Final answer:
Whether an immunoglobulin is secreted or membrane-bound is determined by mRNA splicing, which dictates the form the antibody takes based on the inclusion or exclusion of specific sequences during the processing of the gene's primary transcript.
Step-by-step explanation:
The answer to the student's question on whether an immunoglobulin is secreted or membrane-bound is directly determined by mRNA splicing. This process influences how the primary transcript of the immunoglobulin gene is processed, ultimately deciding whether the antibody will be membrane-bound or secreted. During B cell development and activation, immunoglobulin genes undergo extensive modifications. The primary change at the DNA level is DNA rearrangement, which creates a unique variable region that allows antibodies to bind to a specific antigen. Once the variable region is established, alternative mRNA splicing can result in different forms of immunoglobulins—one for membrane-bound receptors on naive B cells and another for antibodies secreted by plasma cells. The same genes encode both forms, and the difference is determined during mRNA splicing when a particular sequence is included or excluded, resulting in either a membrane-bound form with a hydrophobic tail or a secreted form without the hydrophobic region.