Final answer:
Double-negative thymocytes give rise to T-cell receptors through a process called V(D)J recombination, where gene segments rearrange to form unique TCR specificities. Following recombination, thymocytes are selected based on their ability to recognize self-MHC molecules, leading to mature T cells that express CD4 or CD8.
Step-by-step explanation:
The process by which a double-negative thymocyte gives rise to an alpha/beta or gamma/delta T-cell receptor involves V(D)J recombination. This genetic mechanism generates the diversity necessary for the immune system to recognize a vast array of different antigens. Initially, thymocytes do not express CD4 or CD8 molecules and are termed double-negative. During maturation in the thymus, these cells undergo V(D)J recombination where Variable (V), Diversity (D), and Joining (J) gene segments rearrange to form the unique antigen-binding region of the T-cell receptor (TCR).
This recombination is mediated by enzymes including DNA recombinase, which facilitates the random excision of the gene segments. The resulting rearrangement is critical for the specificity of the TCRs. As these thymocytes develop, they subsequently undergo positive selection, which tests their ability to recognize self-major histocompatibility complex (MHC) molecules presented by the thymic epithelial cells. Successful thymocytes become single-positive, expressing either CD4 or CD8, and exit the thymus as mature T cells capable of responding to antigens.