Question

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

Answer 1

Restriction enzyme also known as restriction endonuclease is a protein produced by the bacteria that can cleave or cut the DNA at specific sites.

Complete answer:
In a bacterial cell, a bacteria produces restriction enzymes to protect itself from the infectious bacterial viruses known as bacteriophage. When a bacterial cell is attacked or infected by a bacteriophage, the phage inserts its DNA into the cell that can further replicate. This replication of DNA can be prevented by the restriction endonuclease produced by the bacteria that cut the viral DNA at specific sites cleaving it into many fragments.
Restriction endonuclease can read and recognize the specific short sequences of DNA or nucleotide bases. These nucleotides sequences are called restriction sites that are randomly distributed along the DNA sequence. When a restriction endonuclease recognizes the restriction site on the DNA it hydrolyses the bond between the adjacent nucleotides. While this process bacteria protect their own DNA by modifying the recognition site on its own DNA by adding a methyl group to adenine or cytosine within the recognition site. The enzyme methylases are used for this process. In a particular bacterial species, a restriction enzyme with methylase is known as a restriction-modification system.
Once the DNA of the bacteriophage is cut into fragments it cannot infect the bacteria. Hence we can say that the restriction enzyme has limited or restricted the effect of a virus on a bacterial cell.

Note:
There are different types of restriction endonuclease designated as type I, II, III, and IV. All of these have a different structure, cleavage site, specificity, etc. Restriction enzymes are also used as an effective tool in genetic engineering.