The reason that apples go brown when sliced or bruised is because the injury introduces oxygen into the flesh of the fruit. When this happens, an enzyme called polyphenol oxidase (PPO) reacts with certain ingredients in the tissue called phenolic compounds, oxidizingthem to a precursor molecule that then gets converted into a brown-colored secondary product.
To prevent this from happening, Okanagan scientists engineered their Arctic apples so that they produce significantly less of the PPO enzyme. But rather than snipping out the DNA segments that are responsible for this oxidation, they actually added in extra copies of the PPO genes which cause the apple to respond by switching off the lot of them. Of course, the apples can still go brown from rotting, but the immediate browning reaction is thwarted.
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