E.coli bacterium has restriction endonucleases which can restrict the growth of bacteriophages by cleaving the viral DNA. However, the host DNA is not cleaved because

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host bacterial DNA does not have restriction sites.

host bacterium cleaves its own restriction enzymes.

restriction enzymes are not active within the host bacterium.

restriction sites of host bacteria are protected by methylation

answer is A.

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Detailed Solution

Restriction endonucleases do not cut the DNA of their host bacteria because within the host DNA, restriction sites are masked by methyl groups.

The primary role of restriction endonucleases is to protect its host (the bacterial cell in which it is present) from viral infection.

By cleaving foreign DNA, restriction endonucleases provide a defense mechanism against bacteriophages and other foreign genetic elements. When a bacteriophage injects its DNA into a bacterial cell, the restriction endonucleases can quickly recognize and cleave the viral DNA, rendering it inactive.

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