The great debate over the classification of viruses rages on. Original classification of organisms was based on visual observations used in deciphering life from non-life. Once Von Leeuwenhoek created lenses for a microscope capable of viewing organisms unseen by the naked eye, evidence of microbial life began to emerge.
Prior to Chatton identifying eukaryotes in the early 20th century, all living particles visible under the microscope were termed microbes. In the 1970’s Carl Woese determined the prokaryote/eukaryote classification actually should be in 3 domains. eubacteria, archaea, and eukarya.
Viruses however, were still considered the “step-child” of living organisms. So why is the debate of virus classification still upon us? There are 5 main ideas that support a new viral classification.
1. Viral Parasites
Virophages are tiny viruses that infect larger viruses. Such viruses use the genome of the infected virus to replicate their genetic material, just as a virus does to its infected host cells. Some virophages can kill the viral host by inhibiting its growth, allowing the original viruses host cell to survive.
2. Viral Transposons
Viruses called transpovirons can insert themselves into a virual genome for replication in the original viruses host cell. Additionally, the transposon style gene shuffling transpovirons participate in, encourages a vast array of diversity throughout viral specimens.
3. Viral Giants
It has long been believed viruses did not exist in sizes larger than 0.2 micrometers. Then in 2003, a giant virus (originally thorugh to be a bacterium) was discovered with a 0.4 micrometer diameter. The mimivirus had 1200 genes compared to the largest viral geneome of 400 genes in 2004. Pandoravirus prior to 2013 was through to be a eukaryotic parasite with a 2.5 million base pair genome.
Scientists have had much success sequencing the genomes of organisms across the domains of life. Giant viruses sequenced have shown parts of prokaryotic, eukaryotic, and viral DNA in their genomes.
5. Phylogenetic Analysis
Although viruses do not carry ribosomal genetic material, they do have tRNA and RNA polymerase. Early organisms also had an RNA based means of protein expression, tracing viral organisms to early life with prokaryotes, eukaryea and archaea.
Some scientists speculate viruses helped transform the RNA world of early organisms to the DNA plethora we see today. It is possible that parasitic viral organisms evolved DNA as a means to eliminate host resistance.
Since genomes between prokaryotes and viral organisms are interchanged among other species, a mechanism for DNA emergence in prokaryotes and eukaryotes is very plausible.
With many European scientists believing the eukaryotic nuclei originated with viruses, the debate seems to continue. Should viruses be considered a fourth domain in the hierarchy of life? Should a superdomain be introduced including the viral organisms as a branch?
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