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DNA’s stretching mystery was only partly solved. A landmark 2011 experiment showed that the abrupt overstretching transition does not require a nick or free end from which one strand peels away. Later single-molecule studies showed why that result did not settle the entire debate: under different sequences, temperatures, salt concentrations and topological constraints, DNA can peel apart, form internal melting bubbles or remain base-paired in an elongated state.
In ordinary pulling experiments, double-stranded DNA suddenly becomes about 70% longer at roughly 60–70 piconewtons (pN). That jump is not one universal molecular event. It is a force-driven choice among several structures, and the experiment’s design determines which pathway is favored.
What happens when a DNA molecule is pulled?
DNA behaves first like a flexible polymer. As force rises, the molecule straightens and extends smoothly. Near a narrow force range, many constructs undergo an abrupt overstretching transition: the end-to-end length increases by approximately 70% even though the applied force changes little. A piconewton is one trillionth of a newton, so optical-tweezer experiments are measuring forces far below what ordinary mechanical instruments can detect.
Mixed-sequence DNA commonly shows this transition around 60–70 pN. The often-quoted value of approximately 65 pN and a 70% increase comes from the NIST account of work by JILA researchers; the account was published in 2011 and updated February 3, 2025. It describes a characteristic laboratory measurement, not a fixed threshold for every DNA molecule. Sequence, salt, temperature, torsional freedom and the presence of backbone discontinuities can all move the transition or change what the molecule looks like afterward.
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The transition is reversible in some constructs and partly irreversible or hysteretic in others. That behavior is an important clue, but it does not identify a mechanism by itself. Researchers combine force-extension curves with fluorescence imaging and details of the DNA construct before deciding whether they observed peeling, melting or an elongated paired state.
For background on the original experiment and its significance, see NIST’s “Stretching, the Truth” account.
What the 2011 “mystery solved” result actually established
An influential model proposed that overstretching began when one strand peeled from the other at a free end or a nick in the sugar-phosphate backbone. Once a small section separated, the unpaired strands could unzip progressively as tension increased. That model explained why many experiments saw single-stranded DNA appear during the transition.
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The JILA experiment tested whether such a discontinuity was essential. Researchers made a DNA construct in which one end was looped around a bead and both strands were attached at the other end. The arrangement preserved rotational freedom but removed the loose end or nick that would provide a starting point for peeling. A comparison molecule retained a nick.
Both constructs entered the overstretched state at essentially the same force. The no-nick molecule therefore demonstrated that strand peeling from a free end or nick is not necessary for the approximately 65-pN transition. It did not show that peeling never occurs, nor did it prove that every overstretched molecule has the same structure.
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NIST quoted senior author and single-molecule-mechanics expert Tom Perkins: “Overstretching is a really amazing phenomenon, and even after 15 years, people are still debating it.” The associated paper is D. H. Paik and T. T. Perkins, “Overstretching DNA at 65 pN does not require peeling from free ends or nicks,” published online January 5, 2011, in the Journal of the American Chemical Society.
Does stretched DNA melt or stay base-paired?
Both outcomes are possible. “Melt” means that hydrogen-bonded base pairs open and the duplex locally becomes single-stranded. A different possibility is an elongated duplex often called S-DNA: the two strands remain paired while the molecule adopts a longer geometry. A third route combines the two, with internal single-stranded bubbles surrounded by still-paired DNA.
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Peeling from a nick or free end
If a molecule has a free end or backbone nick, tension can expose an initiation site. One strand then unzips from its complement, producing a growing single-stranded region. The 2009 fluorescence-imaging study by van Mameren and colleagues observed conversion from double-stranded to single-stranded DNA during overstretching and often saw it start at nicks or free ends. In a constrained construct lacking those easy entry points, the researchers observed a higher force and melting that began throughout the molecule instead of at one obvious end.
Peeling is therefore a real pathway, but the no-nick experiment shows that it cannot be a requirement for every overstretching event.
Internal melting bubbles
When end peeling is blocked, base pairs can still open at internal weak points. These local denaturation bubbles create single-stranded segments without requiring a free terminus. Fluorescent probes that prefer single-stranded DNA can reveal such regions while force is applied.
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Internal melting is sensitive to sequence. AT base pairs have fewer hydrogen bonds than GC pairs and generally provide less resistance to thermal or mechanical opening. A long, designed AT-rich duplex can therefore follow a different force pathway from a GC-rich or mixed-sequence molecule of the same length.
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An elongated, base-paired state (S-DNA)
In 2012, Bosaeus and colleagues used short, designed duplexes to show that an overstretched molecule need not become single-stranded. Their high-AT construct underwent force-induced denaturation at 62 pN and reached approximately 70% extension. A 60%-GC construct instead displayed a reversible transition to an elongated state with approximately 51% extension while retaining base pairing. The results demonstrate that a force-extension jump is not, by itself, proof of complete melting.
The 2013 fluorescence study by King and colleagues made the conditional picture explicit. It reported three competing pathways: unpeeling from backbone discontinuities, internal melting bubbles and S-DNA. The balance among them changed with DNA topology, local sequence stability and ionic strength, with temperature also affecting comparisons to calorimetric measurements.
