A single unpaired strand of nucleotides protruding from the end of a double-stranded DNA molecule. It is able to join with a complementary single strand, e.g. the sticky end of another DNA molecule, thus forming a single large double-stranded molecule. Sticky ends provide a means of annealing segments of DNA in many applications, e.g. in packaging of vectors in genetic engineering, and in hybridizing primers with target DNA in the polymerase chain reaction.