Why do you put the x-ray film on top of nylon membrane in tray?

In the context of molecular biology techniques such as Southern blotting or Northern blotting, X-ray films are not typically placed on top of nylon membranes within the trays. Instead, nylon membranes containing electrophoretically separated nucleic acids (DNA or RNA) are often transferred onto solid supports, such as Whatman papers, followed by baking or UV crosslinking to permanently immobilize the nucleic acids onto the membranes.

X-ray films are placed next to or in close contact with these nylon membranes, usually wrapped separately in plastic wrap, when autoradiography is performed. Autoradiography allows the detection and visualization of radioactively labeled nucleic acids on the membranes by placing X-ray films next to them in cassettes (enclosed light-proof carriers that hold the film closely to the membrane).

When the radioactive material decays, it releases energy in the form of photons or electrons, which interact with the silver halide crystals on the X-ray film. This leads to the development of dark spots or bands on the X-ray film after processing, corresponding to the positions of the radiolabeled nucleic acids on the membrane.

Therefore, the X-ray film is not placed directly on top of the nylon membrane but is used separately to capture the radiation emitted by the radioactive probes hybridized to the nucleic acids on the membrane.