Practice question

Terminal deoxynucleotidyl transferase, also called TdT, is a unique template-independent DNA polymerase expressed in pre-B and pre-T lymphocytes where it adds N-nucleotides during V(D)J recombination. In vitro, it catalyzes addition of deoxyribonucleotides preferentially to the 3'-hydroxyl terminus of single or double-stranded DNA, extending the strand without a complementary template. This property underlies homopolymer tailing for cDNA cloning, 3'-end labeling with dideoxynucleotides, and generation of sticky ends from blunt fragments, as poly-A and poly-T tails can anneal to facilitate recombinant construction.