Practice question

Surface sterilization removes epiphytic and surface-borne microorganisms from explants without killing plant tissues, essential prerequisite for establishing contaminant-free cultures. Most effective protocol uses sequential treatment with 70 percent ethanol for 30-60 seconds followed by sodium hypochlorite solution 0.5-2 percent available chlorine for 10-20 minutes containing surfactant Tween-20 0.1 percent to improve wetting. Ethanol acts as degreasing agent dissolving epicuticular waxes, disrupting microbial lipid membranes and denaturing proteins facilitating penetration of subsequent sterilant. Sodium hypochlorite dissociates in water to hypochlorous acid HOCl strong oxidizer chlorinating amino groups of proteins, oxidizing sulfhydryl groups, nucleotides, and causing lethal damage to bacterial cell walls and fungal spores. Concentration and exposure time optimized for explant type, tender meristems need shorter treatment to avoid phytotoxicity bleaching. Autoclaving at 121 degrees would kill explant cells, dry heat similarly destructive, UV radiation alone has poor penetration into crevices and causes DNA mutations. Repeated sterile distilled water washes after sterilization remove residual chlorine preventing growth inhibition in nutrient medium.