In immunology a major topic of discussion early on is the rearrangement of variable gene segments to form the complete variable region, either the heavy or light chains. Remember from general biology that an antibody is formed from the make-up of two light chain segments and two heavy chain segment with both heavy and light having a variable region. Every immature B-cell becomes uniquely designed to become an antibody generating machine from complex rearrangements of the DNA with the result of a specific variable region and a specific antibody. Therefore a mature B-cell provides no help determining the number of potential variable regions because rearrangement has already occurred, however we can use a germline cell. In my example is the haploid germline organization of the IgH locus of the channel catfish (Ictalurus punctatus).
The image is a little complicated to read, however it is a real example problem from a difficult immunology examination. The vertical black lines indicate functional exons, but the lighter grey lines represent pseudo exons (My professor enjoyed challenging us). So now our problem is to determine how many unique catfish heavy chain variable regions could be produced by deletional rearrangement assuming all the exons point in the same reading frame. To determine this number of rearrangements we need to determine the number of V, D, and J segments and multiply the numbers together. Although the image is barely legible you should be able to count 26 VHregions, additionally the prompt of the problem provides that 6 DH exons and 12 JH exons. From here the problem is simply multiplying the (26V)(6D)(12J) = 1872 unique heavy chain variable regions for the haploid chromosome. However a catfish is a diploid organism and the combinations multiplies by two for the two chromosome with heavy chain variable genes, 1872 x 2 = 3744 Unique catfish heavy chain variable regions can be produced by deletional rearrangement.
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