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Question: How can viruses mutate if they're not considered alive? Is it just that our scientific definition of what's "alive" needs to change? First review a few characteristics of life: • All living things are composed of one or more cells • Living things grow and reproduce • They carry out metabolism (chemical transformations) • They maintain a state of homeostasis Where Do Viruses Fit? • Not a cell • Nucleic acids in protein shell • Do not grow, do not maintain homeostasis, and do not metabolize on their own • Use host cell to replicate Question: How can viruses mutate if they're not considered alive? Is it just that our scientific definition of what's "alive" needs to change? Review Chapter 21 figures on the Lytic and Lysogenic life cycles. Viruses mutate during the process of DNA or RNA replication. Viruses that contain DNA genetic material tend to be stable through time because of the proofreading process that DNA undergoes during replication. Viruses that contain RNA genetic material are dynamic (that is mutate relatively quickly) because RNA replication does not have the same proofreading mechanism in place. Source: How viruses mutate Question: I know that we can kill bacteria using anti-bacterial sanitizer and penicillin and whatnot, but what's the life expectancy of one bacterial cell, and what exactly is it that it "dies" of? It depends. Bacteria experience cell death when they can no longer reinitiate cell division. So environmental conditions that do not favor cell division, or otherwise disrupt the cell, will cause cell death. However, some bacteria can survive in a dormant state indefinitely as endospores. Source Question: Is there a lifespan of a virus? How would a virus die if it's not considered alive? Could a virus continue to "live" if it's host died? Using the definition of life given previously, viruses do not die per se but rather cease to replicate. A virus can only continue to replicate as long as the host survives. Therefore, the longer the host survives, the more opportunities the virus has to infect new hosts. Question: Theoretically, could bacteria be used to kill/consume harmful bacteria and possibly viruses? My understanding is that bacteria can push out an existing population of bacteria (i.e., competitive exclusion), but can they kill each other? Yes, bacteria do not play nice. They can excrete wastes that are harmful to other bacteria strains as well as release restriction enzymes that can cut up the DNA of its competitors. Source: Bacteria Fights to the Death Question: Which one came first? The virus or the host? Good question… Genetic material that became independent of bacteria cells Evolved completely independently of cellular life Evidence that this happened very early on. Source: Origins of viruses Bacteria and humans as symbionts We have 10-100 trillion bacteria in our guts. These bacteria are critical to our ability to digest various foods. Thousands of years ago, humans in Japan started to eat nori (a form of seaweed). In the process they also ingested marine bacteria that can process nori carbohydrates. Through horizontal gene transfer, gut bacteria received the genetic instructions to make the same enzymes, thus facilitating the digestion of nori by humans. Bacteria and humans benefit each other (mutualism) Source: How the gut bacteria evolved to feast on sushi Puzzling increase in autoimmune disorders, particularly allergies in the last two decades. Working Hypotheses? • Humans changed (gene pool) • Environment changed (genetic modifications, additives, etc.) • Increased awareness and reporting • Under-stimulation of the immune system • Overstimulation of the immune system Hygiene hypothesis Could our environment be too clean? (under-stimulation of immune system hypothesis). Research in East and West Germany immediately after reunification indicated that children raised in the dirtier environments were less likely to suffer from autoimmune disorders. Mechanism: appears to be the lack of challenge to the immune system at an early stage. Immune system fails to distinguish nonpathogenic environmental stimuli from genuine threats (pathogens). More information about the Immune System and Allergies Univ. of Virginia Health System National Institute of Health Immune System Info Page The Hygiene Hypothesis Clearinghouse