Fig. 1. Mitochondrial DNA localizes to punctate structures that are distributed throughout the mitochondrial compartment. Primary skin fibroblasts were labeled with antibodies against DNA, COX2 and/or BrdU under normal conditions (A) or after a 20 hour pulse with BrdU (B). Insets 1 and 2 depict enlargements of the boxed areas. (A) Monoclonal antibodies against DNA label punctate structures that are distributed throughout mitochondria labeled with antibodies against the inner membrane protein COX2. Only a few mitochondria appear devoid of DNA-labeling (arrowheads). (B) Antibodies against the thymidine analogue BrdU label the nucleus as well as punctate mitochondrial structures labeled with DNA-antibodies. Some of the mitochondrial DNA-positive structures are devoid of BrdU (arrowheads) and very few BrdU-positive structures escape detection with DNA-antibodies (arrows).