The term exonuclease refers to any of a group of enzymes that cleave or attach to the nucleotides from the end of a polynucleotide chain, DNA or RNA. Since exonucleases cleave the nucleotides from the back, they hydrolyze the phosphodiester bond at the 3 and 5 arms of the polynucleotides. On the other hand, the term endonuclease refers to a group of enzymes that cleave to the nucleotides from the interior of the polynucleotides. They work by hydrolyzing the phosphodiester bonds that are within a polynucleotide chain.
Unlike exonucleases that hydrolyze at the 3 or 5 hydroxyl group of the nucleotides present in the polynucleotide chain, endonuclease enzymes do not need to cleave to the 3 or 5 hydroxyl group of the nucleotides present in the polynucleotide chain before they can cause the hydrolysis of a RNA of DNA chain. Unlike endonucleases that produce oligonucleotides, exonucleases lead to nucleosides.
Both Endonuclease and the exonuclease are actually nuclease enzymes which function is to catalyze the hydrolysis of single nucleotides, which is present in a chain of DNA. Nucleases do play vital roles in the analysis of the sequence of nucleotides in DNA and RNA.
Exonuclease does cause the hydrolysis of a nucleotide at the ends in which there is the presence of a free 3’ or 5’ hydroxyl group in the polynucleotide chain, but the exonuclease does not actually require a free 3’ or 5’ hydroxyl group to effect the hydrolysis of the polynucleotide chain. The activity of exonuclease usually results in nucleosides, while that of endonuclease results in oligonucleotides.
The activity of exonuclease results in the smaller units of the polynucleotide chain immediately, but endonuclease activity goes through a lag phase before they release oligonucleotide groups. Snake venom and the spleen phosphodiesterase are examples of exonucleases, but deoxyribonuclease is examples of endonucleases.