Crossing over happens when chromosomes pair up and exchange bits during that first long phase of meiosis, Prophase I. Then the two divisions work like this: first division separates the chromosome pairs, and second division separates the sister chromatids, finally giving cells with half the original number. I only learned this years later, helping my son with his own biology revision. The textbook made it sound so simple, but in my exam hall panic, all those tangled threads just reminded me of unruly hair I could never quite straighten.
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