Enzyme catalysing DNA recombination. Computer model of the enzyme flippase recombinase (FLP recombinase, yellow, blue, orange and green) mediating the recombination of strands of DNA (deoxyribonucleic acid, green) at a Holliday Junction. This recombination is a natural genetic process known as crossing over, and involves the mixing of two strands of DNA. Recombinase enzymes snip open each DNA strand, starting the process going and forming a four-way alignment known as a Holliday Junction (at centre). This enzyme acts as both a ligase (bond-forming enzyme) and a lyase (bond-breaking enzyme).