Recognition Site
DNA molecules contain restriction sites, also known as restriction recognition sites, that are particular (4–8 base pair long) nucleotide sequences that are recognized by restriction enzymes. Because restriction enzymes often bind as homodimers, these sequences are typically palindromic. A particular restriction enzyme may cut the sequence between two nucleotides within its recognition site or somewhere adjacent.
For example, when the palindromic sequence GAATTC is recognized, the common restriction enzyme EcoRI makes a cut between the G and the A on both the top and bottom strands. On each end of AATT, a sticky end—a segment of a DNA strand with no connected complement—remains as a result. A segment of DNA with a complementary overhang can then be ligated into the overhang (another EcoRI-cut piece, for example).
Restriction Enzymes
Restriction enzyme is a bacterial protein that cleaves DNA at particular locations, these sites are called restricted sites. The restriction enzymes guard against bacteriophages in living bacteria. They identify the bacteriophage and cleave it at its restriction sites, destroying its DNA. Important genetic engineering tools include restriction enzymes. They may be separated from bacteria and applied in research facilities. The recognition sequences, or short and distinct nucleotide sequences, are recognized by restriction enzymes in DNA. When a DNA sequence is recognized by the restriction enzyme, it hydrolyzes the bond between neighboring nucleotides and cleaves the DNA molecule. The bacteria use the enzyme methylases to add the methyl group at the adenine or cytosine bases within the recognition sequence, preventing the DNA sequences from disintegrating.