Final answer:
The immunoglobulin loci in non-B cells are transcriptionally silent because the rearrangement of the immunoglobulin genes occurs during B cell development in the bone marrow, not in non-B cells. This rearrangement process leads to the expression of unique antibody variable regions.
Step-by-step explanation:
The immunoglobulin loci in non-B cells are transcriptionally silent because the rearrangement of the immunoglobulin genes, which is necessary for the production of antibodies, occurs during B cell development in the bone marrow. This rearrangement involves the recombination of different gene segments, such as V, D, and J, to create a diverse repertoire of antibody molecules.
This process is called VDJ recombination, and it leads to the expression of a unique antibody variable region capable of binding a specific antigen. In non-B cells, this rearrangement process is not active, resulting in the immunoglobulin loci being transcriptionally silent.
For example, during B cell maturation, an enzyme called DNA recombinase randomly excises V and J segments from the light chain gene. Splicing at the mRNA level further rearranges the genes, resulting in the production of antibodies with unique variable regions.