Summary

During the first 24 h of light-induced chloroplast development in Ochromonas danica, the single plastid nucleoid increases 4-fold in volume. During this interval, the concentration of DNA within the nucleoid, as determined by eye and by counts of fibrils per µm² of nucleoid sectioned, remains constant. Thus, on morphological grounds, it appears that the amount of plastid DNA increases 4-fold during greening.

To determine whether the chloroplasts of light-grown cells contain more DNA than the proplastids of dark-grown cells, exponentially growing cultures of dark- and light-grown cells were each labelled for exactly one generation with [³H]thymidine. After fixation, the cells were embedded in Araldite, and serial 1-µm sections through entire plastids and nuclei were prepared for autoradiography. In this study, the chloroplasts of light-grown cells incorporated almost 4 times as much label into DNA as the proplastids did, whereas the nuclei of the dark- and light-grown cells were equally labelled. Another study showed that light-grown cells have slightly more total cell DNA than dark-grown cells. These 2 studies provide prima facie evidence that chloroplasts contain more DNA than proplastids and support the hypothesis that an increase in plastid DNA accompanies chloroplast development in Ochromonas.