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Procedure

Preparation
	Dissolve 3 g sodium chloride and 10 ml detergent with 100ml distilled water. Cut the onion into small pieces and grind in the above detergent-salt mixture.
High temperature incubation
	Incubate the mixture in a 60℃ water bath for 15 minutes with gentle stirring.
Ice bath incubation
	Incubate the mixture in ice bath for 5 minutes, with gentle stirring.
Blending
	Blend the mixture in a blender for 5 seconds, and filter the mixture.
Filtration
	Collect the filtrate with a test tube, add 4 drops of protease into 6 ml filtrate.
Alcohol precipitation
	Slowly add the ice-cold absolute alcohol at the side of the test tube onto the filtrate's surface. Use a dropper to remove the intermediate milky layer -- the DNA fraction, and test the pH of it with universal indicator.