What is the reason for cutting a vector and insert with the same enzyme?

Correct answer: To make compatible ends in both

The direct answer is A: cutting both pieces with the same enzyme makes compatible ends. A restriction enzyme recognizes a particular DNA sequence. If the vector and insert are cut with the same enzyme, the cuts can expose complementary single-stranded overhangs on both pieces. These ends can base-pair, bringing the insert into the vector in a suitable position. DNA ligase then joins the sugar-phosphate backbones. Option A is correct because compatibility is the practical reason for using the same enzyme. Option B is wrong: DNA is not converted into water. Option C is wrong: cutting does not change the colour of DNA in the relevant cloning process. Option D is wrong: restriction digestion cuts DNA but does not generally make both molecules biologically inactive. Also, the same enzyme may sometimes cut at more than one site, so the exact fragments depend on the map; nevertheless, the stated main reason remains compatible ends. Memory cue: same enzyme gives matching ends; ligase completes the join.