Abstract
1. 1. Pulse labelling of exponentially growing Tetrahymena pyriformis has been used to identify intermediates in the replication of its mtDNA. 2. 2. Pulse-labelled mtDNA sediments more rapidly than normal Tetrahymena mtDNA: its buoyant density in neutral CsCl and its chromatographic properties in benzoylated-naphthoylated-DEAE-cellulose indicate that it consists of duplex molecules having only short stretches of single-stranded DNA. Alkaline sucrose sedimentation velocity analysis demonstrates that the pulse label is incorporated into DNA varying in size from short pieces to full-length single strands. Longer molecules were not observed. 3. 3. Comparative electron microscopy of mtDNA sedimenting faster than the bulk of Tetrahymena mtDNA and of total mtDNA shows that the former is enriched in molecules containing one symmetrical, centrally positioned, predominantly duplex replication bubble (an “eye”). Oligomeric molecules (circular or linear) were absent from all preparations. 4. 4. The results indicate that the “eyed” molecules are intermediates in replication rather than in repair or recombination.
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