Question about restriction enzyme mapping

[jocelyn074]

Hi,

I am working on this problem as practice for my exam next week and i am stuck on this one. The question is:Someone gives you a plasmid, you digest it with Pst1 and get 3 bands that are 6kb, 4kb, and 3kb. You digest with Xho and get 1 band that is 4 kb long.

Ok so I was thinking that it would be 13kb long with the numbers from the Pst1 enzymes. But I was thinking that if Xho had only 1 cut that the band would be more like 13 kb long. I don't really understand the numbers so I must be doing it wrong.

I thought the addition of the bands=the plasmid size so I don't know why the band from xho is only 4kb.

Can somone please help?

[JackBean]

What about, if Xho digest three-times? Always like 4 kbp away from each other? So you will get one band with three pieces actually