(a) Bivalent hyperstretching and separation before segregation errors. Maximum z-projection images of aged (16 months old) BDF1 oocytes expressing 2mEGFP-CENP-C (KTs, green) and H2B-mCherry (chromosomes, red). KT signals are peak-enhanced and background-subtracted. Arrowheads indicate S-chromosome KTs. The 3D plots show KT positions (spheres) and the connection between homologous KTs (lines). In (i)−(ii), the S-chromosome is coloured in green. In (iii), the hyperstretched S-chromosome is coloured in orange. In (iv)−(vi), the positions (spheres) and trajectories (lines) of the S-chromosome KTs exhibiting independent movements are coloured in red and purple. Intact bivalents are coloured in grey. The insets show the predivision of sister chromatids. Note that the spindle rotated before (vi). Time after metaphase entry (h:mm). Scale bar, 5 μm. See also Supplementary Movie 4. (b–e) Characteristic S-chromosome dynamics suggests bivalent hyperstretching followed by premature separation. KT distance from the spindle equator (b), inter-homologous KT distance (c), angle between inter-homologous KT axis and the spindle axis (d) and KT speed (e). The data for S-chromosomes are coloured as in a. The data for bivalents are coloured in grey. Error bars, s.d. (f) Bivalent separation precedes the majority of segregation errors during MI. The KT trajectories that underwent segregation errors in oocytes from aged BDF1 (16 months old) and CD-1 (11 months old) mice were analysed.