Sticky end/blunt end

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In biology, sticky end (cohesive end) and blunt end are the two possible configurations resulting from the breaking of double-stranded DNA. DNA exhibits a stabilizing interaction between complementary base pairs, providing specificity to the pairing of two strands of DNA. If two complementary strands of DNA are of equal length, then they will terminate in a blunt end, as in the following example:

5'-CTGATCTGACTGATGCGTATGCTAGT-3'
3'-GACTAGACTGACTACGCATACGATCA-5'

However, if one strand extends beyond the complementary region, then the DNA is said to possess an overhang:

5'-ATCTGACT-3'
3'-TAGACTGACTACG-5'

If another DNA fragment exists with a complementary overhang, then these two overhangs will tend to associate with each other and each strand is said to possess a sticky end:

5'-ATCTGACT      + GATGCGTATGCT-3'
3'-TAGACTGACTACG        CATACGA-5'

which can form complementary base-pairs:

           GATGCGTATGCT-3'
5'-ATCTGACT     CATACGA-5'
3'-TAGACTGACTACG

The two fragments may then be covalently bonded by DNA ligase. Blunt ends may also be ligated, but the reaction is significantly slower. These terms are most commonly used to describe the product of a restriction enzyme digestion of DNA. Different restriction enzymes yield different overhanging ends; sticky ends are only 'sticky' with respect to ends with complementary sequence. Designing matching or unmatching ends is often used in genetic engineering.

Double-strand breaks caused by DNA damaging agents often have overhangs. The non-homologous end joining pathway of DNA repair uses base pairing between these "sticky" overhangs to guide accurate repair of the break.