Restriction endonucleases :-
- Are used for in vitro DNA synthesis
- Are present in mammalian cells for degradation of DNA when the cell dies
- Are synthesized by bacteria as part of their defense mechanism
- Are used in genetic engineering for ligating two DNA molecules
Answer
(3) Are synthesized by bacteria as part of their defense mechanism
Solution
Restriction endonucleases are produced by bacteria; they restrict the growth of bacteriophages by cutting the foreign (viral) DNA at specific recognition sequences, while the bacterium's own DNA is protected by methylation.
⇒ They are thus part of the bacterial defence mechanism.
∵ DNA synthesis in vitro needs DNA polymerase and joining of DNA fragments needs DNA ligase, options (1) and (4) are wrong; they are not mammalian enzymes, so (2) is wrong.
Answer: (3)