The Chloroplast Nucleoid in Ochromonas Danica

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.