Abstract
About three-quarters of the DNA synthesis occurring in pachytene pollen mother cells of Lilium henryi takes place in proplastids. Only around 15% can be attributed to mitochondrial labelling and 10% to nuclear DNA synthesis. Label was identified in the proplastid genome by its location in electron microscopic autoradiographs, by its buoyant density (1.698 g/ml), and by its specific hybridization to chloroplast DNA sequences from spinach. Proplastids, while apparently not dividing at pachytene, may be replicating their DNA in readiness for subsequent proliferation in developing microspores. The annealing properties of plastid DNA closely parallel those of labelled pachytene DNA sequences implicated in meiotic exchange events.

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