Figure 10-2: The nucleotide sequences recognized and cut by five widely used restriction nucleases. As shown, the target sites at which these enzymes cut have a nucleotide sequence and length that depend on the enzyme. Target sequences are often palindromic (that is, the nucleotide sequence is symmetrical around a central point). In these examples, both strands of DNA are cut at specific points within the target sequence. Some enzymes, such as Hae III and Alu I, cut straight across the DNA double helix and leave two blunt-ended DNA molecules; for others, such as Eco RI, Not I, and Hind III, the cuts on each strand are staggered. Restriction nucleases are usually obtained from bacteria, and their names reflect their origins: for example, the enzyme Eco RI comes from Escherichia coli.