Human Biochemistry, Volume 2 - Murray R. 1993

Structure, Function, and Replication of Information Macromolecules
RNA Synthesis and Processing
Nucleases

Enzymes capable of degrading Nucleic Acids have been known for a long time, and several Classification systems exist for them. Enzymes exhibiting specific activity toward Deoxyribonucleic Acids are called deoxyribonucleases, whereas those hydrolyzing Ribonucleic Acids are termed ribonucleases. Within each of these groups, there are enzymes that cleave intra-chain phosphodiester bonds, yielding either 3'-hydroxyl and 5'-phosphoryl ends or, conversely, 5'-hydroxyl and 3'-phosphoryl ends. Such enzymes belong to the Class of endonucleases. Some of them can hydrolyze both strands of double-stranded molecules, while others cleave only a single strand of nucleic acids. There are also nucleases capable of hydrolyzing only single strands that lack a duplex Structure, as well as those that cleave strands involved in forming a double helix. Additionally, a class of endonucleases has been described that recognize strictly defined sequences in DNA; the majority of these are Restriction Endonucleases (restriction enzymes), which in recent years have become a powerful tool for researchers working in Introduction/32.html">Genetic Engineering.

A list of some well-known restriction enzymes widely used today is given in Table 36.1.

Some nucleases are capable of cleaving NUCLEOTIDES exclusively from the free ends of molecules; these are referred to as exonucleases.

Exonucleases can hydrolyze a nucleic acid molecule in only one direction (3'→5' or 5'→3'). In Bacteria, 3'→5' exonuclease is an essential component of the METABOLISM/36.html">DNA Replication machinery, serving to correct mismatch errors by removing improperly incorporated nucleotides from the chain.

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