Exonuclease are enzymes that cleave the nucleotides at the end of the DNA molecule.The cleavage by exonuclease results in the formation of nucleotides or nucleosides. For example- BAL31 Endonuclease are enzymes that cleave the bonds of the DNA from within the molecule(middle part) at specific site. The cleavage by endonuclease results in the formation of oligonucleotides(Four nucleotide residues).For example- S1 nuclease, DNAses, Restriction endonucleases.