Symmetrical cleavage occurs when a restriction endonuclease, such as HaeIII, cuts a section of DNA and leaves no overhanging bases (known as Sticky Ends). The enzyme cuts between two bases directly opposite each other on the complementary strands of the double stranded DNA. This cleavage results in the formation of blunt ended DNA fragments.
Blunt Ends are made commonly at a specific site when trying to make a piece of recombinant homologous DNA. This is so only recombinant pieces of DNA will be readable, and increases the chance of an organism being recombinant substantially. These blunt ends will be made in vitro by an artificially engineered restriction endonuclease, designed to cut in a very specific place in the genome.