DNA content and structure of (double) minutes of a methotrexate-resistant cell line.

We have determined the DNA content of intact double minutes (DMs) and of single minutes (SMs) by fluorometry of the individual chromatin bodies in metaphase spreads after staining with Feulgen-Schiff pararosaniline. We find that the intact DMs and SMs of the methotrexate-resistant mouse cell line 3T6R50 contain 4.4 megabase pairs (Mb) and 2.6 Mb DNA respectively, using the DNA content of E. coli (4.7 Mb) as a reference. As the pulsed field gradient gel electrophoresis experiments by van der Bliek et al. (1988) have indicated that the minutes of 3T6R50 cells contain a homogeneous population of 2.5 Mb DNA circles, we conclude that a SM contains one circular double strand DNA molecule of approximately 2.5 Mb, whereas DMs contain two.