Where breaking in half quickly is the point
Biology is an inspiring subject. The variety of living organisms, from the smallest bacterium to the largest Sequoia tree, to processes such as birth that are often described as miraculous, have captivated intellectual and poetic interest since ancient times.
And it isn’t just the outside of living organisms and the systems of which they form part that amaze us. The human heart can pump blood continuously for over a century. Ants that weigh no more than a few hundred milligrammes can build coherent structures that weigh hundreds of kilogrammes each. We currently share the Earth with the largest animal ever to have existed, the blue whale, that gives birth to young that are typically over two tonnes in weight after a gestation of a little over eleven months.
The fundamental processes on which these spectacles rely have been gone into in some detail. We know much about how the heart works, and how ants move sand and soil around. Studies of fertility and the cell cycle have told us about where babies come from. This satisfies much of our curiosity, but some questions remain. For example, how can a cell that will go on to become a blue whale or a person, prepare itself for being broken in half billions of times in succession to create an individual?
This question is not as sarcastic or as niche as it may sound. In order to get from one cell to two, and thus from a fertilised ovum into a healthy baby, cells must divide. This is described as binary fission (bacteria) or cytokinesis (mammalian cells), but it amounts to the same thing: the controlled partitioning of one cell to leave two, living cells. There are bacteria that can do this every twenty minutes, and the billions of cells in a newly-born blue whale have all grown from just one cell, a fertilised ovum.This demonstrates that this process can happen repeatedly and consistently. It raises the question of how the cell manages its structure to ensure this process happens reliably, including fission of the plasma membrane amongst others.
Recent work by Keidel et al.  and Zhao et al.  investigated both membrane scission and fusion. Insodoing, they have provided evidence for how the membrane part of cell division proceeds. The data published provide evidence for hemi-fission and hemi-fusion intermediates, one in which the inner monolayer of the membrane is broken first, followed by the outer monolayer. This is consistent with an hypothesis generated in biophysics some time ago (Figure), and so this has probably lifted a scientific purgatory.
However, further challenges in understanding the structural changes in the cell cycle remain. The limited window for the light-dependent part of photosynthesis means that several species of algae time their cell division around light and dark periods [3, 4]. Furthermore, under favourable conditions, several species of algae are able to undergo so called multi-fusion . This means in effect breaking one cell into as many as sixteen daughter cells. It’s a huge structural task that must happen in as controlled and consistent a manner as binary fission, in order not to kill the cells that attempt it. Multi-fission must also work around the division of chloroplasts so that all of the daughter cells have enough chloroplasts to make best use of the light by the next light period commences.
Further work is required to understand these, however study of the algal cell cycle is tantalising. It may yet shed light on binary fission, its speed and limits, and indeed how the whole process of cell division has evolved.
- A. Keidel, T. F. Bartsch and E.-L. Florin, Scientific Reports, 2016, 6, 23691. 10.1038/srep23691. http://www.nature.com/articles/srep23691#supplementary-information.
- W.-D. Zhao, E. Hamid, W. Shin, P. J. Wen, E. S. Krystofiak, S. A. Villarreal, H.-C. Chiang, B. Kachar and L.-G. Wu, Nature, 2016, advance online publication. 10.1038/nature18598. http://www.nature.com/nature/journal/vaop/ncurrent/abs/nature18598.html#supplementary-information.
- M. Vítová, K. Bišová, D. Umysová, M. Hlavová, S. Kawano, V. Zachleder and M. Čížková, Planta, 2010, 233, 75-86. 10.1007/s00425-010-1282-y.
- V. Zachleder, K. Bišová, M. Vítová, S. Kubín and J. Hendrychová, European Journal of Phycology, 2002, 37, 361-371. doi:10.1017/S0967026202003815.
- V. Zachleder, K. Bišová and M. Vítová, in The Physiology of Microalgae, eds. A. M. Borowitzka, J. Beardall and A. J. Raven, Springer International Publishing, Cham, 2016, pp. 3-46.
- P. R. Cullis, M. J. Hope and C. P. S. Tilcock, Chemistry and Physics of Lipids, 1986, 40, 127-144. http://dx.doi.org/10.1016/0009-3084(86)90067-8.
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