Recombinant DNA is DNA taken from two different sources and fused into a single DNA molecule. Special DNA cutting enzymes, called restriction enzymes, cut the DNA at specific sites. Each restriction enzyme recognizes a different nucleotide sequence. DNA that is cut with a restriction enzyme will have single-stranded ends, called “sticky ends”. Two molecules of DNA cut with the same restriction enzyme will have the same exposed nucleotides and will undergo complementary base pairing.