The Randomness of Life

PZ has a great post on a recent Nature Genetics paper that explores the startling connection between longevity and luck. (Or, as scientists like to stay, stochasticity). It turns out that genetically identical worms survive for very different amounts of time. The length of their life depends upon random cellular processes, which arise from "fundamental thermodynamic and statistical mechanical considerations."* Here's PZ:

The fascinating thing to me is that they [the scientists] are finding so much significant (I think a 50% increase in average life span is certainly significant!) variation in animals that is not a consequence of heredity, but is clearly an outcome of early, random processes in development and physiology.

I think the discovery of stochasticity within biological systems (and its crucial influence in determining phenotypes, like longevity) is one of most interesting trends in biology. One of my favorite papers on this subject remains a 2002 Science paper entitled "Stochastic Gene Expression in a Single Cell" by Michael Elowitz of Cal-Tech. He demonstrated that biological "noise" is inherent in gene expression. Elowitz began by inserting two separate sequences of DNA stolen from fireflies into the genome of E.coli. One gene encoded a protein that made the creatures glow neon green. The other gene made the bacterium radiate red. Elowitz knew that if the two genes were expressed equally in the E.coli (as classical biological theory predicted), the color yellow would dominate (red plus green equals yellow). That is, if life were devoid of intrinsic noise, all the bacterium would be colored by the same neon hue.

But Elowitz discovered that when the red- and green-light genes were expressed at ordinary levels, and not over-expressed, the noise in the system suddenly became visible. Some bacteria were yellow (the orderly ones), but other cells, influenced by their intrinsic disorder, glowed a deep turquoise or orange.

All the variance in color was caused by an inexplicable variance in fluorescent-protein level: The two genes were not expressed equally. Although the cells were technically the same, the randomness built into their system produced a significant amount of florescent variation. This disparity in bacterial hue was not reducible. The noise had no single source. It was simply there, an essential part of what makes life living.

*When molecular biologists refer to thermodynamics, they're generally signaling some sort of surrender: this sort of randomness is beyond their purview.