Nucleases

enzymes that cleave DNA (DNases) or RNA (RNases). Each cell contains several different nucleases, which can be divided into two major classes:

• Endonucleases: cut inside the DNA or RNA strand and break the molecule into small fragments.
• Exonucleases: break down DNA from either the 3' or 5' end of the molecule by removing one nucleotide at a time.

Nucleases have very different functions in the cell. Among the most important tools of DNA recombination technology are the so-called restriction endonucleases, which cleave foreign DNA molecules at very specific sites either unspecifically or with sequence accuracy. They are part of the organism's defence system and are widely used in molecular biology. Non-specific nucleases play a role, for example, in digestion, but are also important for the destruction of DNA during programmed cell death ( apoptosis), such as DNase I (streptodornase), which is found in the pancreas, liver, kidney and thrombocytes. The stability of mRNA in the cell nucleus or cytoplasm is also influenced by the presence of RNases. The controlled degradation of mRNA is an important process for the regulation of gene expression.