Question

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

Answer 1

to cut DNA into fragments of different lengths

Restriction enzymes (restriction endonucleases) recognize specific DNA sequences and cleave the DNA there, producing fragments (often with "sticky" or blunt ends). Joining fragments is done by DNA ligase.