Genomic DNA from the nematode worm Caenorhabditis elegans

Question

Genomic DNA from the nematode worm Caenorhabditis elegans is organized by nucleosomes in the manner typical of eukaryotic genomes, with 145 bp encircling each nucleosome and approximately 55 bp in linker DNA. When C. elegans chromatin is carefully isolated, stripped of nonhistone proteins, and placed in an appropriate buffer, the chromatin decondenses to the 10-nm fiber structure. Suppose researchers mix a sample of 10-nm–fiber chromatin with a large amount of the enzyme DNase I that randomly cleaves DNA in regions not protected by bound protein. Next, they remove the nucleosomes, separate the DNA fragments by gel electrophoresis, and stain all the DNA fragments in the gel.
How do the expected results support the 10-nm–fiber model of chromatin?

How do the expected results support the 10-nm–fiber model of chromatin?

Accepted Answer

Hi. All. Let's look at our next problem. It says in E Caros to contain the DNA inside the nucleus, it is wrapped around nuclear proteins called his stones. The DNA along with his stones is then wrapped into a 30 nanometer spiral called choice A nucleosome B solenoid C chromatin or D double helix. Well, let's recall that our DNA wrapped around histone proteins that our question mentions that is our beads on a string structure. Put that in quotation marks there not a scientific description. And then those string of beads are then wrapped further around in coils. That's the 30 nanometer spiral we're asked about. And that structure is called choice B A solenoid. Now to keep this straight and distinguish it from our other answer choices. Sometimes it can be helpful to look at a diagram just recalling the different orders of structure of DNA as it becomes more and more condensed. So choice D is double helix and that's of course, our most basic level of DNA structure there, the two complementary strands in a double helical shape. So that's not our answer choice as that's lower down the line. And then our DNA double helix proceeds to nucleosome, that double helix wraps around the his stones leading to those little beads of nucleosome connected by a string that linker DNA. Then as we said, then we go to the level of the solenoid. So the nucleosome are all about 11 nanometers around the the little histone core. The solenoid is then this sort of 30 nanometer fiber. And then in the next level up, those solenoids become looped even further into these looped domains to become chromatin fibers. And these chromatin fibers are about 300 nanometers. We can see how we're just packing it in further and further. Now, an interesting thing about our solenoids here doubling back a little bit to the point we're asked about. This is what state our DNA is in an interface when it's not condensed into chromosomes. But as we go further, getting ready for mitosis, we need to condense our DNA into chromosomes. So the next step after that solenoid are these loop domains that become chromatin fibers. And then from that point compacted even further to become a chromatid. And that's how we get to our chromosomes. So that can be a helpful little diagram as we think about DNA and chromosome structure. But then going back to our answer, choices, we see that choice. A the nucleosome is a step before our solenoid in the compaction scale. And then choice C chromatin is a step past our solenoid solenoid gets looped even further to become chromatin fibers. So that's why choice C isn't our answer. So again, when the DNA wrapped around his stones becomes wrapped further into a 30 nanometer spiral that is called Choice B. The solenoid structure see you in the next video.

Key Concepts

Nucleosome Structure

10-nm Fiber Model

DNase I Cleavage

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