Taking the mystery out of biological networks

Abstract

Fifty years ago, our grandparents considered it a luxury to make a long‐distance telephone call or to travel by plane. Today, we can speak to and e‐mail each other instantly, and meet face‐to‐face in a matter of hours despite the intercontinental distances that often separate us. The smooth functioning of this new technological world relies heavily on the complex sets of connections between its parts. Microchips contain an array of components that are linked to each other to build computers, which are then connected through the Internet. Interconnected, cross‐referenced and hyperlinked: we live in a networked world. Complex systems are often networked and biology is no exception. The follow‐up experiments to the genome sequencing projects, such as those using microarrays or the yeast two‐hybrid system, show that molecules in living organisms are also highly connected. This interconnectivity helps to explain how such great complexity can be achieved by a comparatively small set of molecules either in a single organism or in nature as a whole. Most real‐world networks, including those based on social acquaintances, the World Wide Web and those that are revealed by biological data, share certain intrinsic properties that are described as ‘scale‐free’ behaviour (Barabasi & Albert, 1999). First, the distribution of the number of connections per node (that is, an element in the network) follows a power law: most nodes have few connections with an increasingly small number of ‘hubs’ being highly connected. Indeed, most personal home pages are linked to the hub ‘Google’, but they do not usually connect to each other. Second, …