Fig. 2. Mitotic and meiotic chromosomes have a contiguous DNA connection, which is dissolved by 4 bp restriction enzymes, but only weakened by 6 bp restriction enzymes.

Image pairs show pipette positions untreated (native isolated) chromosomes when relaxed and stretched (a), and chromosomes following enzyme treatments (b–d). Vertical blue lines mark positions of force pipettes. Force pipette deflection by pulling (horizontal blue lines) indicates mechanical connection; no movement (no horizontal blue line) indicates no mechanical connection. Red notches mark positions of stiff pipettes. Bars are 5 µm. b Both mitotic and meiotic chromosomes were weakened, but not fully digested after treatment with PvuII (cut sequence CAG˅CTG). c Both mitotic and meiotic chromosomes lost connectivity after treatment with AluI (cut sequence AG˅CT; for 1 of 4 trials meiotic chromosomes were not fully digested by AluI). d Both mitotic and meiotic chromosomes lost connectivity when treated with MNase (cleaves all DNA sequences). e Quantification of chromosome stretching elasticity after no treatment or after being treated with PvuII, AluI, and MNase. No treatment caused a 13 ± 4% weakening of mitotic chromosomes (N = 10) and a 1 ± 4% weakening of meiotic chromosomes (N = 10). PvuII treatment caused a 70 ± 8% reduction in stiffness for MEF chromosomes (N = 4) and 70 ± 9% reduction in stiffness for meiotic chromosomes (N = 4). One of four AluI treatments of meiotic chromosomes caused a 90% reduction in stiffness (rather than fully digesting), while AluI treatment digested 4 of 4 mitotic chromosomes. All MNase treatments caused full digestion of mitotic and meiotic chromosomes (N = 4 in both cases). All averages are reported as mean value ± SEM. Bars are 5 µm.