An endonuclease is an enzyme that cuts the phosphodiester bonds within a nucleic acid strand, such as DNA or RNA. This action allows the enzyme to break the nucleic acid into smaller fragments, which is essential for various biological processes, including DNA repair, replication, and recombination. These enzymes play a crucial role in molecular biology and genetic engineering, where they are used to manipulate DNA for cloning and sequencing. Some well-known endonucleases include restriction enzymes, which are used to cut DNA at specific sequences, facilitating the study and modification of genetic material.