Nucleases: An Overview

Enzymes able to digest nucleic acids are of course essential to molecular biology, indeed the whole technology was founded on the discovery of bacterial enzymes that cleave DNA molecules in a base-specific manner. These enzymes, the type II restriction endonucleases, are perhaps the best studied of the nucleases as to both their in vivo role and their use as tools in the techniques of molecular biology. However, the nucleases are ubiquitous in living organisms and function in all situations where partial or complete digestion of nucleic acid is required. These situations not only include degradation and senescence but also replication and recombination, although it must be noted that, to date, evidence for the involvement of nucleases in the latter two processes in eukaryotes is largely circumstantial. The significance of nucleases in the functioning of nucleic acids as the genetic material can be gaged however by considering that several enzymes implicated in DNA replication, recombination, and repair have integral exo- or endodeoxy-ribonuclease activity. For example, the 5′-3′ and 3′–5′ exonuclease activity of DNA polymerases and the endo-DNase activity of topoisomerases (e.g., see ref. 1 ).