The conformation and dynamics of circular polymers is a subject considerable theoretical experimental interest. DNA an important example because it occurs naturally in different topological states, including linear, relaxed circular, supercoiled forms. A fundamental question how the diffusion coefficients isolated scale with molecular length they vary for topologies. Here, D supercoiled, linear molecules L ranging from approximately 6 to 290 kbp were measured by tracking Brownian motion...

Over the past few decades, microrheology has emerged as a widely used technique to measure mechanical properties of soft viscoelastic materials. Optical tweezers offer powerful platform for performing measurements and can rheological at level single molecules out near macroscopic scales. Unlike passive methods, which use diffusing microspheres extract properties, optical probe nonlinear response, space- time-dependent heterogeneous, nonequilibrium In this Viewpoint, I describe basic...

Microtubules enable organized myosin-driven contraction by providing flexural rigidity and enhanced connectivity to actin networks.

When long polymers such as DNA are in a highly concentrated state they may become entangled, leading to restricted self-diffusion. Here, we investigate the effect of molecular topology on diffusion solutions and find surprisingly large effects, even with molecules modest length concentration. We measured coefficients linear relaxed circular by tracking Brownian motion single fluorescence microscopy. Four possible cases were compared: surrounded molecules, circles, circles circles. In...

Microrheology has emerged as a powerful approach for elucidating mechanical properties of soft materials and complex fluids, especially biomaterials. In this technique, embedded microspheres are used to determine viscoelastic via generalized Stokes–Einstein relations, which assume the material behaves homogeneous continuum on length scale probe. However, condition can be violated if macromolecular systems form characteristic scales that larger than probe size. Here we report observations...

Chain topology has a profound impact on the flow behavior of single macromolecules. For circular polymers, absence free ends results in unique chain architecture compared to linear or branched chains, thereby generating distinct molecular dynamics. Here, we report direct observation DNA dynamics transient and steady flows for sizes spanning range 25.0–114.8 kilobase pairs (kbp). Our show that longest relaxation times rings follow power-law scaling relation with weight differs from chains....

Understanding the dynamics of ring polymers is a particularly challenging yet interesting problem in soft materials. Despite recent progress, complete understanding nonequilibrium behavior has not been achieved. In this work, we directly observe flow DNA-based rings semidilute linear polymer solutions using single molecule techniques. Our results reveal strikingly large conformational fluctuations extensional long after initial transient stretching process terminated, which observed even at...

Self-diffusion coefficients (D) of DNA molecules varying length and concentration were measured by tracking the Brownian motion individual fluorescently labeled tracer molecules. Four possible cases examined: linear surrounded (L−L), circular tracers linears (C−L), circles (L−C), (C−C). With 6 11 kilobasepair (kbp) D was largely insensitive to topology varied consistent with Rouse scaling (D ∼ L-1C-0.5). In contrast, 25 45 kbp had a strong influence. At 1 mg/mL we found DC-C > DL-C ≫ DL-L...

Oligomerization of the HIV-1 protein Rev on Response Element (RRE) regulates nuclear export genomic viral RNA and partially spliced mRNAs encoding for structural proteins. Single-molecule fluorescence spectroscopy has been used to dissect multistep assembly pathway this essential ribonucleoprotein, revealing dynamic intermediates mechanism assembly. Assembly is initiated by binding a high-affinity site in stem-loop IIB RRE proceeds rapidly addition single monomers, facilitated cooperative...

We optically drive a trapped microscale probe through entangled DNA at rates up to $100\ifmmode\times\else\texttimes\fi{}$ the disentanglement rate ($\mathrm{Wi}\ensuremath{\approx}100$), then remove trap and track subsequent recoil motion. identify unique crossover nonlinear regime $\mathrm{Wi}\ensuremath{\approx}20$. Recoil dynamics display rate-dependent dilation complex power-law healing of reptation tube. The force response during strain exhibits key features such as shear thinning...

Abstract The cytoskeleton precisely tunes its mechanics by altering interactions between semiflexible actin filaments, rigid microtubules, and crosslinking proteins. We use optical tweezers microrheology confocal microscopy to characterize how varying motifs impact the mesoscale mobility of actin-microtubule composites. show that, upon subtle changes in patterns, composites can exhibit two distinct classes force response – primarily elastic versus more viscous. For example, a composite which...

