scanning" or "entry-site" model, RNA polymerase (or another component of the transcription machinery) was proposed to bind at the enhancer and then scan along the DNA until it reached the promoter. These two models were tested using an enhancer on one piece of DNA and a $\beta$ -globin gene and promoter on a separate piece of DNA (Figure $\mathrm{Q} 8-9$ ). The $\beta$ -globin gene was not expressed when these two separate pieces of DNA were introduced together. However, when the two segments of DNA were joined via a linker (made of a protein that binds to a small molecule called biotin), the $\beta$ -globin gene was expressed. Does this experiment distinguish between the DNA looping model and the scanning model? Explain your answer.
When enhancers were initially found to influence transcription many thousands of nucleotide pairs from the promoters they control, two principal models were invoked to explain this action at a distance. In the "DNA looping" model, direct interactions between proteins bound at enhancers and promoters were proposed to stimulate transcription initiation. In the "scanning" or "entry-site" model, RNA polymerase (or another component of the transcription machinery) was proposed to bind at the enhancer and then scan along the DNA until it reached the promoter. These two models were tested using an enhancer on one piece of DNA and a $eta$ -globin gene and promoter on a separate piece of DNA. The $eta$ -globin gene was not expressed from the mixture of pieces. However, when the two segments of DNA were joined via a linker (made of a protein that binds to a small molecule called biotin), the $eta$ -globin gene was expressed. Does this experiment distinguish between the DNA looping model and the scanning model? Explain your answer.
Enhancers are sequences that affect the in itiation of the transcription of genes that are hundreds or thousands of nucleotides away. Transcriptional activator proteins that bind to enhancers usually interact directly with transcription factors at promoters by causing the interven ing DNA to loop out. An enhancer of bacteriophage T4 does not function by looping of the DNA (D. R. Herendeen et al. 1992. Science 256: $1298-1303$ ). Propose some additional mechanisms (other than DNA looping by which this enhancer might affect transcription at a gene thousands of nucleotides away.
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