Moving pictures and pulsed-field gel electrophoresis show linear DNA molecules from chloroplasts and mitochondria

Summary

The structure of the cytoplasmic genomes in plants has been investigated by using fluorescence microscopy to make moving pictures of ethidium-stained DNA fractionated by pulsed-field gel electrophoresis (PFGE) and emerging from organelles lysed within gelled agarose. For watermelon chloroplasts, PFGE fractions contained linear molecules representing monomeric to tetrameric lengths of the unit genome (155 kilobase pairs, kb) and linear DNA of at least 1,200 kb, whereas circular molecules were clearly identified only with chloroplast agarose inserts. For pea, the oligomeric series extended only to the trimer. Most of the DNA from watermelon mitochondria was in the form of 50–100 kb linear molecules with some DNA of at least 1,200 kb, but no band was seen at the size of this genome (330 kb) and no circular molecules were identified in either PFGE fractions or mitochondria embedded in agarose. Most mitochondrial DNA from cauliflower also consisted of 50–100 kb linear molecules with some much longer linear forms, but no genome-sized (220 kb) PFGE band was evident. The possible relevance of the very long linear DNAs to previous cytological and genetic observations is discussed.