The Chloroplast Nucleoid in <i>Ochromonas Danica</i>

The 3-dimensional structure of the plastid nucleoid was determined from serial sections of the plastid of dark-grown, greening, and light-grown cells of Ochromonas danica. In light-grown and greening cells, the chloroplast nucleoid forms a continuous cord or ring which closely follows the rim of each lateral lobe of the chloroplast and is continuous across the top and bottom of the bridge connecting the 2 chloroplast lobes. The nucleoid always lies just inside the chloroplast girdle bands where they loop around the rim of the plastid. It was demonstrated by electron-microscopic autoradiography of greening cells labelled with [3H]-thymidine that all the plastid DNA is localized in this peripheral ring-shaped nucleoid. In the proplastid of dark-grown cells, the nucleoid also forms a ring-shaped structure lying just inside the single girdle thylakoid, although frequent irregularities, such as gaps, are present. It is postulated that the girdle bands determine the shape of the chloroplast nucleoid, possibly by having specific attachment sites for the plastid DNA molecules. A survey of the literature shows that a peripheral ring-shaped chloroplast nucleoid is a characteristic feature of the 5 classes of algae whose chloroplasts possess girdle bands, namely the Raphidophyceae, Chrysophyceae, Bacillariophyceae, Xanthophyceae, and Phaeophyceae, and has never been observed in plants whose plastids lack girdle bands.</jats:p

The Chloroplast Nucleoid in <i>Ochromonas Danica</i>

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.</jats:p