* Modification of Recognition Sites: Restriction enzymes often recognize specific, short DNA sequences called recognition sites. These sites are typically 4-8 base pairs long. To prevent self-cutting, bacterial DNA (where restriction enzymes originate) has a built-in defense mechanism. These DNA sequences are chemically modified (usually by methylation) by other enzymes. This modification "hides" the recognition site from the restriction enzyme, preventing it from binding and cutting.
* Methylation: Methylation is the process of adding a methyl group (CH3) to a base in the DNA sequence. This modification alters the shape of the DNA and makes it unrecognizable to the restriction enzyme.
* Specificity and Protection: Each restriction enzyme has a specific recognition sequence, and its modification system is tailored to match. This ensures that the enzyme cuts foreign DNA (which is usually unmethylated) while leaving its own DNA intact.
In Summary: It's not that restriction enzymes can't cut their own DNA, it's that their host cells have evolved mechanisms (primarily methylation) to protect their own DNA from the enzymes they produce. This ensures that the restriction enzyme can effectively defend against invading viruses or other foreign DNA, without harming the host's own genetic material.
