Mechanical stability of single DNA molecules.

Area of Science:

• Biophysics
• Molecular Biology
• Materials Science

Background:

• The mechanical stability of DNA is crucial for its biological functions.
• Understanding DNA's response to mechanical forces can reveal insights into its structure and dynamics.

Purpose of the Study:

• To investigate the mechanical stability of individual double-stranded (ds) DNA molecules using atomic force microscopy (AFM).
• To characterize the force-induced melting transition of DNA and its dependence on various factors.

Main Methods:

• Overstretching of individual dsDNA molecules using a modified AFM.
• Measurement of DNA mechanical stability under varying ionic strengths, temperatures, and force-loading rates.
• Analysis of DNA melting and reannealing kinetics.

Main Results:

• A force-induced melting transition of the DNA double helix was observed, distinct from the B-S transition.
• This melting transition showed significant dependence on force-loading rate and hysteresis, indicating a nonequilibrium process.
• Low salt concentrations destabilized DNA, while high salt concentrations stabilized it.
• Increased temperature reduced the mechanical energy the DNA could withstand before melting.
• DNA sequence influenced melting force, with poly(dG-dC) requiring higher forces (~300 pN) and poly(dA-dT) lower forces (~35 pN) compared to lambda-phage DNA (65-200 pN).

Conclusions:

• Force-induced DNA melting is a nonequilibrium transition.
• The mechanical stability of the DNA double helix is modulated by ionic strength, temperature, and base composition.
• Results correlate mechanical properties with thermodynamic parameters like base-pairing free enthalpy.