Endonuclease vs Exonuclease

Endonuclease and exonuclease are nuclease enzymes which catalyze the hydrolysis of single nucleotides present in a chain of DNA. Nucleases play a vital role in analyzing the sequence of nucleotides in DNA and RNA.

Exonuclease
Exonuclease enzymes are a category of enzymes that cleave to the nucleotides at the ends of the DNA molecule. The two strands of DNA are complementary to each other. They are represented as 3’ and 5’ arms. The phosphodiester bridges of DNA and RNA are attacked by two classes of enzymes which are represented as “a” and “b.” The enzymes of group “a” specifically hydrolyze the ester linkage between 3’ carbon, and the phosphoric group and the enzymes of group “b” hydrolyze the ester linkage between 5’ carbon and the phosphoric group.

A good example of a group “a” exonuclease enzyme is the venom of a rattlesnake and Russell’s viper. This venom hydrolyzes all the 3’ bonds in DNA or RNA liberating the nucleotides units as nucleoside 5’ phosphates.

The class “b” enzymes are represented by the spleen’s phosphodiesterase which hydrolyzes all the “b” or 5’ linkages of both DNA and RNA and thus liberates only nucleoside 3’ phosphates.

Endonuclease
Endonuclease enzymes are enzymes that cleave to the bonds of the DNA from within the molecule. They do not require a free 3’ or 5’ hydroxyl group at the end of the polynucleotide chain. The endonucleases attack specific 3’ or 5’ linkages wherever they occur in the polynucleotide chain.

Endonucleases are also categorized into groups “a” and “b.” The deoxyribonuclease I of a bovine pancreas is categorized as a class “a” enzyme catalyzing the hydrolysis of some of the 3’ linkages of DNA to yield oligonucleotides containing about four nucleotide residues on an average.

Deoxyribonuclease II is another endonuclease of class “b.” It is isolated from the spleen and thymus of various bacteria and causes the hydrolysis of 5’ linkages resulting into a group of nucleotides.

Summary:

1.Exonuclease causes the hydrolysis of a nucleotide at the ends where a free 3’ or 5’ hydroxyl group is present in the polynucleotide chain while the exonuclease does not require a free 3’ or 5’ hydroxyl group to cause hydrolysis of the polynucleotide chain.
2.Exonuclease activity results into nucleosides while endonuclease activity results in oligonucleotides.
3.The exonuclease’s activity results in the small units of the polynucleotide chain almost immediately while endonuclease activity undergoes a lag phase before releasing oligonucleotide groups.
4.Snake venom and spleen phosphodiesterase are examples of exonucleases while deoxyribonuclease I and II are examples of endonucleases.

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