Visualizing the formation and collapse of DNA toroids

Area of Science:

• Biophysics
• Molecular Biology
• Structural Biology

Background:

• DNA condensation is crucial for packaging genetic material within confined volumes in organisms.
• Positively charged multivalent ions, like spermine, are known to induce DNA toroidal structures in vitro and in viruses.

Purpose of the Study:

• To investigate the dynamics and stability of DNA toroids formed by spermine.
• To visualize the real-time process of spermine-induced DNA condensation and decondensation in single DNA molecules.

Main Methods:

• Utilizing optical tweezers to apply controlled tension to single DNA molecules.
• Employing fluorescence imaging for real-time visualization of DNA condensation and decondensation dynamics.
• Developing an analytical model to interpret experimental observations.

Main Results:

• Observed that spermine-induced DNA (de)condensation occurs in a quantized manner.
• Identified that quantized steps involve the addition or removal of individual DNA loops from the toroid.
• Reported toroidal condensates significantly smaller than those observed in previous studies.
• Demonstrated an apparent force plateau during the (de)condensation process.

Conclusions:

• The study reveals a quantized mechanism for DNA toroidal condensation driven by spermine.
• The findings provide a high-resolution understanding of DNA packaging dynamics under tension.
• The developed analytical model qualitatively explains key experimental observations, including force plateaus.