Restriction endonucleases are enzymes that cut DNA at specific sequences, known as recognition sites. These enzymes are naturally found in bacteria, where they serve as a defense mechanism against viral DNA. By recognizing and cleaving foreign DNA, they help protect the bacterial cell from infections. Scientists utilize restriction endonucleases in molecular biology for various applications, including DNA cloning and genetic engineering. By cutting DNA at precise locations, researchers can insert, delete, or modify genes, facilitating the study of gene function and the development of genetically modified organisms.