A circular bacterial plasmid consists of 1000 base pairs (bp). A specific restriction enzyme has three recognition sites within this plasmid. The sites are at 150 bp, 250 bp and 950 bp. After complete restriction, what size are the fragments?
A. 100 bp, 200 bp and 700 bp only
B. 100 bp, 150 bp, 200 bp only
C. 50 bp, 100 bp, 150 bp and 700 bp only
D. 50 bp, 100 bp and 700 bp only
E. 50 bp, 100 bp, 150 bp, 200 bp and 700 bp
Before we work our way to the correct choices, let’s eliminate some considerations: Please do not be confused, this is a circular bacterial plasmid. Which is why although there are 3 recognition sites, do not mistakenly think that there are 4 fragments, which would be the case if this was a linear and not circular arrangement. Since this is a circular plasmid, there will be only 3 fragments, thus, you can eliminate choices C and E from your considerations.
How can we then begin working out this question?
First, it would be useful to look at 1-1000 like a scale arranged as a circle, so that it would be easier for you to visualize the different fragments.
You can start discerning some of the sizes of the fragments given the 3 recognition sites. The distance of 150 to 250 bp is 100 bp, thus, you can be sure that one of the fragments has a size of 100 bp.
The distance of 250 to 950 bp is 700 bp, which you can now safely say that one of the fragments has a size of 700 bp.
Here’s a bit of a tricky part, where you have to discern the distance between 950bp and 150bp. Do not be tempted to do simple subtraction and answer 800bp, which is incorrect, but instead, remember the reminder to imagine 1-1000 like a scale arranged in a circle. The distance between 950-1000 bp is 50 bp, and since this is a circle, you will have to continue along this circle, and add the distance until 150bp, which is 200bp in total. Thus, one of the fragments has a size of 200 bp.
Therefore, the fragments will have a size of 100, 200, and 700 bp only, which makes choice A as the only correct choice.