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Unformatted text preview: CREDITS: III-I‘l- I: VLADIMIR K- 'INNG HINDI UNWHIT“ "I‘l- HPNTT'USH PHIL-LIN“ HT? CIF HTTILI‘GH from these amino acids to living cells remain to be elucidated. For example, how did small monomers polymerize into longer chain spe- cies, such as nucleic acids {4}, and how were the developing reaction networks compart- mentalized into protocells {5}? None ofthese processes contradicts or is inherently incom- prehensible in terms of our current under- standing ofthermodynamics, chemical kinet- ics, and catalysis. Yet, a coherent, explana- tory, conceptual synthesis is far enough away that it still seems reasonable to characterize early life as emergent. Spirals and worms. {Al Segmented spirals emerge from about one million interartlng neoodroos of aqueous BI solution in an oiloslater mlcroernulslon [15}. Frame site, 3.12 x 4.32 mm'. {Bl In a simular tion {16}. a BI gel “norm” autonomously reorient-1 Itself to localize 1n the dart reglon. Dne popular example of emergence is Conway‘s “Game of Life“ {ii}. In this game, a rectangular grid of sites is populated by “cells.” A simple set of rules determines whether those cells live, die, or multiply, based on the number of neighboring sites that are populated. Depending on the ruleschosen and the initial conditions specified, repeated iteration yieldsa remarkable variety ofmov- ing and stationary patterns that seem farmore intricate than the simple elements and rules of the game should allow. This game is a simple example ofceilular automata { 3"}, whose rich- ness continues to surprise. More generally, computer programming provides an attractive metaphor for emer- gent behavior in chemical and biological sys- tems. If one imagines molecules and reac- tions as analogous to the primitive operations and instructions in a programming language, one can build complex chemical “program s" to accomplish specified tasks. Because mol- ecules, such as the complementary base pairs in DNA, can have natural affinities for one another without forming covalent bonds {3}, much ofthe “work" ofthis sort of program- ming occurs very efficiently via self-assem- bly. Complex supramolecular structures can be designed from DNA “origami" seeds and tiles {9]. Deoxyribozyme-based molecular automata can play tic-tac-toe {m}. Even relatively simple chemistry can yield a remarkable array ofdynamic behav- ior resembling that displayed by multicellular organisms. For example, when a few dozen nickel electrodes undergoing electrodisso- lution are coupled to each other, they form a small set of clusters that oscillate in phase with one another, resembling behavior seen in brain slices { H}. Another system, catalyst- impregnated beads in a solution containing the reactants of the Belousov-Zhabotinslsy {BE} reaction, displays the phenomenon of "quorum sensing:“ when the bead density reaches a critical value, the population of independent, quiescent beads undergoes a sharp transition to coherent oscillation {f2}. Biomolecules—notably proteins and nucleic acids—provideparticularly impressive exam— ples of emergent phenomena. Networks of such molecules can perform functions analo- gous to associative learning U3}. The various threads of the above research could provide useful guidelines for creating artificialcells thatcan simultaneously perform multiple functions, such as self-propulsion, self-sensing, and a form of communication that leads to cooperative activity. Research- ers have designed self—propelled microscopic "swimmers" and colloids and identified con- ditions where hydrodynamic interactions between these objects drive them to swim together in a concerted manner U4}. Such self—propelled entities could exhibit greater "intelligence“ if they incorporated some inter- nal chemical dynamics, such as provided by the BE reaction. In the case of the oscillating bends U2} and that of microemulsions con- sisting ofnanodrops of aqueous BI solution suspended in an oil phaser}, the state ofone entity affects the behavior of the rest, and, in this respect, different units communicate with each other (see the figure, panel A}. Further- PERSPECTIVES more, simulations have shown that light can be used to direct the movement ofa small El-Z gel “worm“ [see the figure, panel B} {to}. It may also be possible to exploit recent synthetic advances to direct the conglomer- ates to fight or flee—two other hallmarks of biological entities. For example, magneti- cally driven microspheles at liquid-liquid interfacescan be manipulated such that some microspheres raid and attack others, result- ing in the formation of larger snakelike fig— ures {in Under certain conditions, these snakes divide into smaller entities, thereby undergoing a simple form of reproduction. Finally, advances in manipulating DNA [9} may be harnessed to further direct such “intelligent“ microscopic objects. For exam- ple, functionalization ofcolloid particieswith complementary strands of DNA {Hi} leads to a new class ofsynthetic materials designed to self-replicate and, as a result, grow exponen— tially. Coupling this technology to beads that communicate, fight,flee,and consume energy would open the door to objects displaying essential hallmarks of living systems. The notion of emergence as an area of scientific inquiry is inherently paradoxical, in that emergent phenomena are defined as those whose origins we do not completely understand. As we probe more deeply, what is perceived to be emergent necessar— ily recedes into the distance as we begin to see more clearly how interactions between components give rise to more complex struc- tures and behaviors. Nonetheless, how life emerged from atoms and molecules is a question that, whatever we choose to call it, will challenge scientists for many years, if not centuries, to come. Relerences and Notes L Emrgeneetn chmkoflystems an. tlolv. olnlaska. Androrage. Alaska. 22 to 26. June zoos.- see enumath. uaa.alss|ramwnafkjmhemkalemergencelhdaphp. 2. 5. Rasmussen a! d1..5fltlrtt'93l13, 953 lEIJDIl]. wwwscienoemagflrg SCIENCE UDLEL'ZE 25 SEPTEMBERQUD‘? Pathway“ Downloaded from wwwsciencemsgorg on October 9. 2009 1533 ...
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