The pairing of the two organisms was, to put it mildly, extremely successful. So much so that the vast majority of multicellular life, from comb jellies to chimpanzees, is made up of eukaryotic cells. Each cell has a distinct nucleus for passing on physically expressed genetic traits and specific organelles, including the power-producing mitochondria.
Since the mitochondrion acts as the cell's power source, think of it as a factory or coal-burning power plant. Like any power plant, there are inputs and output. The inputs for the mitochondria include sugars (usually in the form of broken-down carbohydrates) and fatty acids. The mitochondria turn this fuel into energy. To continue with the power plant analogy, the "burning" of the lumps of coal produces inevitable output. The intra-cellular output is a combination of adenosine triphosphate (ATP), water, heat, and molecules called "free radicals."
ATP is an extremely special molecule in that it is able to store and transport chemical energy within cells—consider it the electrical current that is generated by the power plant. (ATP is also known by biochemists as the "molecular currency" of intracellular energy transfer.) The free radical molecules could be thought of as the "smoke" that escapes the plant as the fuel is burned.1
In chemistry, free radicals are atomic or molecular species with unpaired electrons or an otherwise open-shell configuration, making them especially likely to take part in unwanted chemical reactions. Because of this, the free radicals that are produced can, over time, seriously damage the DNA of the mitochondria. This damage limits the lifespan of the cell, and in turn the lifespan of the entire organism.
Doug Wallace made the point very clearly over lunch that day. "From my point of view, biology is information. Information is encoded in information-rich molecules. Oxygen radicals—reactive oxygen species—will oxidize any kind of released compound. But if you damage a protein, you can remake it. As long as you have the information you can make another one just like it. What really matters is when [a free radical] damages the information."
Wallace noted that with all of the cells that were constantly being replaced at the end of their lifespan and those that were being created anew, the "Michael" sitting before me shares almost no atoms with the "Michael" that was born. All human beings are constantly undergoing the same simultaneous buildup and teardown. From a biological viewpoint, there was nothing inherent about the physical structure of a person. What was important was the information that was specific to creating you.
By that token, if oxygen radicals damaged the mitochondrial DNA and this information is needed to maintain the power plant, then serious long-term consequences would take place. "You can damage a power plant, say damage the wall, but as long as you have the blueprints to repair the wall, that's no problem. But if you destroy the blueprints, then you damage the wall, the wall stays damaged, and that's what causes aging."
1. Ironically, given the nature of the analogy being used, the most commonly observed "free radical" reaction is combustion.
While admitting that his argument as to what truly controls the biological clock is still highly speculative, Wallace is emphatic in his belief that he sees the key to what drives the clock's mechanism. "It's the accumulation of the damage to the information in the mitochondrial DNA that erodes the ability of the system to repair itself. Once it can't repair itself, it will go off line, and come to a stop."
In a prior interview with The Orange County Register, Wallace makes this point even more vividly. If, as he hypothesizes, the mitochondria act as cellular power plants, then they allow the city to run. "If the blueprints are sound and the contractors built them well, all will be fine. But if the contractors were lazy or sloppy, the power plants would spew smoke into the air, sputter, and fail. The city would go dark, and all would collapse into chaos."
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