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Scientists are not sure whether viruses are living or non-living. In general, scientists use a list of criteria to determine if something is alive. Let’s look at some traits of living things and see if viruses also have those traits.
Living things have cells. Viruses do not have cells. They have a protein coat that protects their genetic material (either DNA or RNA). But they do not have a cell membrane or other organelles (for example, ribosomes or mitochondria) that cells have.
Living things reproduce. In general, cells reproduce by making a copy of their DNA. Unlike cells, viruses do not have the tools to make a copy of their DNA. But they have found other ways to make new viruses. This is done this by inserting virus genetic material into a host cell. This causes the cell to make a copy of the virus DNA, making more viruses.
Many scientists argue that even though viruses can use other cells to reproduce itself, viruses are still not considered alive under this category. This is because viruses do not have the tools to replicate their genetic material themselves.
More recently, scientists have discovered a new type of virus, called a mimivirus. These viruses do contain the tools for making a copy of its DNA. This suggests that certain types of viruses may actually be living.
Living things use energy. Outside of a host cell, viruses do not use any energy. They only become active when they come into contact with a host cell. Once activated, they use the host cell’s energy and tools to make more viruses.
Because they do not use their own energy, some scientists do not consider them alive. This is a bit of an odd distinction though, because some bacteria rely on energy from their host, and yet they are considered alive. These types of bacteria are called obligate intracellular parasites.
Living things respond to their environment. Whether or not viruses really respond to the environment is a subject of debate. They interact with the cells they infect, but most of this is simply based on virus anatomy. For example, they bind to receptors on cells, inject their genetic material into the cell, and can evolve over time (within an organism).
Living cells and organisms also usually have these interactions. Cells bind to other cells, organisms pass genetic material, and they evolve over time, but these actions are much more active in most organisms. In viruses, none of these are active processes, they simply occur based on the virus's chemical make-up and the environment in which it ends up.
When scientists apply this list of criteria to determine if a virus is alive, the answer remains unclear. Because of this, the debate of whether viruses are living or non-living continues. As the understanding of viruses continues to develop, scientists may eventually reach a final decision on this question.
Should viruses be included with other living things? After you decide why you think they should or should not be considered alive, listen to biochemist Nick Lane and Dr. Biology discuss if they think viruses are alive.
Ebola Virus image by NIAID
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