Question

Restriction endonuclease of bacteria cut the DNAof bacteriophage but not the bacteria itself.Why?

Answer 1

The modification enzyme of restriction-modification system adds a methyl group to one or two bases within the recognition sequence of the restriction enzyme. Addition of a methyl group modifies the DNA sequence. As a result, the restriction enzymes fails to recognize and could not cut that DNA. This is how a bacterium modifies and protects its own DNA from cleavage by these restriction enzymes.