What is "exonuclease"

n. (context enzyme genetics English) Any of a group of enzymes which cleave single nucleotides from the end of a polynucleotide (DNA or RNA) chain.

n. a nuclease that releases one nucleotide at a time (serially) beginning at one of a nucleic acid

Exonucleases are enzymes that work by cleaving nucleotides one at a time from the end (exo) of a polynucleotide chain. A hydrolyzing reaction that breaks phosphodiester bonds at either the 3’ or the 5’ end occurs. Its close relative is the endonuclease, which cleaves phosphodiester bonds in the middle (endo) of a polynucleotide chain. Eukaryotes and prokaryotes have three types of exonucleases involved in the normal turnover of mRNA: 5’ to 3’ exonuclease, which is a dependent decapping protein; 3’ to 5’ exonuclease, an independent protein; and poly(A)-specific 3’ to 5’ exonuclease.

In both archaebacteria and eukaryotes, one of the main routes of RNA degradation is performed by the multi-protein exosome complex, which consists largely of 3' to 5' exoribonucleases.