Which is the role of restriction enzymes? to isolate the selected gene
to cut DNA into fragments of different lengths
to move and separate the strands of DNA
to join the sticky ends of DNA fragments

to cut DNA into fragments of different lengths

Explanation:

A restriction enzyme is a protein that recognizes a specific, short nucleotide sequence and cuts the DNA only at that specific site, which is known as restriction site or target sequence.

More than 400 restriction enzymes have been isolated from the bacteria that manufacture them. In live bacteria, restriction enzymes function to defend the cell against invading viral bacteriophages. Restrictions sites in the viral genome (a "happy accident" of nature, as far as the bacteria are concerned, since they don't appear to have any specific function in the virus) are cleaved by the bacterium's restriction enzymes, fragmenting and destroying the DNA of invading bacteriophages before it can incorporate into the host's genome and take over the cell.

A bacterium is immune to its own restriction enzymes, even if it has the target sequences ordinarily targeted by them. This is because the bacterial restriction sites are highly methylated, making them unrecognizable to the restriction enzyme.

Isn't evolution fantastic?