Final answer:
Bacterial defense mechanisms include enzymes that specifically target and cut unmethylated DNA to protect against foreign DNA, such as from bacteriophages, thereby providing an immune-like defense system.
Step-by-step explanation:
A bacterial defense mechanism includes enzymes that cut up unmethylated DNA because these enzymes, known as restriction endonucleases, serve as a protective measure against foreign DNA, such as that from bacteriophages (viruses that infect bacteria). The specific sequence of DNA that is recognized by these enzymes is usually short and palindromic, which allows the bacterial cell to modify its own DNA through methylation by DNA methylase. This modification protects the host bacterial DNA from being cut since the restriction endonuclease will only cleave unmethylated DNA, which is typically characteristic of foreign, invading DNA.
In contrast, foreign DNA, such as that introduced during transformation or bacteriophage infection, is not methylated in the same way and is therefore recognized and digested by these restriction endonucleases. This degradation of invasive DNA helps to prevent its incorporation into the bacterial genome, thereby acting as an immune-like response to protect the bacterium from potentially harmful genetic material. Notably, this system enables the differentiation between self and non-self DNA, with only the latter being selectively degraded.