Final answer:
Genetic rearrangement in the thymus allows for the production of a vast population of T cells with millions of specific T cell receptor proteins.
Step-by-step explanation:
The genes that encode for different T cell receptor proteins are produced through a process called genetic rearrangement. This process occurs in the thymus during the first step of thymic selection. The genes that code for the variable regions of the T cell receptors are divided into different gene segments: V (variable), D (diversity), and J (joining). The a chain of the T cell receptor has around 70 different Va segments and 61 different Ja segments, while the ß chain has 52 different Vå segments, 2 different Dß segments, and 13 different Jß segments. Through genetic rearrangement, one Va segment is brought together with one Ja segment to code for the variable region of the a chain, and one VB segment is brought together with one DB segment and one Jß segment to code for the variable region of the ß chain. All the possible combinations of rearrangements between the different segments allow for the production of a vast population of T cells with millions of specific T cell receptor proteins.
For the alpha chain of the TCR there are roughly 70 variable (V) and 61 joining (J) segments, while the beta chain comprises approximately 52 V, 2 diversity (D), and 13 J segments. A single V and a single J segment are selected to form the alpha chain, while a V, a D, and a J segment form the beta chain. This combination process substantially increases the variety of TCRs beyond the actual gene count.
Therefore, rather than being determined by a finite number of genes, the vast array of TCRs is generated through a combinatorial process known as V(D)J recombination, occurring within the thymus.