Cell Biology. Using cell-to-cell variability--a new era in molecular biology.

425 PERSPECTIVES self-shielding (9– 11). The principle is similar to that of laser isotopic separation of gases, but uses either interstellar ultraviolet (UV) radiation or the light of the growing Sun. In either case, UV radiation dissociates CO molecules. However, owing to the abundance of C 16 O molecules, the radiation penetrating the system runs out of photons capable of breaking up this isotopic combination before it runs out of photons that dissociate C 17 O or C 18 O. The result is that in such regions, 17 O and 18 O are free to react, either directly with dust grains or with H atoms to form H 2 O that eventually reacts with the dust. Meanwhile, the extremely stable C 16 O sequesters the 16 O in the gas phase and prevents it from reacting with silicates. As long as CO remains stable, the self-shielding mechanism remains a viable explanation for the mass-independent fractionation of oxygen isotopes throughout the solar system. Unfortunately, Ciesla and Sandford demonstrate that CO is not a stable sink for 16 O, and that such molecules will be converted into complex organics even in the cold outer reaches of the solar nebula, just as they are in cold molecular cloud cores. If CO is converted into organic molecules by radiation processing in icy grains in the outer nebula, just as it is via Fischer-Tropsch–type reactions (12– 14) on the surfaces of warmer grains in the inner nebula, and if such organics are free to react with silicate dust or to react with the bulk sili-cate in planetesimals, then the effects of pho-tochemical self-shielding will be minimized or even completely erased. A new mechanism may be needed to explain the oxygen isotopic fractionation observed in the solid bodies of the solar system. Our understanding of the chemistry in pro-tostellar systems has made enormous progress over the last few decades, fueled by an increased awareness of the complex dynamics of these evolving energetic nebulae. We can no longer consider just the simple local environment (15) to explain the composition of a planet, asteroid, or comet as was done in the past, but must now consider chemical processes that might take place within the nebula as a whole as well as the probability of transport and mixing the products of such reactions throughout the system. Just as we now fi nd it impossible to explain the complex chemistry …