Exonucleases
Cuts from an end, not in the middle, so an intact circle is immune.
Exonucleases degrade DNA from its ends, and that single fact does most of the work in this section. A circular molecule with no nick in it has no ends at all, so it is simply immune, which is why an undamaged plasmid survives a treatment that destroys everything linear in the tube. Many of them are non-specific, which is what makes them useful for clearing DNA out of an RNA prep, and some will take RNA as well. Section divider: Exonucleases. The lede says they cut from an end rather than in the middle, so an intact circle is immune.