(approx. 10 mg/ml) Denatured fragmented salmon sperm DNA should be used at a concentration of 100 pg/ml in prehybridization and hybridization solutions involving essentially any type of hybridization experiment. To prepare:
1. Dissolve salmon sperm DNA (Sigma type III sodium salt) in H2O at a concentration of 10 mg/ml. If necessary, stir the solution on a magnetic stirrer for 2-4 hours at room temperature to help the DNA to dissolve.
2. Adjust the concentration of NaCl to 0.1 M, and extract the solution once with phenol and once with phenol:chloroform.
3. Recover the aqueous phase, and shear the DNA by passing it 12 times rapidly through a 17-gauge hypodermic needle.
4. Precipitate the DNA by adding 2 volumes of ice-cold ethanol, recover by centrifugation, and dissolve at a concentration of 10 mg/ml in H2O.
5. Determine the A260 of the solution and calculate the approximate concentration of the DNA.
6. Boil the solution for 10 minutes and store at -20°C in small aliquots. Just before use, heat the solution for 5 minutes in a boiling-water bath and chill quickly in ice water.
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