Partial digest in restriction mapping

[Mycelium]

Homework Statement

The question:
Scientists need to take precautions when they carry out restriction mapping. They need to make sure that the enzyme they have used has completely digested the DNA. One check they may carry out is to add the sizes of the fragments together. How could scientists use this information to show that the DNA has not been completely digested?

Homework Equations

The Attempt at a Solution

I know that it would be indicative of a partial digest if the sum of the fragments was greater than the size of the original fragment, but I don't understand why it would be so?

 

[Ygggdrasil]

Well, let's say you have a 10 kb linear DNA that gets cut into a 4 kb and 6 kb piece. What bands do you expect to see in a complete digest, and what is the sum of the sizes of the bands? If the digest is incomplete, what fragments do you expect to see, and what is the sum of the sizes of the bands?

 

[Mycelium]

Well, let's say you have a 10 kb linear DNA that gets cut into a 4 kb and 6 kb piece. What bands do you expect to see in a complete digest, and what is the sum of the sizes of the bands? If the digest is incomplete, what fragments do you expect to see, and what is the sum of the sizes of the bands?

Okay, I think I get it. So if it was complete, you would see only 4kb and 6kb fragments. If incomplete, there would be 10kb pieces along with the 4 and 6 kB pieces so it would seem as though you had a 20kb piece originally?

 

[Ygggdrasil]

Yup. Incomplete digestion essentially gives "extra" bands that will make the sum of the sizes of the bands larger than the original piece of DNA.