A 3.5-kb segment of DNA containing the complete sequence of a ...

Question

A 3.5-kb segment of DNA containing the complete sequence of a mouse gene is available. The DNA segment contains the promoter sequence and extends beyond the polyadenylation site of the gene. The DNA is studied by band shift assay, and the following gel bands are observed.

Match these conditions to a specific lane of the gel.
3.5-kb fragment plus TFIIB, TFIID, TFIIF, and RNA polymerase II

Gel electrophoresis image showing five lanes with distinct DNA band patterns for a 3.5-kb mouse gene segment.

Match these conditions to a specific lane of the gel.

3.5-kb fragment plus TFIIB, TFIID, TFIIF, and RNA polymerase II

Accepted Answer

Welcome back, everyone. Here's our next problem. It says the band shift assay was used to examine a segment of DNA containing the whole sequence of a gene including the promoter and terminator regents. The following image shows the gel bands that were observed and we have a gel with five lanes each with different bands of different sizes. With the exception of lane one and lane four, which look about the same size. And then the problem says lane one represents the control containing only naked DNA, determine which of the following conditions corresponds to lane four choice A DNA plus T F two B, choice B DNA plus T F two D as in dog, choice C DNA plus R N A polymerase two four choice D DNA plus T F two B like boy plus T F two D like dog. So when we think about the different sizes of our bands here, we want to always remember that the more proteins bound to the DNA, the lower the mobility. So the higher bands equal the most proteins bound. And then conversely as we see lane one, the control, which has only naked DNA with no proteins bound. No proteins represents the smallest band that has traveled the furthest. It's the lowest on the gel. Now, when we look at lane four, which is what we're supposed to identify, we see that that band is equally small. It looks exactly the same size as the control. Why would that be? We know that among our answer choices, they all have at least one protein in the mix. Well, the answer is that R N A polymerase two cannot bind to the promoter without the help of a transcription factor of A T F. So when we look at choice C, it's a DNA plus R N A polymerase two, but there are no transcription factors in that sample. Therefore, the R N A polymerase two will not be bound to the DNA. And that means that indeed we would see a band that is the exact same size as the control. The DNA doesn't have a protein bound to it. And so it will look just the same. And that is what we see in lane four. So when we're supposed to identify which condition corresponds to that way it is choice C DNA plus R N A polymerase two, all of our other answer choices involve transcription factors in our sample with the DNA. And those would bind to the DNA and lessen its mobility resulting in higher bands on the gel. So that's why the other answer choices are not correct. And we're looking at choice c see you in the next video.

Key Concepts

Transcription Factors

Gel Electrophoresis

Polyadenylation Site

Watch next