In this chapter, we focused on how DNA is organized at the chromosomal level. Along the way, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions: How do we know that viral and bacterial chromosomes most often consist of circular DNA molecules devoid of protein?

Video transcript

Hi everyone. Welcome back. Here's our next question. It says If a double stranded linear molecule has a helix containing 20 complete turns, what linking number should it have for it to be considered as energetically relaxed? If we change the linear molecule into a closed circle. Well the key here is we have to imagine if our molecule is linear Then we assume it automatically. Is that the you know, most favorable uh helical arrangement in terms of number of base pairs per turn, which is 10.4 base pairs per turn. So we know we have 20 turns at this optimal arrangement and now we've turned it into a circle. Are linking number or l it's just the number of turns or better described in the case of a circular piece of DNA. The number of times the strands cross each other in that circle. And this molecule is energetically relaxed If it has that ratio of 10.4 base pairs per turn, which is the most favorable arrangement that DNA naturally adopts in its linear form. So we know that since the linear form is already in the most efficient number of turns, we're going to expect that we're still going to see 20 turns when we put it in a circular form. So therefore our linking number will have to equal For our molecule to be energetically relaxed. So our answer here is going to be choice B-20. Now it's good to know that there is a formula for calculating linking number. Um if we weren't given that 20 turns information. And that formula is that l the linking number. And for a relaxed piece of circular DNA. Ask to equal the number of base pairs divided by the number of base pairs per turn. Let's make that a little more clear here. Doesn't look like base pairs there. So in this case we don't know the number of base pairs but we don't need to know the number of base pairs because we've been told how many turns we could then say it's 20 turns times 10.4 base pairs per turn. That would give us the number of base pairs but then we're dividing again by the number of base pairs per turn. So we're going to end up with the number 20 using that formula. But if you're given the number of base pairs and you're trying to calculate the linking number, you would use this formula here. But again, since we know how many turns are in that arrangement on the linear molecule, we know for it to be energetically relaxed, there must be the same number of turns or strands crossing over each other in the circular version. So the linking number will be choice B-20. See you in the next video