How the zebra got its stripes
How the zebra got its stripes
Have you ever noticed how regular nature can be; that there are patterns hidden everywhere? Sea shells have a nice spiral pattern, trees show subtle fractal patterns, and some plants have their petals arranged in a very specific way. But some patterns are a lot more obvious. Take the zebra for example, the black-and-white striped cousin of the horse.
Each one of its billions of cells contains a very detailed set of instructions that defines its specific job. Certain cells will have instructions to become part of a liver, some will go towards forming bones and others will have instructions to become a fur coat. But how does the tell-tale stripey pattern form on the zebra’s coat? Is it all down to their DNA? You’d think so, wouldn’t you?
DNA is very simply a chemical code, which when translated by the cell, gives specific instructions to make different proteins. Nowhere in the code does it give any instructions for defining characteristics like four legs, one tail, two ears, or stripes. These details are down to morphogenesis; a phenomenon that determines biological structure.
Professor Andrea Sella, a synthetic chemist from UCL has taken a particular interest in this subject, “At the heart of the problem lies chemistry, physics, and mathematics, since structure emerges naturally in physical and chemical systems simply as a result of the interplay of different processes… many of the processes by which structures in biology form - think of the cell membranes, for example, have little or nothing to do with DNA - they self-assemble spontaneously simply because of their molecular structure,” he explained.
These simple processes are reaction and diffusion. Simple chemical reactions, for example, turning oxygen and glucose into carbon dioxide and water, eventually become stable. Diffusion occurs when a substance moves from an area of high concentration to one of a lower concentration. This will also reach an equilibrium, or stable point. But when reaction and diffusion processes are combined to a reaction-diffusion system, all of a sudden an unstable state is generated.
It is in this unstable state that patterns begin to form. During the embryonic phase, genes can be activated by a specific chemical signal called a morphogen to produce, for example a darkly pigmented patch of skin. If there is a high concentration, and even distribution of this morphogen, the result is a very even colour, like the black panther. If the morphogens are unevenly spread, spots or stripes will form, like the leopard and the zebra.
It is these morphogens form the basis of the patterns, and how they propagate along a body. They are often part of a standing wave pattern, which is the result of an inhibitor-activator sequence that controls their production.
Alan Turing, probably better known for his pioneering work in computing, was one of the first scientists to derive a mathematical model for how morphogenesis works. In his paper The Chemical basis of morphogenesis, Turing suggested “that a system of chemical substances, called morphogens, reacting together and diffusing through a tissue, is adequate for the main phenomena of morphogenesis.”
Patterns, regularities of form, are seen throughout nature. They appear on animals, in plants, and even in landscapes. That many of these patterns are formed by chemical reactions is astounding, as they are often required for very specific purposes: animals have certain patterns on their fur mostly for camouflage. That nature, a very complex, and sometimes disordered state can create such simple, organised patterns is astounding.
Professor Sella will be looking at the underlying chemistry and physics of this phenomenon in his talk ‘How the zebra got its stripes’, which you can see on Saturday September 8 from 14:00 -15:00 in the Spiegeltent.
Julie Guld is the Science in Society Assistant for the British Science Association. She is starting the Science Communication MSc this autumn at Imperial College, London. She has a blog and you can follow her on Twitter @JuliePCGould.