Seeing Life in Viruses
Artist’s representation of the Severe Acute Respiratory Syndrome (SARS) virus. This RNA virus has killed hundreds of people since it was first reported in 2003.
We all try to avoid viruses due to the havoc they can wage on our health. Some viruses do more than create temporary discomfort: it is estimated the influenza virus of 1918 killed somewhere between 50 to 100 million people around the world. We might think differently about viruses, however, if we discovered that rather than just being dangerous to life, they could be the basis for life itself.
Kirsi Lehto of the University of Turku in Finland studies plant viruses with an eye toward their role in the origin and evolution of life.
"I am inclined to see present-day viruses as hypothetical models for primitive RNA organisms," says Lehto. Like primitive RNA-based organisms, RNA viruses use ribonucleic acid as their genetic information molecule. The SARS virus, influenza, and hepatitis C are all RNA viruses.
Despite their tendency to cause ill health in humans and others, Lehto thinks such viruses might hold clues about how early life transitioned from being RNA-based to DNA-based.
There are 6,000 million base pairs of DNA in the nucleus of almost every one of your cells. The DNA is packaged into 46 bundles called chromosomes. These long strands of information encode two sets of 80,000 different genes, one set inherited from each of your parents. A single gene ranges in size from a hundred base pairs to millions of base pairs.
Credit: The Science Museum, UK.
In the beginning, life had to start out in a very simple form, and the current DNA-based life is very complex. Part of the problem with DNA-based life is the interdependent relationship with proteins. Nowadays, the instructions for making proteins are coded in DNA, but DNA needs proteins to exist. Enter RNA, which is still the mediator between DNA and protein synthesis. Early in the life’s history, this mediator molecule may have played the role of both the proteins and the genetic code.
"There had to be an era when the current machinery was not invented yet," says Lehto. "Life had to be functioning without proteins, and RNA would have been the central part of this system – the information is in the RNA, and RNA makes up the core of the machinery for translating the information. So it is believed that prior to the time that proteins were invented, RNA was doing everything by itself."
Life may have evolved into the DNA world we know today because of the limitations of the RNA world. RNA is a fragile molecule, easily broken apart. It makes a lot of errors when it copies information. Also, RNA can’t hold that much information in the first place.
"In the early world, RNA fragments by themselves may have been only up to a few hundred nucleotides long," says Lehto. Nucleotides are the repeating units of sugars, phosphates, and nitrogenous bases that make up the genes that form strands of RNA or DNA. "Now the shortest functional RNA viruses are some 4,000 nucleotides long, and contain three functional genes. The longest viral RNA genomes contain up to 20 genes."
The earliest life may have used RNA for functions now fulfilled by DNA and proteins.
Compare that to DNA’s ability to hold hundreds of millions of nucleotides, and thousands of genes. The human genome, for instance, is composed of more than 30,000 genes.
Still, larger isn’t always better. Lehto says that viruses have the smallest replicating genomes known today, and some of them survive with a minimal number of self-encoded gene functions.
"Different viruses utilize different strategies and molecular mechanisms for their survival, and many of these mechanisms are really ingenious and efficient," says Lehto.
RNA viruses have managed to overcome the fragility and replication errors that early RNA-based life was probably prone to. Lehto says that the strategies observed in these viruses may be similar to how early RNA life may have survived, reproduced, and fought off competitors and parasites, and eventually "associated themselves with membraneous structures, leading to the development of cell membranes, and of cellular life."
Lehto is not the first scientist to look to the RNA viruses as a model for early life. However, she says more research needs to be done on the various ways viruses make do with so little. Armed with this sort of information, we may get a better idea of just what the first life on Earth was like, and how it may have functioned.
Deep-sea hydrothermal vents, such as this one on the Juan de Fuca Ridge off the Pacific coast of Washington State, spew super-hot mineral-rich fluids into the surrounding ocean water. Vents such as these are thought to be a possible location for life’s origin and early evolution.
"The life cycles of present-day viruses depend on the environment inside their host cells," says Lehto. "This feature might be reminiscent of the hypothetical environment of early life, which apparently had to provide a rich supply of all the components needed."
One such hypothetical environment for life’s origin is volcanic sea vents. These hydrothermal vents are home to thermophiles, bacteria and archaea that can live in hot temperatures. Genetic analysis has shown that thermophiles are among the most ancient life forms known. Viruses have been detected at vent sites as well, although little is known about their provenance and history.
Regardless of their role in life’s history, most scientists do not consider viruses to be life forms themselves because they are parasites that need a host to survive. Peter Ward, a paleontologist at the University of Washington and author of the book, "Life As We Do Not Know It," thinks otherwise, and he has proposed that some viruses are living fossils from the ancient RNA world. He suggests the creation of a new taxonomic category for life forms that are based on RNA. Having this new category for defining life could come in handy in the future, he says, because it’s possible we could find RNA life on other worlds.
"What kind of life can we expect to find on other planets?" asks Ward. "Perhaps DNA life will be common on Mars. We have tests that can find DNA life, but there may be RNA life that we would not find using current tests."
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