For many years, the scientific community has held a misbelief about the behavior of DNA as it navigates through extremely small openings known as nanopores. A team of researchers from the University of Cambridge, in collaboration with scientists from around the globe, has uncovered an essential mechanism that dictates how DNA interacts with these tiny passages. This discovery is not just a minor detail; it plays a significant role in various biological functions and could greatly enhance the efficiency of DNA sensing technologies currently in development. Their work sheds light on an often-ignored structural feature of DNA called plectonemes, which may pave the way for groundbreaking advancements in genomics and biosensing methodologies.
Nanopores serve as minuscule gateways that enable single strands of DNA to pass through while generating electrical signals that scientists can use to analyze genetic material with precision. Surprisingly, previous interpretations of these signals contained significant inaccuracies.
Why Did Scientists Misinterpret DNA Signals?
For decades, researchers assumed that the intricate electrical patterns observed during experiments with nanopores were indicative of DNA forming knots as it transited through these narrow conduits. This analogy was straightforward: much like pulling a shoelace through a tight hole can lead to tangling, scientists believed that DNA behaved similarly. Thus, any irregularities in the electrical signal were attributed to the strand knotting up within the pore.
This misconception shaped the interpretation of nanopore data for a long time. However, the new study, published in Physical Review X, reveals that this assumption was often misguided. Instead of creating actual knots, DNA frequently twists upon itself when passing through nanopores. These twisted formations, referred to as plectonemes, resemble a coiled telephone cord rather than a tied knot.
This distinction is crucial because the presence of twists and knots produces markedly different effects on the electrical signals generated. Lead author Dr. Fei Zheng from the Cavendish Laboratory elaborated, "Our experiments demonstrated that as DNA is pulled through the nanopore, the ionic flow inside induces twisting of the strand, resulting in torque accumulation and the formation of plectonemes rather than mere knots. This 'hidden' twisting structure leaves a distinctive, long-lasting mark on the electrical signal, unlike the fleeting signature left by knots."
What Experiments Revealed About DNA Behavior
To arrive at this conclusion, the researchers conducted extensive experiments utilizing both glass and silicon nitride nanopores, examining a diverse range of voltages and experimental conditions. They discovered that instances where multiple sections of DNA occupied the pore simultaneously—termed "tangled" events—occurred far more frequently than what knot theory could account for. As the voltage increased and the DNA strands became longer, these tangled occurrences rose significantly, suggesting an additional force was influencing the behavior.
How Water Flow Influences DNA Twists
The researchers identified that this twisting effect stems from electroosmotic flow—the movement of water induced by electric fields within the nanopore. As water currents flow past the DNA, they exert a rotational force on the helical structure of the molecule. This twisting torque propagates along the strand, causing portions of the DNA outside the pore to coil into plectonemes.
Unlike knots, which constrict under tension and usually vanish quickly, plectonemes have the capacity to grow larger and maintain their form throughout the entire process of translocation. Computer simulations that applied realistic physical forces corroborated this behavior, demonstrating that the formation of plectonemes relies on DNA's capability to transmit twisting forces along its length.
Confirmation Through Manipulating DNA
To validate their findings further, the research team created "nicked" DNA strands—those deliberately interrupted at specific points. These interruptions hindered the spread of twisting along the DNA molecule, leading to a substantial reduction in the formation of plectonemes during their tests. This outcome confirmed that the ability to propagate twist is integral to the entire process and hints at novel applications for nanopores in detecting DNA damage, as breaks in the strand disrupt twisting behavior.
Enhanced Precision in Reading DNA Signals
"What’s truly exciting here is our newfound ability to differentiate between knots and plectonemes in the nanopore signal based on their duration," remarked Prof. Ulrich F. Keyser, also affiliated with the Cavendish Laboratory and a co-author of the study. "Knots traverse quickly, akin to a brief bump, while plectonemes linger, producing extended signals. This opens pathways to more detailed and nuanced insights into DNA organization, genomic health, and potential damages."
Implications for Biology and Technology
The implications of this research extend well beyond nanopore-based sensing technologies. Inside living cells, DNA often undergoes twisting and tangling as enzymes interact with it, and both knots and plectonemes are crucial for maintaining genome organization and stability. Gaining a deeper understanding of how these structures form could enhance models predicting cellular DNA dynamics.
In the realm of diagnostics and biosensing, the capability to detect or manipulate DNA twisting could result in the development of more sensitive tools that can identify subtle genetic alterations and early indications of DNA damage associated with various diseases.
"From a nanotechnology standpoint, our research underscores the immense potential of nanopores, not solely as advanced sensors but also as innovative instruments for manipulating biopolymers in unprecedented ways," concluded Keyser.
