Which option gives the correct source of restriction enzymes?

Correct answer: Bacteria

The direct answer is A: bacteria. Restriction enzymes were first identified in bacteria, where they form part of a defence system against foreign DNA, such as DNA introduced by bacteriophages. Step by step: a bacterium may encounter foreign genetic material; the enzyme recognizes a suitable foreign DNA sequence; it cuts that DNA; this can help protect the bacterial cell. Bacteria also have their own protective pattern, commonly involving methylation, so their important DNA is not normally cut by their own restriction system. Option A is correct because bacteria are the original biological source of these enzymes used in biotechnology. Option B, sand, is an inorganic material and does not produce enzymes. Option C, metal, can be a chemical element or material but is not the living source of restriction enzymes. Option D, glass, is an artificial non-living material and cannot synthesize proteins. Scientists may now produce or use enzymes through laboratory methods, but their natural discovery and source are bacterial organisms. Memory cue: bacteria use restriction enzymes as defence scissors against foreign DNA.