Fig. 2. Topo II is not required for the structural integrity of mitotic chromosomes. Shown are representative images following the expansion and refolding assay. Stages (1) to (3) represent the first cycle of expansion and refolding, and (4) is the refolded status after cycle 2. Stage (1): colcemid-arrested metaphase before treatment; stage (2): first-round expansion in the low-salt plus EDTA TEEN buffer; stage (3): first-round contraction in the low-salt plus Mg²⁺ RSB buffer; stage (4): appearance after a second round of expansion and contraction. Topo-II-depleted chromosomes [both -only depleted (120 hours exposure to dox) and -plus β-depleted (120 hours exposure to dox plus 72 hours exposure to siRNA)] recover their normal starting morphology, which is a slightly thinner and more ribbon-like appearance than that displayed by the control chromosomes (Carpenter and Porter, 2004). All images are shown at the same magnification, with some regions enlarged for clarity. Bar, 10 µm.