Endo/Exo-Nuclease catalyze the hydrolysis of the phosphodiester bond

Endo/Exo-Nuclease with Type I, II, III

catalyze the hydrolysis of the phosphodiester bond
cut the nucleotides from the middle of DNA or RNA molecules
cut the nucleotides from the ends of DNA and RNA molecules
cannot work on circular DNA
can work on circular DNA
may form blunt ends or sticky ends
form sticky ends
result in oligonucleotide
result in single nucleotide

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Type I is an enzyme that has 3 subunits for restricted digestion and methylase activity. It cuts nucleotides at the site away from the recognition site with the help of ATP and S-adenosyl-methionine (SAM).

Type II endonuclease means there are two different enzymes, one is to cleave the nucleotides, one is to modify the recognition sequence. They do not use ATP for their activity. It cleaves the sequence at a short specific distance within or from the recognition site.

Type III is a single enzymes that contains two subunits. They cut at a site that is close to the identification site. Same with the type I, they also require ATP.

With the help of Type I exonuclease, the single-stranded DNA is broke in a 3′ to 5′ direction. The phosphoryl group of the DNA strand needs to be first unblocked. If not, the chains cannot be cleaved because they do not have terminal 3′-OH groups.

Type II exonuclease is linked with DNA polymerase I. DNA polymerase I contain a 5′ exonuclease that helps in cutting RNA primers directly upstream of the DNA synthesis site in a 5′ to 3′ direction.

Type III exonuclease catalyzes the removal of single nucleotides, one by one, from the 3′-OH end of the polynucleotide chain.


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