Enzyme catalysing DNA recombination. Molecular model of the enzyme CRE (cyclization recombination) recombinase (green and pink) mediating the recombination of strands of DNA (deoxyribonucleic acid, pale red) 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. It is this structure that other recombinases, like CRE, attach to as they continue the process.