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MECHANISM OF VIRAL MUTATION
Just as natural selection has shaped the evolution of humans, plants, and all living things on the planet, natural selection shapes viruses too. Though viruses aren’t technically living ( they need a host organism in order to reproduce) they are subject to evolutionary pressures.
One way hosts protect themselves from a virus is to develop antibodies to it. Antibodies lock onto the outer surface proteins of a virus and prevent it from entering host cells. A virus that appears different from other viruses that have infected the host has an advantage: the host has no pre-existing immunity, in the form of antibodies, to that virus. Many viral adaptations involve changes to the virus’s outer surface.
Below we look at two special cases in viral evolution: how evolution occurs in influenza viruses and in the human immunodeficiency virus (HIV, the virus that causes AIDS). Both of these viruses are RNA viruses, meaning that their genetic material is encoded in RNA, not DNA.DNA is a more stable molecule than RNA.They manage to use the host cell to verify viral DNA replication. If the virus makes a mistake in copying the DNA, the host cell can often correct the mistake. DNA viruses, therefore, do not change, or mutate, much. RNA, however, is an unstable molecule, and RNA viruses don’t have a built-in proofreading step in their replication. Mistakes in copying RNA happen frequently, and the host cell does not correct these mistakes. RNA virus mutations are frequent and can have important consequences for their hosts.
Influenza Viruses Influenza viruses are simple entities belonging to one of three types: A, B, or C. Antigenic Drift Influenza viruses can evolve in a gradual way through mutations in the genes that relate to the viral surface proteins hemagglutinin and neuraminidase (HA and NA in shorthand). These mutations may cause the virus’s outer surface to appear different to a host previously infected with the ancestor strain of the virus. In such a case, antibodies produced by previous infection with the ancestor strain cannot effectively fight the mutated virus, and disease results. In such a case, antibodies produced by previous infection with the ancestor strain cannot effectively fight the mutated virus, and disease results. As mutations accumulate in future generations of the virus, the virus “drifts” away from its ancestor strain. Antigenic drift is one reason that new flu vaccines often need to be created for each flu season. Antigenic Shift Antigenic shift is a process by which two or more different types of influenza A combine to form a virus radically different from the ancestor strains. The virus that results has a new HA or NA subtype. Antigenic shift may result in global disease spread, or pandemic, because humans will have few or no antibodies to block infection. However, if the new influenza A subtype does not easily pass from person to person, the disease outbreak will be limited. HIV The virus that causes Acquired Immune Deficiency Syndrome (AIDS) is a highly genetically variable virus, for several reasons. First, it reproduces much more rapidly than most other entities. It can produce billions of copies of itself each day. As it makes rapid-fire copies of itself, it commonly makes errors, which translate into mutations in its genetic code. The rapid rate of HIV evolution has important consequences. HIV can quickly develop resistance to anti-HIV drugs. Scientists try to predict which changes are likely to occur to currently circulating flu viruses. They create a vaccine designed to fight the predicted virus. Sometimes the prediction is accurate, and the flu vaccine is effective. Other times the prediction misses the mark, and the vaccine won’t prevent disease.
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