Biomedical Engineering Reference
In-Depth Information
Figure 7
. Mechanism of gene duplication and divergence model. At each time step a gene
is randomly duplicated, retaining all of its links (blue nodes and edges). Subsequently,
interactions of the duplicated gene are deleted or newly added with probabilities E and B,
respectively (green edge).
databases (22,35,52,65), and the conformational spaces of RNA (64), also re-
flect a modular architecture.
4.5. Mechanisms of Proteome Evolution
The origin of scale-free behavior in biological networks continues to offer
some unresolved questions. Recently, however, it has been shown that a simple
model based on gene duplication can lead to the experimentally observed scale-
free topology of protein-protein interaction networks (42,51,53,56). In the
model, at each time step a gene is randomly chosen and duplicated. The copied
gene retains all interactions of the original gene. To mimic the potential loss or
gain of interactions due to random mutations, interactions of the duplicated
genes are deleted or newly added with probabilities E and B, respectively (Figure
7). The emerging network can be shown analytically to have a power-law degree
distribution, a high clustering coefficient, and a visual structure similar to the
protein-protein interaction network shown in Figure 5.
5.
CONCLUSIONS
A power-law degree distribution, the quantitative signature of a scale-free
network, has emerged as one of the few universal laws characterizing cellular
networks. Of even greater immediate importance is the intriguing possibility of
using the insights provided by scale-free models as a framework to facilitate
