This question is asking about VDJ rearrangement which happens in the bone marrow. The genes are all chopped up because the B cell is trying to generate a unique combination for its receptor
simple concepts... odd wording
Chapter 3 of "how the immune system works" - awesome book
I believe (pls correct me if I'm wrong) you would have similar Southern blot results seen in B cells undergoing somatic hypermutation, but that takes place in the secondary lymphoid tissue, not in the bone marrow.
You can look at this question strategically.
That's important, because now you know they are looking at the constant region of immunoglobulin aka Fc of the heavy chain.
So why is this cDNA that encodes Fc, lighting up only once in other tissues, but the same cDNA lights up multiple times in lane 2 (bone marrow)? This is because, at the bone marrow random recombination of light or heavy chain occurs, and our cDNA is present in different random combinations with light chains. That specific cDNA sequence they used didn't change between the tissues or even within the bone marrow, and it will only bind to the one gene sequence. The fact it's lighting up multiple times tells you that the same sequence is present, your question is why so many times.
Also a side note, southern blots are time consuming, so labs use other methods to do the same thing (like PCR), but southern blots are still the best when it comes to checking immunoglobulin and T cell receptor gene rearrangement.
Southern blots are commonly used in immunological studies, as the southern blot allows for the study of DNA alterations. What is normally one gene configuration related to immune globulins in most tissues demonstrates multiple different bands in the bone marrow, indicative of gene rearrangement. This is basically how we create new antibodies. Reactive processes are polyclonal (multiple bands); leukemia, in contrast, is monoclonal (single band).