molecules (2)

Soap-Like Properties...


1 Soap, shampoo, and worm-like micelles Soaps and shampoos are made from amphiphilic molecules with water-loving (red) and water-hating (blue) parts that arrange themselves to form long tubes known as “worm-like micelles”. Entanglements between the tubes give these materials their pleasant, sticky feel. b The micelles can, however, disentangle themselves, just as entangled long-chain polymer molecules can slide apart too. In polymers, this process can be modeled by imagining the molecule sliding, like a snake, out of an imaginary tube formed by the surrounding spatial constraints. c Worm-like micelles can also morph their architecture by performing reconnections (left), breakages (down), and fusions (right). These operations occur randomly along the backbone, are in thermal equilibrium, and are reversible. (Courtesy: Davide Michieletto)

Topics: Biology, Biotechnology, DNA, Molecules

DNA molecules are not fixed objects – they are constantly getting broken up and glued back together to adopt new shapes. Davide Michieletto explains how this process can be harnessed to create a new generation of “topologically active” materials.

Call me naive, but until a few years ago I had never realized you can actually buy DNA. As a physicist, I’d been familiar with DNA as the “molecule of life” – something that carries genetic information and allows complex organisms, such as you and me, to be created. But I was surprised to find that biotech firms purify DNA from viruses and will ship concentrated solutions in the post. In fact, you can just go online and order DNA, which is exactly what I did. Only there was another surprise in store.

When the DNA solution arrived at my lab in Edinburgh, it came in a tube with about half a milligram of DNA per centimeter cube of water. Keen to experiment with it, I tried to pipette some of the solutions out, but they didn’t run freely into my plastic tube. Instead, it was all gloopy and resisted the suction of my pipette. I rushed over to a colleague in my lab, eagerly announcing my amazing “discovery”. They just looked at me like I was an idiot. Of course, solutions of DNA are gloopy.

I should have known better. It’s easy to idealize DNA as some kind of magic material, but it’s essentially just a long-chain double-helical polymer consisting of four different types of monomers – the nucleotides A, T, C, and G, which stack together into base pairs. And like all polymers at high concentrations, the DNA chains can get entangled. In fact, they get so tied up that a single human cell can have up to 2 m of DNA crammed into an object just 10 μm in size. Scaled up, it’s like storing 20 km of hair-thin wire in a box no bigger than your mobile phone.

Make or break: building soft materials with DNA, Davide Michieletto is a Royal Society university research fellow in the School of Physics and Astronomy, University of Edinburgh, UK

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Drops in Cells...


Liquidated3672 (2021), Theodore Lee Jones,

Topics: Applied Physics, Biology, Microscopy, Molecules

A major challenge in cell biology remains to unravel is how cells control their biochemical reaction cycles. For instance, how do they regulate gene expression in response to stress? How does their metabolism change when resources are scarce? Control theory has proven useful in understanding how networks of chemical reactions can robustly tackle those and other tasks.1 The essential ingredients in such approaches are chemical feedback loops that create control mechanisms similar to the circuits that regulate, for example, the temperature of a heating system, the humidity of an archive, or the pH of a fermentation tank.

Theories for the control of biochemical reactions have largely focused on homogeneous, well-stirred environments. However, macromolecules inside cells are often highly organized in space by specialized subunits called organelles. Some organelles, such as the cell nucleus, are bound by a membrane. By contrast, another class of organelles—biomolecular condensates—show the hallmark physical properties of liquid-like droplets, and they provide chemically distinct environments for biochemical reactions.2–4

Such droplets can act as microreactors for biochemical reactions in a living cell (see figure 1). Their liquid nature sustains the fast diffusion of reactants while their specific composition gives rise to the partitioning of reactants in or out of the droplets. In general, the concentrations of reactants inside condensates differ from the concentrations outside. Those differences modify reaction fluxes, which, in turn, can dramatically affect reaction yield and other properties of chemical reactions. Just how such modified fluxes govern the biochemistry inside cells remains poorly understood.

Drops in Cells, Christoph Weber, Christoph Zechner, Physics Today

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