High Quality Content by WIKIPEDIA articles! Endonucleases are enzymes that cleave the phosphodiester bond within a polynucleotide chain. There are a small amount of significant classes of endonucleases that cleave only at the specific nucleotide sequences (such as the restriction endonucleases which are so vital in biotechnology).. At the extreme ends of a sequence there are restriction endonucleases, usually called restriction enzymes. These are endonucleases from eubacteria and archea that recognize a specific DNA sequence. The nucleotide sequence recognized for cleavage by a restriction enzyme is called the restriction site. Typically, a restriction site will be a palindromic sequence of about four to six nucleotides long. Most restriction endonucleases cleave the DNA strand unevenly, leaving complementary single-stranded ends. These ends can reconnect through hybridization and are termed "sticky ends." Once paired, the phosphodiester bonds of the fragments can be joined by DNA ligase. There are hundreds of restriction endonucleases known, each attacking a different restriction site. The DNA fragments cleaved by the same endonuclease can be joined together regardless of the origin of the DNA. Such DNA is called recombinant DNA; DNA formed by the joining of genes into new combinations.. Restriction endonucleases (restriction enzymes) are divided into three categories, Type I, Type II, and Type III, according to their mechanism of action. These enzymes are often used in genetic engineering to make recombinant DNA for introduction into bacterial, plant, or animal cells, as well as in synthetic biology.:375-390