Question

How is the action of exonucleases different from that of endonuclease?

Answer 1

Both enzymes belong to a large class of enzymes called nucleases. One of them cut the DNA sequences at the end whereas the other one cut the DNA sequences at the specific site.

Complete answer:
Nucleases are an enzyme that cleaves or digests the nucleic acids that are DNA and RNA. There are two kinds of nucleases – exonucleases and endonucleases.
Exonucleases
It is enzymes that cleave the nucleic acids (DNA sequences) in a polynucleotide chain from either 5’end or 3’end one at a time. It is a hydrolyzing enzyme that cleaves the phosphodiester bond present between the nucleotide.
Exonucleases are an important enzyme in the process of replication, as one of the enzymes work with RNA polymerase II. These enzymes degrade the RNA primer that is present on a new transcript and replaced by DNA nucleotides.
It also plays an important role in editing and proofreading of the DNA for errors.
There are three kinds of exonucleases in eukaryotes and prokaryotes – a decapping 5’to3’ exonucleases, polyA-specific 3’to5’ exonucleases and independent 5’to3’exonucleases. All of these are involved in the replication, formation and transcription of RNAs.
Unlike endonuclease, exonucleases do not have a lag period.
Endonucleases
These groups of enzymes cleave the DNA sequence within a polynucleotide chain. They are capable of breaking the chain from the middle.
Like exonucleases, it is also a hydrolyzing enzyme that cleaves the phosphodiester bond.
These enzymes can be specific or nonspecific to the sequence being cleaved. The enzymes that are specific to a particular site or sequence are known as restriction endonucleases.
The restriction endonucleases identified the specific palindromic nucleotide sequence in the DNA to cut.
The restriction endonuclease plays an important major role in Recombinant DNA technology.

Basis for comparison	Exonucleases	Endonucleases
Cleavage	Cleave the nucleotide from the end	Cleave the nucleotide from the middle
Lag period	Does not have lag periods before action	Some of the enzymes of these groups have a lag period before action.
Ends	Resulting in the sticky end	Resulting in either sticky or blunt end
Effect on circular DNA	Less active toward the circular DNA as compared to linear DNA.	Cleave the specific site within a circular DNA.
Examples	Xrn1, exonuclease I and snake venom	EcoRI, BamHI, etc

Note: Exonucleases do not have defensive properties against the pathogens. Whereas some of the endonucleases have the defensive function and prevent the pathogen from entering. Exonucleases are found in snake and lizard venom, which cleave the DNA that is coding for essential protein within the body.