Final answer:
Restriction enzymes evolved in bacteria as a protection mechanism against bacteriophages by recognizing and cutting specific DNA sequences in the phage DNA. They do not cut host bacteria's own DNA because the bacterial DNA is methylated at specific sequences which are not recognized by the restriction enzymes as targets for cleavage.
Step-by-step explanation:
Why Restriction Enzymes Evolved
Restriction enzymes, or restriction endonucleases, evolved in bacteria as a defense mechanism against invading phages (viruses that infect bacteria). These enzymes cut and destroy foreign DNA, such as that brought by bacteriophages, by recognizing and digesting specific sequences within the phage DNA, thus protecting bacteria from phage infection and limiting their replication.
Each restriction enzyme cuts DNA at a characteristic recognition site, which is a specific, palindromic DNA sequence.
Protection of Host Bacteria from Restriction Enzymes
The reason restriction enzymes do not cut the DNA of the host bacteria is due to a protective mechanism involving DNA methylation. Bacteria possess another enzyme called DNA methylase, which methylates their own DNA at specific sequences, usually the same sequences recognized by the accompanying restriction enzymes.
Due to this modification, the restriction enzymes do not recognize or cut the bacteria's own methylated DNA, while still being able to cleave unmethylated, foreign DNA. This selective cutting protects the integrity of the bacterial genome while targeting and restricting the genetic material of potential bacterial phages.