ABSTRACT
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 /tm2 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 1μm 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.
In another autoradiographic experiment on log-phase light-grown cells (Gibbs & Poole,73)10 to 12 serial 1μm sections were employed. In that case, the entire chloroplast was virtually always sectioned in those cells which were cut at the level of the nucleolus in the middle section. The average chloroplast in that experiment occupied 6 · 9 sections rather than 5 8 sections. Thus, as a rough estimate, the number of grains per plastid in the light-grown cells would have been approximately 20% higher than the observed value given in Table 3 had a longer series of sections been employed. 6 or 7 sections. The precise value varies depending on the angle at which the chloroplast is cut, but is almost always less than 100 μm2. Clearly, then, the number of background grains found over an entire chloroplast is insignificant. Since the average nucleus is only 18 μm3 in volume, and the average proplastid, only 12 μm3, background labelling of these organclles is also negligible.
