A cloned DNA sequence was used to probe a Southern blot. There were 2 DNA sample

Question

A cloned DNA sequence was used to probe a Southern blot. There were 2 DNA samples on the blot, one from white blood cells and the other from a liver biopsy of the same individual. Both samples had been digested with HpaII. The probe bound to a single 2.2 kb band in the white blood cell DNA, but bound to 2 bands (1.5 and 0.7 kb) in the liver DNA.

a ) What are possible explanations for this difference?

b ) How would it affect your answer if you knew that white blood cell and liver DNA from this individualboth showed the 2 band pattern when digested with MspI ?

Explanation / Answer

Interesting question . . . I'm not positive that I know the answer, but since no one else has tackled this question, I'll share my thoughts. The first thing that comes to my mind is a mutation in the germline that gave rise to either the liver or the WBCs (there's no indication in the question whether the single band or the double band is the wild-type, so let's assume that the liver's pattern is wild-type). Perhaps the HpaII recognition site was mutated in the WBC lineage. The second part of the question puts paid to this explanation, though -- since MspI and HpaII have the same recognition sequence, mutation in only one cell type can't be the answer. I don't know how detailed the material is in your course, but I'm wondering if the answer has something to do with methylation. I might look up how the two restriction enzymes deal with methylation and see if that sheds any light on the problem. Good luck!

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