How does a restriction enzyme limit or restrict the effect of a virus on a bacterial cell?

By cleaving particular DNA sequences known as palindromic sequences—which read the same in both directions, like the name HANNAH—restriction enzymes bind foreign DNA. These target sequences are typically six nucleotides long.

The sequence typically only appears in low abundance in the bacterial cell. Sal1 (GTCGAC), a restriction enzyme I frequently used, caused this sequence to appear once every 4096 bases.

Therefore, upon virus invasion, transcription of the genome and protein translation are required. Since the genome of Bacteriophage Lambda is approximately 48,500 bases long, Sal1 would appear in this genome about 12 times.

This genome would continue to be cut by the Sal1 enzyme, which would prevent the virus from replicating.

A restriction enzyme limits the effect of a virus on a bacterial cell by recognizing specific sequences of DNA within the virus's genome and cutting it at those sites, preventing the virus from replicating or integrating into the bacterial cell's DNA. This defense mechanism is part of the bacterial immune system known as the restriction-modification system.