Practice question

Positively charged nylon membranes contain quaternary ammonium groups that initially attract negatively charged phosphate backbone of DNA through electrostatic interaction during Southern or Northern transfer. For permanent immobilization, membrane is exposed to ultraviolet irradiation at 254 nm. Photochemical reaction creates covalent bonds between thymine residues of DNA and amine groups of nylon matrix. This covalent fixation withstands high stringency hybridization, high temperature washing, and repeated stripping cycles. By contrast, nitrocellulose relies solely on hydrophobic interactions, and neutral nylon without crosslinking shows lower retention, explaining why UV-induced covalent linkage is standard for nylon membranes.