Properties of biological networks.A, The connectivity of random networks is reflected by a Poisson distribution, whereas a scale-free (or clustered) network is defined by a power law distribution. As a result, there exist nodes that are sparsely linked as well as nodes that are more highly linked (hubs). B, As depicted by a hypothetical complex electric circuit, the detailed analysis of 1 or 2 resistors in isolation allows for little insight into the behavior of the circuit as a whole. Thus, similar to scale-free networks, this circuit displays emergent behavior. Moreover, detailed analysis of every resistor in the circuit is not necessary but rather can be inferred to understand the circuit as a whole. Thus, scale-free networks and this circuit are overdetermined. C, Depiction of the 3 modularity concepts of biological networks. Topological modules (left) are defined by locally dense neighborhoods of the interactome. Functional modules (gray nodes, center) are defined by network neighborhoods in which there is a statistically significant segregation of nodes of related function. Disease modules are groups of nodes whose perturbation (mutations, deletions, copy number variations, or expression changes) can be linked to a particular disease phenotype and can cross functional molecules, shown as red nodes. Adapted from Barabasi et al,1 with permission. |
