The Origin of Cell Division

thesis The origin of cell division The origin of life is among the deepest make the time for achieving a minimal unexplained mysteries. How did the first The study suggests RNA set grow only algebraically in N, self-replicating entities emerge, providing that chemically rather than exponentially (preprint the material on which the selective at http://arxiv.org/abs/1612.00825; mechanism of evolution could then active droplets 2016). Merging of physically distinct operate? The most primitive known self- could be plausible as protocells could aid the discovery of a replicating forms of life are far too complex self-replication machinery. to have sprung from the pre-evolutionary prebiotic protocells. This raises a further question: are there environment through chance alone. natural conditions that might lead to such Consequently, for the last century, scientists acting as containers to protect and feed a protocell merging? There are basic physical have sought mechanisms that would make prebiotic metabolism. processes leading to droplet merging. the originating event more routine, perhaps This theoretical work remains to be But other recent work by mathematicians even likely, given enough time. confirmed in real experiments. But it Ole Peters and Alexander Adamou Perhaps a single breakthrough idea rests on well-understood principles, and (preprint at http://arxiv.org/abs/1506.03414; will one day settle the matter. More likely, suggests that droplets that grow, divide and 2015) points to a basic statistical the truth may emerge piecemeal as a half proliferate like cells could be natural in the diversification that could also have acted in dozen or so ideas from different settings fit pre-evolutionary environment. This could the pre-evolutionary era to make merging together with an unforeseen synergy. One be part of a pathway to life’s emergence. But events more common. such component idea now getting increasing another feature — the occasional merging of Imagine some prebiotic world in which attention is that cell-like geometrical such cells — may also be key to developing many growing and dividing protocells enclosures can exert control over key self-replication based on inherited containing RNA float about while harvesting molecular processes. Modern living cells genomic information. basic chemicals crucial to their growth. The depend on myriad closed containers — from One of the leading scenarios for life’s growth of such cells won’t be steady and the cell itself and complex organelles to emergence rests on RNA molecules, which constant, but stochastic, influenced by noise much simpler vesicles — to constrain the can both store information in the sequence linked to environmental fluctuations. One movement and interaction of molecules. of their base pairs and also act as enzymes would expect different cells to experience Such confinement, as several recent to catalyse reactions. These dual features different conditions. As larger cells can strands of research suggest, may also have motivated the hypothesis — the ‘RNA harvest more material to support growth, encouraged life-like chemistry in the pre- world’ scenario — that life started when one would also expect the mathematical evolutionary past. RNA molecules evolved to form sets able growth process to be multiplicative — cells One obvious requirement for life to catalyse their own reproduction. In this grow in proportion to their current size. The is the emergence of physical cells that setting, a key issue is the time required for natural mathematical model for such growth are able to grow, by taking resources chance molecular events to bring together a is geometric Brownian motion and, as Peters from the environment, and also divide, minimal autocatalytic set of RNA molecules. and Adamou have shown, the time-average thereby increasing their numbers. A The longer the time, the less plausible the growth rate for a population of such growing natural idea — going back to biochemist scenario. Earlier this year, mathematical cells is significantly enhanced by occasional Alexander Oparin in the 1920s — is that biologist Sam Sinai and colleagues argued merging events. droplets forming under the right conditions that physical compartmentalization, In effect, the existence of merging events in complex liquids might already act this combined with occasional merging, might means that cells experiencing bad outcomes way, and could have played a role as early greatly reduce the time, making the RNA may get temporary help from resources chemical reaction centres. A recent study scenario more plausible. gathered by others that were recently more by David Zwicker et al. demonstrates As they reasoned, if N is the minimal fortunate. The effect simply reflects a sharing conceptually one way that basic physics number of component RNAs, and p the of resources, and diversification of risks, might cause such growth and division small probability for any one component to made possible through merging. (Nat. Phys. 13, 408–413; 2017). be present, then the baseline probability of Again, merging brings a surprising The team studied a simple theoretical formation of the minimal set is ~pN. Hence, benefit. If any simple mechanism could model for the behaviour of liquid droplets in the time needed grows exponentially support merging in a population of already systems driven away from thermodynamic with N. However, if RNA components growing and dividing protocells, then it equilibrium by an applied energy flow. don’t float around freely in the primordial should be amplified and spread through the As a result of a chemical disequilibrium, soup, but reside within protocells, their population, becoming the norm. chemical reactions tend to drive droplets contents remain together for longer This is an admittedly speculative linking to add key material, making them grow. periods, making interaction more likely. together of several ongoing strands of At the same time, however, droplet growth Even more crucially, if protocells can merge research. We’ll wait to see if some, or all, eventually leads to instabilities — linked to together, they can also assemble larger of these ideas play an important part in the changing shape of the droplets — that sets of components from smaller ones. our eventual understanding of the true make cells divide into progeny. The study Sinai and colleagues showed that merging origins of life. ❐ suggests that chemically active droplets of rudimentary protocells — in the form could be plausible as prebiotic protocells, of droplets or anything else — would MARK BUCHANAN 526 NATURE PHYSICS | VOL 13 | JUNE 2017 | www.nature.com/naturephysics ©2017 Mac millan Publishers Li mited, part of Spri nger Nature. All ri ghts reserved. .

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