Tension induces a base-paired overstretched DNA conformation

Area of Science:

• Biophysics
• Molecular Biology
• Genetics

Background:

• Mechanical overstretching of DNA by ~70% at 60-70 pN has been observed.
• A long-standing debate exists whether this involves a new DNA form or denaturation.
• Previous studies provided conflicting evidence on DNA's mechanical behavior under tension.

Purpose of the Study:

• To resolve the controversy surrounding DNA overstretching.
• To investigate the role of DNA base composition in mechanical response.
• To identify distinct structural changes in DNA under force.

Main Methods:

• Utilized optical tweezers to apply controlled force to single DNA molecules.
• Engineered short DNA duplexes (60-64 bp) with designed base compositions (AT-rich vs. GC-rich).
• Measured force-induced extension and structural changes at the single-molecule level.

Main Results:

• High AT-content DNA (~70%) undergoes force-induced denaturation at 62 pN with ~70% extension.
• GC-rich DNA (~60% GC) exhibits a reversible overstretching transition to a distinct form with 51% extension.
• This GC-rich form remains base-paired, representing a novel stretched DNA structure.

Conclusions:

• Demonstrated the existence of a force-induced, stretched, base-paired DNA form.
• Showed that DNA base composition dictates mechanical response under tension.
• The observed reversible transition in GC-rich DNA may be relevant to homologous recombination processes involving RecA and Rad51.