Bacteria have small loops of DNA called plasmids. In genetic engineering, plasmids can be cut open with restriction enzymes, and DNA from other organisms can be inserted. Restriction enzymes recognize specific nucleotide sequences at which they make cuts in the DNA molecule. They cut in such a way as to leave jagged ends with unpaired bases. These are called sticky ends because they will hydrogen bond to complementary nucleotides. A section of foreign DNA with the appropriate bases on its own sticky ends can bind to the plasmid and DNA ligase helps join the ends. The result is recombinant DNA.

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