What is needed for restriction digestion after DNA isolation?

Correct answer: enzyme-compatible clean DNA / Enzyme-compatible clean DNA

The direct answer is A: clean DNA that is compatible with the restriction enzyme reaction is needed. Restriction enzymes recognize specific DNA sequences and cut them under defined buffer, salt, temperature, and ionic conditions. The reasoning is: DNA is isolated from cells; remaining proteins, alcohol, salts, or other contaminants may interfere with enzyme activity; purified DNA is placed in the enzyme’s compatible buffer; the correct temperature and time allow sequence-specific cutting. Option A is correct because clean DNA and suitable reaction conditions are essential for reliable restriction digestion. Option B is wrong because protein-contaminated DNA may inhibit or complicate digestion rather than improve it. Option C is wrong because excess ethanol can interfere with enzymes and must be removed or reduced before the reaction. Option D is wrong because salt crystals alone are not DNA and cannot provide the required substrate. Restriction digestion also needs the correct enzyme, buffer, DNA amount, and incubation conditions, so “clean DNA” is necessary but not the only requirement. Memory cue: “restriction enzymes need the right DNA and the right reaction environment.”