Light stimulates the incorporation of [3H]uridine and [3H]thymidine in addition to plastid replication in germinating Polytrichum spores. Significant amounts of [3H]uridine and [3H]thymidine are incorporated in darkness but not to the same level as in light. Plastids continue to produce nucleic acids when their capacity to multiply is suspended due to the absence of light. However, a higher amount of DNA synthesis in the light is correlated with the result that chloroplast replication begins again in the light after prolonged dark incubation. An imperfect association of plastid replication and nucleic acid synthesis is suggested by the lack of stimulation of DNA synthesis by light during plastid replication in the first 8 h of incubation. A temporal separation can be demonstrated for chloroplast and nuclear DNA synthesis at the beginning of germination in Polytrichum spores, with DNA synthesis in the chloroplasts preceding that in the nucleus. The mitotic S phase is longer than 16 h for at least half of the nuclei.

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