Lecture: Aging of course: Narrator: Listen to part of a lecture in a biology class. Professor: Aging of course, is a natural part of life for all living things. Most theories of aging, why aging occurs, fall into one of two categories. First, you have the biological clock category. Theories in this category assert that all organisms have a sort of internal biological clock, that they're sort of genetically programmed to age in a certain way at a certain rate. The second category asserts that aging is the result of gradual accumulation of damage to cells and tissues. And in this second category, we have a theory called the free radical theory which we will now examine. According to this theory, an organism's cells, in their regular metabolic processes, uh, by metabolic processes, I mean by cellular respiration and reproduction. In their regular metabolic processes, cells generate oxygen molecules, some of which have an extra electron. That extra, or free electron makes the whole molecule unstable. The unstable molecule is known as a free radical, okay? And according to this theory, these free radicals cause damage to other molecules and structures within the cell, including most significantly, to the cell's DNA, and it's this gradual degradation of DNA and tissues and larger structures that drives the aging process. So, how does this theory hold up? Well, some organisms do have some protection against free radical damage, in the form of substances called anti-oxidants. Anti-oxidants, through a series of chemical reactions, help stabilize free radicals, so they can't damage the cells. One important anti-oxidant is an antioxidant called superoxide dismutase, or SOD. In experiments with fruit flies, when researchers increased the levels of SOD, there was less free radical harm. The fruit flies had longer lifespans, and stayed younger longer. So this seems to show that indeed, at least in some invertebrates, free radical damage IS what causes aging. Okay, and there's another line of thought that at one point was used to support the free radical theory. It was discovered that when organisms such as yeast, or fungus were given a calorie-restricted diet, they lived longer. If their diets included far fewer calories than normal but still included all the necessary nutrients, their lifespan increased! So while researchers explained this by arguing that the limited food intake led the yeast's cells metabolism to slow down. And remember, according to the free radical theory, cell's metabolic process was what brought about the electrons that created free radicals. And, so the argument went at that time, if the cell's metabolism slowed down, the rate of free radical production also slowed down, so there was less cell damage. And that's why the yeast cells aged slower and lived longer. However, later research, this time on round worms, suggested that while it was true that organisms on a calorie-restricted diet lived longer, it wasn't because of the slower metabolism. In fact, the researchers in this round worm study claimed that the special diet actually increased cell respiration, and that what happened was, this increased respiration activated a certain gene, a gene that has been linked to a longer lifespan. And these researchers claims that it was actually the activation of this longevity gene, not the slowing of free radical damage, that lengthens the lifespan.