Circular DNA is a product of the immunoglobulin class switch rearrangement

There are various classes of immunoglobulins including IgD, IgM, and IgG. The genes coding for these molecules undergo changes so that different parts of the genes are expressed at different times during an immune response. One of the portions of the immunoglobulin gene that changes is the constant region of the heavy chain, a component of the immunoglobulin molecule. A study was conducted with a cell that is a premature B lymphocyte, known as the pre-B-cell. The cell switched from expressing the mu heavy chain to expressing the gamma 2b heavy chain. Occasionally, the gene rearranges so that the deoxyribose nucleic acid (DNA) sequence between the mu and gamma 2b sequences are inverted. For this to happen, the DNA must form a loop, which is then is deleted or removed. This produces a loop or circle of DNA that is free from the rest of the genes. These circular forms of DNA have been isolated and have been found to contain the sequences that are deleted when the heavy chains switch from one class of immunoglobulin to another. Therefore, the mechanism of deletion of DNA segments, which occurs during immunoglobulin class switching, involves the formation of circular loops of DNA. (Consumer Summary produced by Reliance Medical Information, Inc.)