Cleaving enzymes that linearize and chop up circular DNA elicit non-equilibrium rheological trajectories are starkly different at small large spatiotemporal scales due to time-varying contributions of entanglements, threading, crowding.

Imaging of single DNA molecules has enabled detailed studies dilute polymer dynamics and rigorous testing assumptions predictions molecular theories. It is interest to extend these methods the study entangled polymers correlate with rheology measurements. Progress in this direction been hampered, however, by a lack available samples sufficient quantities covering wide range lengths. Here we describe preparation suitable set ranging length from ∼3 300 kilobase pairs. These constructs are...

By combining single-molecule tracking with bond-fluctuation model simulations, we show that diffusion is intricately linked to molecular topology in blends of entangled linear and ring biopolymers, namely DNA. Most notably, find a previously unreported non-monotonic dependence the self-diffusion coefficient for DNA on fraction comprising ring-linear blend, which argue arises from second-order effect molecules being threaded by varying numbers molecules. Results address several debated issues...

We drive optically trapped microspheres through entangled F-actin at constant speeds and distances well beyond the linear regime, measure microscale force response of filaments during following strain. Our results reveal a unique crossover to appreciable nonlinearity strain rate [small gamma, Greek, dot above]c ≈ 3 s(-1) which corresponds remarkably with theoretical relaxation entanglement length deformations 1/τent. Above above]c, we observe stress stiffening occurs over very short time...

The entanglement of ring polymers remains mysterious in many aspects. In this Letter, we use electric fields to induce self-entanglements circular DNA molecules, which serve as a minimal system for studying chain entanglements. We show that self-threadings give rise entanglements and can slow down polymer dynamics significantly. find strongly entangled molecules remain kinetically arrested compact state very long times, thereby providing experimental evidence the severe topological...

Ring polymers are a unique class of macromolecules that lack free ends and show qualitatively different dynamics compared to linear chains. Despite recent progress, the nonequilibrium flow behavior ring is not fully understood. In this work, we study polymer in steady shear using combination single molecule experiments Brownian (BD) simulations. particular, DNA studied flow-gradient plane by custom apparatus allows for direct observation stretching tumbling fluorescence microscopy. Using...

Ring polymers in dense solutions are among the most intriguing problems polymer physics. Because of its natural occurrence circular form, DNA has been extensively used as a proxy to study fundamental physics ring different topological states. Yet, torsionally constrained-such supercoiled-topologies have largely neglected so far. The applicability existing theoretical models supercoiled is thus unknown. Here, we address this gap by coupling large-scale molecular dynamics simulations with...

Bulk rheology measurements on concentrated monodisperse ring–linear DNA mixtures are reported for the first time. The entanglement behavior of (with 15 and 50% linear chain fractions, ϕLin) over a range total concentration, ϕTot, from ∼0.8 mg/mL (20C*) to 2 (50C*) is reported. A discussion current problems in ring dynamics/scaling included. concentration-dependent dynamics compared with 100% at same degree entanglement, Z. Dynamic oscillatory tests were conducted understand bulk rheological...

Abstract The cytoskeleton is a major focus of physical studies to understand organization inside cells given its primary role in cell motility, division, and mechanics. Recently, protein condensation has been shown be another intracellular organizational strategy. Here, we report that the microtubule crosslinking proteins, MAP65-1 PRC1, can form phase separated condensates at physiological salt temperature without additional crowding agents vitro. size droplets depends on concentration...

We use optical tweezers to directly measure the intermolecular forces acting on a single polymer imposed by surrounding entangled polymers (115 kbp DNA, 1 mg/ml). A tubelike confining field was measured in accord with key assumption of reptation models. time-dependent harmonic potential opposed transverse displacement, recent simulation findings. tube radius 0.8 microm determined, close predicted value (0.5 microm). Three relaxation modes (approximately 0.4, 5, and 34 s) were following...

We characterize the lengthscale-dependent rheological properties of mucus from ubiquitous Chaetopterus marine worm. use optically trapped probes (2-10 μm) to induce microscopic strains and measure stress response as a function oscillation amplitude. Our results show that viscoelastic are highly dependent on strain scale (l), indicating three distinct regimes at l1 ≤4 μm, l2≈4-10 l3≥10 μm. While is similar water for l1, suggesting rarely contact mesh, l2 distinctly more viscous independent...