Power, Sex, Suicide: Mitochondria and the Meaning of Life

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Authors: Nick Lane
Tags: General, science
bacteria do indeed have a cyto-skeleton as well as a cell wall—they wear a belt and braces, as Henry Fonda put it in
Once Upon a Time in the West
(‘never trust a man who can’t even trust his own trousers’). Unlike Fonda’s risk-averse cowboy, however, bacteria do need both to maintain their shape.
    Many bacteria are spherical (
cocci
) while others are rod-shaped (
bacilli
), filamentous, or helical. Some oddballs have been found that even have triangular or square shapes. Quite what advantages these different shapes might confer is an interesting question, but it seems that the default bacterial shape is spherical, and any other shape requires internal support. Non-spherical bacteria possess protein filaments very similar in microscopic structure to those found in eukaryotes like yeast, as well as in humans and plants. In each case, the cytoskeleton filaments are composed of a protein akin to actin, best known for its role in muscle contraction. In non-spherical bacteria, these filaments form into a helical swirl underneath the cell membrane, which apparently provides structural support. What is clear is that if the genes encoding the filaments are deleted then bacteria that are normally rod-like in shape (bacilli) develop as spherical cocci instead. Impressions resembling bacilli have been found in rocks 3500 million years old, so it is conceivable that the cytoskeleton evolved not long after the appearance of the earliest cells. This reverses the problem. If a cytoskeleton was there all along, then why do so few bacteria survive the loss of the cell wall? We’ll return to this theme in Part 3 . For now, let’s satisfy ourselves with the possible consequences.
‘Discovery’ of the archaea—a missing link?
    Only two groups of cells have thrived in the absence of a cell wall—the eukaryotes themselves, and the
Archaea
, a remarkable group of prokaryotes (cells that lack a nucleus, like bacteria). The
Archaea
were discovered by Carl Woese and George Fox at the University of Illinois in 1977, and named from the Greek for ‘ancient’. Most archaea do, in fact, have a cell wall, but their walls are rather different in chemical composition from those of bacteria, and some groups (such as the boiling-acid loving
Thermoplasma
) do not have a cell wall at all. Curiously, antibiotics like penicillin don’t affect the synthesis of archaeal cell walls, lending support to the idea that cell walls might have been the target of bacterial chemical warfare. Like bacteria, archaea are tiny, typically measuring a few thousandths of a millimetre (microns) across, and they do not have a nucleus. Like bacteria, they have a single circular chromosome. Again, like bacteria, the archaea take on many shapes and forms, and so presumably have some sort of cytoskeleton. One reason why they were discovered so recently isthat archaea are mostly ‘extremophiles’, that is, they thrive in the most extreme and arcane of environments, from boiling acid-baths beloved of
Thermo-plasma
, to putrid marshes (inhabited by marsh-gas producing methanogens) and even buried oilfields. In the latter case, the archaea responsible have attracted commercial interest, or rather annoyance, as they ‘sour’ the wells—they raise the sulphur content of oil, which corrodes the well-casings and metal pipelines. Greenpeace could hardly conceive a more wily sabotage.
    The ‘discovery’ of the archaea is a relative term, as some of them had been known about for decades (particularly the oil-souring archaea and swamp-gas producing methanogens), but their small size and lack of nucleus meant that they were invariably mistaken for bacteria. In other words, they were not so much discovered as reclassified; and even now, some researchers prefer to classify them with the bacteria, as just another diverse group of inventive prokaryotes. But the painstaking genetic studies of Woese and others have convinced most impartial observers that the archaea really do differ

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