Ultraviolet light harms cells of all types, including yeast cells. A common laboratory experiment for college students studying biology involves irradiating yeast cells with ultraviolet light for varying periods of time. The students then plot the fatality rate of the cells versus the time of exposure to radiation and find a direct relationship. The radiation does not directly destroy yeast cells, however; it damages molecules of deoxyribonucleic acid, or DNA, which can affect cells in a number of ways.
Ultraviolet radiation does not kill yeast cells outright. Instead, it damages their DNA. In fact, at lower exposure times, most yeast cells might not die at all, but many will become mutated. Mutations occur when the sequence in DNA is changed. Remember that DNA contains an organism's genes, and genes are responsible for all of an organism's characteristics. When an organism's genes are changed, its characteristics change. Sometimes these changes will be fatal to the organism, but sometimes they can be beneficial. Mutations, after all, are one of the driving forces for evolution by natural selection. So ultraviolet radiation in low doses will affect a yeast population by introducing mutations, and thus genetic variability, into it.
Death by Apoptosis
At higher exposures, the DNA in yeast cells becomes irreparably damaged by the ultraviolet radiation. Eukaryotic cells, like yeast, can undergo a process called apoptosis when their DNA becomes too badly damaged. Apoptosis, or programmed cell death, is essentially a form of cellular suicide wherein the cell digests itself. The result of apoptosis is a group of small membrane packets that contain what used to be the contents of the whole cell. In multicellular animals, apoptosis has the function of preventing cancerous cells from taking over the body. In a single-celled organism like yeast, scientists speculate that apoptosis occurs to make more space in the environment for other cells to live.
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A Surprising Benefit
Scientists have discovered many growth factors that can cause a population of yeast cells to grow more quickly. Among these are nitrogen-rich amino acid complexes that are contained in the cells themselves. For instance, if a scientist breaks open a bunch of yeast cells, then mixes what comes out in with living cells, the living cells will grow faster. Scientists have found that the most effective growth stimulating mixture is obtained from yeast cells that have been killed by ultraviolet radiation. So if half of a yeast colony is killed by ultraviolet radiation, the survivors may experience an enormous growth spurt.