Many restriction enzymes produce sticky ends, which are very useful for genetic engineering. What makes the ends sticky?After the restriction enzyme cuts the DNA strands, additional steps are used to add nucleotides to the ends of each strand. These newly added nucleotides can form hydrogen bonds to join with other strands.When the restriction enzyme cuts, it cuts the two strands of a double-stranded DNA molecule unevenly. This produces overhanging DNA nucleotides on each fragment. These overhanging fragments are complementary to each other or to other fragments produced using the same restriction enzyme.When the restriction enzyme cuts, it separates the two strands of DNA in a similar manner to the way that the strands separate in a replication bubble. Whenever the two strands of a DNA molecule are pulled apart, complementary bases are separated that can easily come back together again.Restriction enzymes cut the phosphodiester backbone of the DNA molecule. The broken bonds can easily re-form, rejoining the separated fragments.

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