arguing that there was no way for them to gain more because of their peculiar way of reproducing. Because viruses hijack cells to make new viruses, they are sloppy about copying their genes. They don’t carry their own repair enzymes that can fix errors, for example. As a result, they are much more vulnerable to lethal mutations. If a virus accumulated thousands of genes, its high mutation rate would wipe it out.
The sizes of virus genomes offered some good reason to believe this was actually true. Viruses carry genes encoded either in DNA, or its single-stranded version, RNA. For a number of reasons, RNA is an inherently more error-prone molecule to copy. And it turns out that RNA viruses, like influenza and HIV, have smaller genomes than DNA viruses.
Forced to carry tiny genomes, viruses could not make room for genes that did anything beyond make new viruses and help those viruses escape destruction. They could carry genes to let them eat, for example. They could not turn raw ingredients into newgenes and proteins on their own. They could not grow. They could not expel waste. They could not defend against hot and cold. They could not reproduce by splitting in two. All those nots added up to one great, devastating Not . Viruses were not alive.
To be alive, many scientists argued, required having a true cell. “An organism is constituted of cells,” the microbiologist Andre Lwoff declared in a lecture he gave when he accepted the Nobel Prize in 1967. Lacking cells, viruses were considered as little more than cast-off genetic material that happened to have the right chemistry to get replicated inside cells that were truly alive. Scientists could purify viruses down to crystals, the same way they could crystallize salt or pure DNA. No one could ever crystallize a maple tree. In 2000, the International Committee on Taxonomy of Viruses declared that “viruses are not living organisms.”
In the decade following that declaration, a number of scientists rejected it outright. The old rules no longer work well in the face of new viruses. Mimiviruses, for example, went overlooked for so long in part because they were a hundred times bigger than viruses are supposed to be. They are also loaded with far too many genes to fit old-fashioned notions of a virus. Scientists don’t know what mimiviruses do with all of their genes, but some suspect that they do some rather lifelike things with them. Some of their proteins, for instance, look a lot like the proteins our own cells use to assemble new genes and proteins. When mimiviruses invade amoebae, they don’t dissolve into a cloud of molecules. Instead, they set up a massive, intricate structure called a viral factory. The virus factory takes in raw ingredients through one portal, and then spits out new DNA and proteins through two others. The viral factory looks and acts remarkably like a cell. It’s so much like a cell, in fact, that La Scola and his colleagues discovered in 2008 that it can be infected by a virus of its own. It was the first time anyone had found a virus of a virus. It was yet another thing that ought not to exist.
Drawing dividing lines through nature can be scientifically useful, but when it comes to understanding life itself, those lines can end up being artificial barriers. Rather than trying to figure out how viruses are not like other living things, it may be more usefulto think about how viruses and other organisms form a continuum. We humans are an inextricable blend of mammal and virus. Remove our virus-derived genes, and we would be unable to reproduce. We would probably also quickly fall victim to infections from other viruses. Some of the oxygen we breathe is produced through a mingling of viruses and bacteria in the oceans. That mixture is not a fixed combination, but an ever-changing flux. The oceans are a living matrix of genes, shuttling among hosts and viruses.
Drawing a bright line between life and nonlife can also
The sizes of virus genomes offered some good reason to believe this was actually true. Viruses carry genes encoded either in DNA, or its single-stranded version, RNA. For a number of reasons, RNA is an inherently more error-prone molecule to copy. And it turns out that RNA viruses, like influenza and HIV, have smaller genomes than DNA viruses.
Forced to carry tiny genomes, viruses could not make room for genes that did anything beyond make new viruses and help those viruses escape destruction. They could carry genes to let them eat, for example. They could not turn raw ingredients into newgenes and proteins on their own. They could not grow. They could not expel waste. They could not defend against hot and cold. They could not reproduce by splitting in two. All those nots added up to one great, devastating Not . Viruses were not alive.
To be alive, many scientists argued, required having a true cell. “An organism is constituted of cells,” the microbiologist Andre Lwoff declared in a lecture he gave when he accepted the Nobel Prize in 1967. Lacking cells, viruses were considered as little more than cast-off genetic material that happened to have the right chemistry to get replicated inside cells that were truly alive. Scientists could purify viruses down to crystals, the same way they could crystallize salt or pure DNA. No one could ever crystallize a maple tree. In 2000, the International Committee on Taxonomy of Viruses declared that “viruses are not living organisms.”
In the decade following that declaration, a number of scientists rejected it outright. The old rules no longer work well in the face of new viruses. Mimiviruses, for example, went overlooked for so long in part because they were a hundred times bigger than viruses are supposed to be. They are also loaded with far too many genes to fit old-fashioned notions of a virus. Scientists don’t know what mimiviruses do with all of their genes, but some suspect that they do some rather lifelike things with them. Some of their proteins, for instance, look a lot like the proteins our own cells use to assemble new genes and proteins. When mimiviruses invade amoebae, they don’t dissolve into a cloud of molecules. Instead, they set up a massive, intricate structure called a viral factory. The virus factory takes in raw ingredients through one portal, and then spits out new DNA and proteins through two others. The viral factory looks and acts remarkably like a cell. It’s so much like a cell, in fact, that La Scola and his colleagues discovered in 2008 that it can be infected by a virus of its own. It was the first time anyone had found a virus of a virus. It was yet another thing that ought not to exist.
Drawing dividing lines through nature can be scientifically useful, but when it comes to understanding life itself, those lines can end up being artificial barriers. Rather than trying to figure out how viruses are not like other living things, it may be more usefulto think about how viruses and other organisms form a continuum. We humans are an inextricable blend of mammal and virus. Remove our virus-derived genes, and we would be unable to reproduce. We would probably also quickly fall victim to infections from other viruses. Some of the oxygen we breathe is produced through a mingling of viruses and bacteria in the oceans. That mixture is not a fixed combination, but an ever-changing flux. The oceans are a living matrix of genes, shuttling among hosts and viruses.
Drawing a bright line between life and nonlife can also
Similar Books
