As an out-class practice, I was working on a restriction map problem since the last night. The problem itself was basically simple, yet I spent rather a long time to do it.
When you digest the genome with enzyme A, and you get 2 fragments; you digest the genome with enzyme B, and you get 3 fragments. Then, you digest the genome with both A and B, and you get 4 fragments. Now you need to analyse the lengths of the fragments. If you see the total length of two fragments from A+B is exactly the same to one fragment from A, you may confirm one of enzyme B cutting sites by measuring the length from the end of the genome/enzyme A cutting site to this enzyme B cutting site.
There were three enzymes used in the problem. I made a draft of two of them, but when I tried to confirm the cutting sites of the last enzyme, I couldn't get all fragment lengths exactly match in other conditions (the combinations of using enzymes, I mean).
But my idea was absolutely correct. I was pretty sure about it. About the instructor of the class, I know he is often in the situation of "I-lost-my-thought." Thus I came to doubt the position of fragment bands in the illustration, which was drawn by the instructor, I guess. Some bands, although they look like at different horizontal levels, is it possible that they actually have the same size? Thinking about it, I tried some changes on the numbers of the sizes of each band. Amazingly, after several times of changing, I got all lengths matched.
I spent much of time due to the stupid badly drawn illustration sure enough. orz…