Fig. 6. Fork stability is not significantly affected by the intra-S-phase checkpoint. (A-C) Sperm nuclei were incubated at 10 ng DNA µl^-1 in Xenopus extract. After 35 minutes (early S phase) extract was supplemented with 40 µM aphidicolin and 0.5 mM roscovitine minus (B) or plus (C) 5 mM caffeine. 0 minutes, 10 minutes, 25 minutes or 55 minutes after this, nuclei were isolated and transferred to fresh extract supplemented with 0.5 mM roscovitine and [-³²P]dATP. Total DNA synthesis was measured at different times after transfer. A schematic outline of the experiment is shown in A. (D) Sperm nuclei were incubated at 10 ng DNA µl^-1 in extract. At 35 minutes, the extract was supplemented with 7.5 µM aphidicolin and 5 mM caffeine, minus or plus 0.5 mM roscovitine. At the indicated times, samples were pulse labelled with [-³²P]dATP for 2 minutes, the DNA was isolated and analysed by agarose electrophoresis and autoradiography. The migration of end-labelled -phage HindIII-digested DNA is also shown.