Fig. 4. The majority of mitochondria remain DNA-positive after fragmentation of mitochondrial filaments in vivo. HeLa cells expressing mtRFP (A-C) and primary skin fibroblasts (D,E) were fixed and decorated with COX2 and/or DNA-specific antibodies under control conditions (A) or after a 4-hour treatment with cccp (B-E). C and E are enlargements of the areas boxed in B and D, respectively. (A) In HeLa cells expressing mtRFP, DNA-specific antibodies label the cell nucleus and punctate intramitochondrial structures. Numerous DNA-positive structures colocalize within elongated mitochondrial filaments. (B-E) Inhibition of mitochondrial fusion with cccp leads to mitochondrial fragmentation in HeLa cells (B,C) and fibroblasts (D,E). The majority of punctate mitochondria are DNA-positive. Arrowheads point to mitochondria devoid of mtDNA. Bars, 10 µm.