Restriction sites, or restriction recognition sites, are particular sequences of nucleotides that are recognized by restriction enzymes as sites to cut the DNA molecule. The sites are generally palindromic, and a particular enzyme may cut between two nucleotides within its recognition site, or somewhere nearby. For example, the common restriction enzyme EcoRI recognizes the sequence GAATTC and cuts between the G and the A on both the top and bottom strands, leaving an overhang (an end-portion of a DNA strand with no attached complement) on each end, of AATT. This overhang can then be used to ligate in (see DNA ligase) a piece of DNA with a complementary overhang (another EcoRI-cut piece, for example). A nucleotide is a monomer or the structural unit of nucleotide chains forming nucleic acids as RNA and DNA. A nucleotide consists of a heterocyclic nucleobase, a pentose sugar, and a phosphate or polyphosphate group. ... A restriction enzyme (or restriction endonuclease) is an enzyme that cuts double-stranded DNA. The enzyme makes two incisions, one through each of the phosphate backbones of the double helix without damaging the bases. ... Space-filling model of a section of DNA molecule Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions specifying the biological development of all cellular forms of life (and many viruses). ... A palindrome is a word, phrase, number or any other sequence of units (like a strand of DNA) which has the property of reading the same in either direction (the adjustment of spaces between letters is generally permitted). ... In molecular biology, DNA ligase is a particular type of ligase (EC 6. ...