Method of Coexpression
In the human Heavy-Chain Locus, 3' of the V-D-J cassette is a series of C (for constant) genes, each conferring to an Ig isotype. The Cμ gene is 3' and closest to the V-D-J cassette, with the Cδ gene appearing 3' to Cμ.
A Primary mRNA transcript will contain the transcribed V-D-J cassette, and the Cμ and Cδ genes, with introns in between them.
Alternative splicing can then occur, causing a selection of either Cμ or Cδ to appear on the functional mRNA (μ mRNA and δ mRNA respectively). Alternative splicing is thought to be possible due to two Polyadenylation sites, one appearing between the Cμ and Cδ, and the other 3' of Cδ (polyadenylation in the latter site would cause Cμ to be spliced away along with the intron). The precise mechanism of how the polyadenylation site is chosen remains unclear.
The resulting functional mRNA will have the V-D-J and C regions contiguous, and its translation will generate either a μ heavy chain or δ heavy chain. The heavy chains then couple with either κ or λ light chains to create the final IgM or IgD antibody.
Read more about this topic: Immunoglobulin D
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