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7. Organization and rearrangement of the antigen receptor genes: Difference between revisions

7. Organization and rearrangement of the antigen receptor genes (view source)

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Starting with the heavy chain, it’s gene looks like this. Each box is a genetic segment that makes up the gene. The red segments are called “variable” (V), the green are called “diversity” (D), the yellow are called “joining” (J), and the blue are constant and are not modified.
Starting with the heavy chain, it’s gene looks like this. Each box is a genetic segment that makes up the gene. The red segments are called “variable” (V), the green are called “diversity” (D), the yellow are called “joining” (J), and the blue are constant and are not modified.
[[File:Ig heavy chain gene.png|center|thumb|536x536px|The organization of the Ig heavy chain gene. The red are called «variable» segments, the green «diversity», the yellow «joining» and the blue «constant». The segments are very underrepresented. There are actually 65 V segments, 27 D and 6 J.]]
[[File:Ig heavy chain gene.png|center|thumb|536x536px|The organization of the Ig heavy chain gene. The red are called «variable» segments, the green «diversity», the yellow «joining» and the blue «constant». The segments are very underrepresented. There are actually 65 V segments, 27 D and 6 J.]]
During B-cell development, this gene (and the gene for the light chain, which we will see later) will be reorganized. When finished, the gene will only contain only one of each type of segment. So, the B-cell will cut out all but one V segment, all but one D segment, and all but one J segment, and put the three (randomly chosen) remaining segments together. These VDJ segments will code for the variable domain on the heavy chain, while the C segments will code for the constant domains (there is one C segment for each heavy chain isotype). Because there are 65 V segments, 27 D segments and 6 J segments to choose from, there are very many possibilities for recombination, roughly 3×1011. Each V, D and J combination will yield an B cell that can produce antibodies that can recognize different epitopes. After recombination, the gene might for example look like this:
During B-cell development, this gene (and the gene for the light chain, which we will see later) will be reorganized. When finished, the gene will only contain only one of each type of segment. So, the B-cell will cut out all but one V segment, all but one D segment, and all but one J segment, and put the three (randomly chosen) remaining segments together. These VDJ segments will code for the variable domain on the heavy chain, while the C segments will code for the constant domains (there is one C segment for each heavy chain isotype). Because there are 65 V segments, 27 D segments and 6 J segments to choose from, there are very many possibilities for recombination, roughly 3×10<sup>11</sup>. Each V, D and J combination will yield an B cell that can produce antibodies that can recognize different epitopes. After recombination, the gene might for example look like this:
[[File:Ig heavy gene after recombination.png|center|thumb|395x395px|After recombination, only one V, one D, and one J remain. The C are untouched.]]
[[File:Ig heavy gene after recombination.png|center|thumb|395x395px|After recombination, only one V, one D, and one J remain. The C are untouched.]]
Because the light-chain can be either κ or λ, there are two genes encoding for it. Their genes are very similar in build-up to the gene for the heavy chain, except the light chains don’t have D segments, just V, J and C.
Because the light-chain can be either κ or λ, there are two genes encoding for it. Their genes are very similar in build-up to the gene for the heavy chain, except the light chains don’t have D segments, just V, J and C.