Bending DNA increases its helical repeat

Area of Science:

• Molecular Biology
• Biophysics
• Structural Biology

Background:

• DNA experiences significant mechanical stress from bending and twisting in biological systems.
• Accurate mechanical models of DNA are crucial for understanding fundamental biological processes like DNA packaging, replication, and gene regulation.
• The DNA double helix's helical repeat (~10.45 base pairs/turn) is traditionally considered independent of curvature.

Purpose of the Study:

• To investigate the mechanical behavior of DNA under varying degrees of curvature.
• To determine if DNA curvature affects its helical repeat.
• To refine the mechanical model of DNA for biological relevance.

Main Methods:

• Development of a ligation assay using nicked DNA circles with controlled, variable curvature.
• Measurement of DNA helical repeat as a function of curvature using the developed assay.

Main Results:

• A strong unwinding of the DNA double helix was observed for tightly bent DNA.
• The helical repeat increased to over 11 base pairs per turn for DNA circles with radii of approximately 3-4 nm.
• This indicates a significant dependence of DNA helical repeat on curvature, contradicting previous assumptions.

Conclusions:

• The study presents a major modification to the standard mechanical model of DNA.
• The findings necessitate a reassessment of the molecular mechanisms and energetics involved in processes utilizing tightly bent DNA.
• This research provides critical insights into DNA structure and function under mechanical stress.