Final answer:
In B cells making IgM, the untranscribed gene segments encoding constant regions of all other isotypes lie downstream of the expressed mu heavy chain segment. The genetic diversity seen in antibodies is due to the recombination of V(D)J gene segments, allowing each B cell to potentially express a unique antibody molecule.
Step-by-step explanation:
In B cells making IgM, the array of untranscribed gene segments encoding constant regions of all other Ig isotypes lies downstream of the expressed μ (mu) heavy chain segment in the B cell's DNA. These genes contain multiple variant V (Variable), D (Diversity), and J (Joining) regions linked to a C (Constant) region. During B cell development, DNA recombinase randomly excises most of these segments from the gene in a process known as V(D)J recombination. This recombination and splicing result in the potential for over 106 possible V(D)J combinations, allowing each differentiated B cell to potentially express a unique antibody due to this immense diversity in the variable regions.
Furthermore, rearrangements of these segments allow the B cell to switch from expressing IgM to other immunoglobulin classes (IgG, IgA, IgD, IgE) through a process called class switch recombination, which occurs in response to cytokine signals during an immune response. The constant region of the heavy chain determines the immunological mechanism of action of the corresponding antibody class, enabling different functions despite binding the same antigen.