A scientist examines a stained squash preparation of an onion (Allium cepa) root tip under a light microscope. She focuses on two cells:
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Cell X: The chromosomes are aligned along the equator of the cell. Each chromosome appears as a pair of sister chromatids held together at the centromere. No distinct nuclear envelope is visible, and fine fibres extend from opposite poles towards the chromosomes.
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Cell Y: The chromosomes are separating, with individual chromatids moving towards opposite poles. Fibres are attached to the centromeres, and a new nuclear envelope appears to be forming around each group of chromatids.
Explain the behaviour of the chromosomes in Cell X and Cell Y, naming the stage of mitosis each cell is in, and describe the associated changes to the nuclear envelope and spindle fibres.