Interruptions in single strands of the DNA in slime mold and other organisms.

The molecular weight of single-stranded DNA from the slime mold Physarum polycephalum has been determined by alkaline gradient centrifugation. The average molecular weight during DNA synthesis ( approximately 1.5 x 10(7) D) is less than that observed in nonsynthetic periods ( approximately 4 x 10(7) D). On the basis of a chromosome number of 50 per nucleus and a DNA content of 1 mumug per nucleus, we are led to conclude that at pH 12 each chromosome dissociates into 300 (single-stranded) pieces of DNA. We have also compared the sedimentation profiles of single-stranded DNA from Escherichia coli, PPLO, and T2 bacteriophage. These data support the conjecture that each bacterial chromosome can be dissociated into 10 or 12 single-stranded pieces of DNA. Dissociation of DNA into multiple pieces under our experimental conditions is best interpreted in terms of interruptions in the continuity of the DNA either by naturally occurring gaps or at alkali-labile bonds.