Final answer:
When terminal deoxynucleotidyl transferase (TdT) expression is turned off in small pre-B cells, there will be P nucleotides present but no N nucleotides in about half of the light-chain genes. TdT is essential for adding N nucleotides that contribute to antibody diversity in the immune system. Lack of TdT leads to less diverse antibodies, affecting immune response.
Step-by-step explanation:
When expression of terminal deoxynucleotidyl transferase (TdT) is turned off in small pre-B cells, the result is the presence of P nucleotides but an absence of N nucleotides in around 50% of light-chain genes. TdT is responsible for the addition of N nucleotides to the DNA during V(D)J recombination, which is a process of creating diversity in antibody genes. Without TdT, V(D)J recombination can still occur, but the additional randomness provided by N nucleotide addition is lost, leading to less diversity in the antibodies produced.
During B-cell maturation, proteins called RAG-1 and RAG-2 are part of the DNA recombinase complex that mediates the rearrangement of V, D, and J segments to generate functional immunoglobulin genes. This recombination, along with further mRNA splicing, creates the necessary diversity for the antibody repertoire. The RAG proteins do not directly add N nucleotides; instead, this is the role of TdT.
In the context of agammaglobulinemia, patients with defective BTK gene lack proper B-cell maturation and cannot proceed beyond the pre-B-cell stage, which is crucial for immunoglobulin production. Patients suffering from this condition experience recurrent infections due to the absence of antibody production, underscoring the importance of B-cell maturation and antibody variability in immune response.