Final answer:
DNA is protected from enzymatic cleavage by methyl groups added to certain bases, which distinguish the organism's own DNA from foreign DNA.
Step-by-step explanation:
DNA is protected from restriction enzymes by being methylated. The bacteria have a defense mechanism known as a restriction modification system, which includes DNA methylase that specifically adds methyl groups to certain bases within the bacterial DNA. This methylation typically occurs at specific sequences that are short and palindromic. As a result of this modification, the bacterial restriction endonucleases recognize these sequences as 'self' and do not cleave the methylated DNA. However, when foreign DNA such as that from a virus enters the bacterial cell and is not methylated in the same manner, the restriction endonucleases are able to recognize and cleave this unmethylated foreign DNA, thereby protecting the bacterium from viral infection. This process ensures that the cell's own DNA is safeguarded, while foreign DNA is cleaved, which is referred to as 'restricted DNA'.