Why different experiments reach different conclusions
Two papers can report different transition forces or structures without either being wrong. The following variables change the mechanical and thermodynamic options available to the molecule.
| Experimental variable | How it changes the result | What to check in a paper |
|---|---|---|
| Sequence composition | AT-rich segments generally open more readily; GC-rich designs can favor a reversible paired state under some conditions. | Was the DNA short and specially designed, or long and mixed-sequence? |
| Nicks and free ends | Provide initiation sites for strand peeling and can lower the force needed for unpeeling. | Were both strands closed at the pulled end, or was a backbone discontinuity present? |
| Rotational freedom | Allows the strands to rotate as tension changes; torsional constraint alters the energetic cost of each pathway. | Could the DNA swivel, or were its ends rotationally fixed? |
| Ionic strength | Salt ions stabilize the negatively charged backbone and can shift the competition between melting and base-paired extension. | What buffer and salt concentration were used? |
| Temperature | Changes duplex stability and the force balance between paired and melted states. | Was the measurement made at room temperature or another controlled temperature? |
| Transition history | Hysteresis or irreversibility can indicate kinetic barriers, but does not identify a structure alone. | Did the molecule return along the same force-extension path? |
These details matter more than the headline number. A value near 65 pN in one construct cannot be used as a universal DNA “melting force,” and a 70% extension does not uniquely identify single-stranded DNA.
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Key measurements and what they mean
| Study or construct | Reported observation | Proper interpretation |
|---|---|---|
| NIST/JILA report, 2011 | Approximately 65 pN and 70% extension; a no-nick, no-free-end construct overstretched at essentially the same force as a nicked comparison. | Peeling from a discontinuity is not necessary for the transition. |
| Bosaeus et al., PNAS, 2012 | Mixed-sequence DNA context: approximately 70% extension at 60–70 pN. | A typical range for common laboratory setups, not a universal constant. |
| 70%-AT designed duplex, Bosaeus et al., 2012 | Denaturation at 62 pN with approximately 70% extension. | Force-induced melting in a short, AT-rich design; do not generalize directly to genomic DNA. |
| 60%-GC designed duplex, Bosaeus et al., 2012 | Reversible transition with approximately 51% extension while base pairing was retained. | Evidence for an elongated paired state under those sequence and solution conditions. |
| King et al., PNAS, 2013 | Approximately 65 pN in torsionally unconstrained DNA, with competing structural pathways. | Topology and solution conditions help determine which mechanism appears. |
The primary sources are Bosaeus et al. (2012) and King et al. (2013). For the earlier direct imaging of duplex-to-single-strand conversion, see van Mameren et al. (2009).
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Force-extension traces
Optical tweezers hold microscopic beads attached to DNA in a focused laser beam. By moving the trap or the sample, researchers increase tension while recording force and molecular extension. A sudden length increase identifies the overstretching transition, while reversibility, hysteresis and the force at which the jump occurs provide clues about the underlying pathway.
Fluorescent labels
Fluorescent dyes can be selected to bind preferentially to double-stranded or single-stranded DNA. Other probes, including proteins that recognize exposed single-stranded regions, provide an independent signal. Imaging these labels during a force ramp shows whether single-stranded regions begin at an end, appear as internal bubbles or fail to appear while the molecule elongates.
Construct design
Researchers deliberately change one variable at a time: they close or nick a backbone, permit or prevent rotation, alter sequence composition, or vary salt and temperature. A no-nick construct tests the necessity of peeling; a torsionally constrained construct tests the role of rotation; designed AT- and GC-rich duplexes expose sequence effects.
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesThese are isolated-molecule experiments under controlled laboratory conditions. They reveal molecular mechanics, not a population statistic or a direct measurement of DNA behavior inside every living cell.
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Why the answer matters beyond a single force curve
DNA-protein interactions can depend on whether a segment is double-stranded, locally melted or mechanically elongated while still paired. A protein that binds exposed single-stranded DNA may recognize a force-induced bubble but not an S-DNA segment. Conversely, a protein that tracks duplex geometry could respond differently when the same bases remain paired in an elongated form.
Single-molecule pulling also gives physicists a way to test force calibration at very small scales. The NIST account connected the carefully designed DNA construct to efforts to improve calibration for tiny forces. Better calibration matters wherever researchers measure molecular motors, chromatin mechanics or other biological systems with optical traps.
The broader lesson is methodological: a dramatic transition in a bulk measurement does not automatically identify a unique microscopic structure. Direct imaging, topology controls and sequence-defined constructs are needed to connect a force jump to what the strands are actually doing.
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The 2011 result solved one narrow but important question: DNA can undergo its characteristic overstretching transition without peeling from a nick or free end. Subsequent studies did not overturn that finding; they placed it in a wider mechanism map. Depending on sequence and experimental conditions, DNA may unpeel, melt internally or enter an elongated base-paired state. “DNA overstretching” names the force-induced transition, not one compulsory molecular structure.
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