Electron microscopy can be used to measure DNA supertwisting.

Area of Science:

• Molecular Biology
• Biophysics
• Microscopy

Background:

• DNA supertwisting is critical for packaging and function.
• Accurate measurement of DNA supertwisting is essential for molecular biology research.
• Electron microscopy (EM) offers potential for direct visualization of DNA structure.

Purpose of the Study:

• To evaluate electron microscopy (EM) as a method for measuring DNA supertwisting.
• To compare EM-based supertwist measurements with traditional gel electrophoresis techniques.
• To determine the reliability of EM for quantifying DNA twists in covalently closed circular DNA.

Main Methods:

• Preparation of DNA topoisomers with varying superhelical densities using Drosophila topoisomerase 1 and ethidium bromide (EtBr).
• Isolation of discrete DNA topoisomers via preparative agarose gel electrophoresis.
• Direct mounting of DNA on EM grids in a spermidine-containing buffer for visualization.
• Counting crossovers in individual DNA molecules observed under EM.
• Comparison of EM crossover counts with supertwist values derived from gel electrophoresis.

Main Results:

• Electron microscopy (EM) provided a reliable estimation of DNA supertwists.
• Measurements of supertwisting by EM correlated well with values obtained from gel electrophoresis.
• The study confirmed EM's utility for DNAs with low-to-moderate superhelical densities.

Conclusions:

• Electron microscopy (EM) is a viable and accurate technique for quantifying DNA supertwisting.
• EM offers a direct visualization method complementing biochemical approaches like gel electrophoresis.
• This study validates EM for structural analysis of DNA topology.