Assume that a viral DNA molecule is a 50-µm-long circular strand with a uniform 20-Å diameter. If this molecule is contained in a viral head that is a 0.08-µm-diameter sphere, will the DNA molecule fit into the viral head, assuming complete flexibility of the molecule? Justify your answer mathematically.

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Welcome back. Let's look at our next problem. It says an equal I blank is the enzyme that reduces the number of negative super coils in a closed circular D. N. A. Well, when we think about needing to reduce negative super coils, we want to think about that. Pulling apart of the double strand helix of DNA for DNA replication. And the illustration often used is that of two strands of rope or string if they were twisted together, the way our D. N. A. Double helix is. And imagine that you were going to pull apart the two ends of this rope. And as you pull these ends apart, you're going to introduce more coils, more tension into the part back here that will coil more and more tightly and begin to super coil upon itself as you separate these strands. So you have to have DNA strategies for reducing that tension when you pull apart those strands. And when we think about dealing with this tingling and tension in D. N. A. We want to look at the topo isom races and we see that choice is A and B. We have topo I summaries one and Topo I summaries to. And the question asks us which enzyme is the one that reduces the number of negative super coils in that D. N. A. And there we wanna look at choice A. Topo I summaries one. Um The one here refers to the fact that it cuts one DNA strand of our two and then really gates it and as a result does reduce the number of super coils that form Choice B. Topo I summaries to um is also part of this process. Um But it actually can introduce super coils into that D. N. A. So that's why it's not our correct choice. The two here refers to the fact that it cuts to do both DNA strands and then really gates them and can therefore introduced super coiling into the strand. I see hell a case here. A case is the enzyme that does the pulling apart so it pulls apart the two strands. So that's why that's not our answer. It's not involved in reducing the number of super coils and finally proliferates the polymer races. Are those enzymes that synthesize chains of polymers including DNA and RNA. DNA polymerase is what comes along and synthesizes the new DNA chain as you pull that apart. So that's not our correct answer here. So again, an E coli choice A top. Why some race one is the enzyme that reduces the number of negative super coils in a closed circular DNA. Hope to see you in the next video

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Key Concepts

DNA Molecular Dimensions and Volume

Volume of a Sphere

Concept of Molecular Flexibility and Packing