A set of four descriptions (W–Z) were recorded from a time-lapse study of mitosis in animal cells. They are not in the correct chronological order.
W: The chromosomes are aligned along the equator of the cell. Each chromosome is attached to spindle fibres from both poles.
X: The nuclear envelope has reformed around each set of chromosomes. The chromosomes are beginning to decondense. The cell surface membrane is pinching inwards.
Y: The chromosomes become visible as two sister chromatids joined at a centromere. The nuclear envelope breaks down into small vesicles. Spindle fibres begin to assemble from the poles.
Z: The sister chromatids separate and move towards opposite poles. The spindle fibres that are attached to the centromeres shorten.
Describe the correct order of these events (stages of mitosis) and for each stage, describe the behaviour of the chromosomes, the nuclear envelope, and the spindle.