Enzymes capable of degrading nucleic acids have been recognized for many years. These nucleases can be classified in several ways. Those that exhibit specificity for DNA are referred to as deoxyribonucleases. Those nucleases that specifically hydrolyze RNA are ribonucleases. Some nucleases degrade DNA and RNA. Within both of these classes are enzymes capable of cleaving internal phosphodiester bonds to produce either 3′-hydroxyl and 5′-phosphoryl terminals or, 5′-hydroxyl and 3′-phosphoryl terminals. These are referred to as endo nucleases. Some are capable of hydrolyzing both strands of a double-stranded molecule, whereas others can only cleave single strands of nucleic acids. Some nucleases can hydrolyze only unpaired single strands, while others are capable of hydrolyzing single strands participating in the formation of a double-stranded molecule. There exist classes of endonucleases that recognize specific sequences in DNA. One class of these DNA cleaving enzymes, the restriction endonucleases, also termed restriction enzymes, do so directly by binding specific (usually) contiguous DNA base pairs (typically 4, 5, 6, or 8 bp), and cleaving both strands of DNA, usually DNA within the binding/recognition sequence element. The second class of enzymes, which are ribonucleoprotein complexes, utilizes a “guide RNA” of specific nucleotide sequence that targets a nuclease to cleave distinct DNA or RNA sequences. These are the CRISPR-Cas family of enzymes.

Some nucleases are capable of hydrolyzing a nucleotide only when it is present at a terminal of a molecule; these are referred to as exonucleases. Exonucleases act in one direction (3′ → 5′ or 5′ → 3′) only. In bacteria, a 3′ → 5′ exonuclease is an integral part of the DNA replication machinery and there serves to edit—or proofread—the most recently added deoxy nucleotide for base-pairing errors.

In addition to their roles in nucleic acid metabolism in living cells, the nucleases described here, in concert with a panoply of other nucleic acid synthesizing and modifying enzymes, coupled with nucleic acid cloning and sequencing techniques represent the essential tools of modern molecular genetics and molecular medicine.

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مواضيع ذات صلة

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Histones Are the Most Abundant Chromatin Proteins

Directions for Future Research in Molecular Biology

The ras-fos-jun Pathway

Recessive Oncogenic Mutations, Tumor Suppressors

Small RNA

Ribosomal RNA

Messenger RNA

Identification of the src and sis Gene

Cellular Counterparts of Retroviral Oncogenes

Retroviruses and Cancer

Identifying a Nucleotide Change Causing Cancer