| Abstract: | Morphological changes in the organellar nucleoids and mitochondria of living Chlamydomonas reinhardtii Dang were examined during the cell cycle under conditions of 12:12 light:dark. The nucleoids were stained with SYBR‐Green I, and the mitochondria were stained with 3,3‐dihexyloxacarbocyanine iodide. An mocG33 mutant, which contains one large chloroplast nucleoid throughout the cell cycle, was used to distinguish between the mitochondrial and chloroplast nucleoids. Changes in the total levels of organellar DNA levels were assessed by real‐time PCR. Each of the G1, S, M, and Smt,cp phases was estimated. At the start of the light period, the new daughter cells were in G1 and contained about 30 mitochondrial and 10 chloroplast nucleoids, which were dispersed and had diameters of 0.1 and 0.2 μm, respectively. During the G1 phase of the light period, and at the start of the S phase, both nucleoids formed short thread‐like or bead‐like structures, probably divided, and increased continuously in number, concomitantly with DNA synthesis. The nucleoids probably became smaller due to the decrease in DNA of each particle and were indistinguishable. The cells in the S and M phases contained extremely high numbers of scattered nucleoids. However, in the G1 phase of the dark period, the nucleoids again formed short thread‐like or bead‐like structures, probably fused, and decreased in number. The mitochondria appeared as tangled sinuous structures that extended throughout the cytoplasm and resembled a single large mitochondrion. During the cell cycle, the numbers of mitochondrial nucleoids and sinuous structures varied relative to one another. |
