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It s all in the cells

It s all in the cells size and shape are the two most obvious characteristics of all living creatures. Elephants are large and have a long proboscis (trunk) and big, flappy ears; mosquitoes are tiny, winged and carry a proboscis that can literally get under your skin. Ultimately, the reasons for this have to be understood in terms of evolution. Large animals have an obvious advantage in terms of both being able to defend themselves against predators and being able to attack others for food or mates. Small animals can multiply more rapidly than large ones and so may be able to compensate for their size. Also, if they are small enough, they may be able to successfully parasitise creatures that are much bigger than themselves.

However, the basic building blocks of all animals, their cells, seem to be of roughly the same size (about one-hundredth of a millimetre in diameter, taking the cell as a sphere).Therefore, elephants are bigger than mosquitoes because they have more cells, not because their cells are bigger. Similarly, if you compare two flies, the one that is bigger has more cells than the smaller one. The reason why all cells are more or less the same size is that as cells assimilate nutrients and grow, they divide only after they reach a critical size.

The basis of that widely-held belief has now to be questioned, thanks to a remarkable finding made by Jacques Montagne and colleagues in the Friedrich Miescher Institute, Basel, Switzerland (Science , Vol 285, p2126-2130).

What these workers have discovered is that fruit flies, that lack the gene for a particular enzyme, develop slowly, and are significantly smaller than normal and have smaller cells rather than fewer cells than normal flies do. The enzyme belongs to a class known as kinases, which are able to catalyse the transfer of a phosphate group from a donor molecule to an acceptor. The enzyme acts in a cell-autonomous fashion. This means that if one cell in a tissue lacks the enzyme, only that cell is smaller than usual and the rest of the tissue is of normal size.

Indirectly, the observations on these mini flies shows how important it must be to have cells of the normal size. Flies that entirely lack this kinase, called S6 kinase, do very poorly indeed. Often they are born dead. Any female that make it to adulthood are sterile. And both males and females that do manage to survive, develop extremely slowly.

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