Restriction enzymes, first described in 1971, are bacterially derived enzymes that cleave DNA. Evolutionarily, restriction enzymes arose as a bacterial self-defense mechanism; the genomes of invading organisms would be degraded, leading to an inability to replicate. Type II restriction enzymes generally recognize an inverted repeat palindrome. This recognition site structure leads to a symmetrical cleavage of both DNA strands and results in either blunt- or sticky-ends of the digested DNA. Blunt ends are universally compatible with other blunt-ended DNA and possess a 5’ phosphate group to promote ligation. Sticky ends, on the other hand, are stretches of single-stranded DNA that is capable of self-ligation or ligation with a complementary region of DNA from another molecule or organism.
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