|WIKI Categories: |
Due to the great diversity of viruses, biologists have struggled to classify these entities and relate them to the conventional tree of life. They may represent previously free-living organisms that have become parasites. They may be the harbingers of life as we know it.
We know that viruses are very diverse. Unlike all other biological organisms, some viruses, such as the poliovirus, have an RNA genome, and others, such as the herpes virus, have a DNA genome. Moreover, some viruses have single-stranded genomes while others have double-stranded genomes.
Are viruses alive?
The upper half of the sphere represents the capsid-coding organisms, which are viruses, and include Archaea viruses, Bacteria viruses, and Eukarya viruses. The lower half of the sphere represents ribosome-coding organisms and includes bacteria, archaea, and cercariae. In addition, populations of living organisms change over time. We all probably realize that viruses reproduce in one way or another.
Likewise, we all probably realize that viruses evolve over time. We need to get the flu shot every year, mostly because the flu virus changes or evolves from year to year. However, viruses do not carry out any metabolic processes. Most importantly, viruses differ from living organisms in that they cannot generate ATP.
Viruses also lack the necessary machinery for translation, as mentioned above. They do not have ribosomes and cannot independently form proteins from messenger RNA molecules. Due to these limitations, viruses can only replicate within a living host cell. As a result, According to a strict definition of life, they are non-living.
Where do viruses come from?
The virus-first hypothesis states that viruses arose before or co-evolved with their current cellular hosts.
The Progressive Hypothesis
A schematic diagram shows the life cycle of a retrovirus as it infects a eukaryotic host cell, replicates, and releases its progeny. The main structural features of the virus are shown with a simplified outline of a host cell with a nucleus. When the virus penetrates a host cell, a viral enzyme, reverse transcriptase, converts this single-stranded RNA into double-stranded DNA. This viral DNA then migrates into the nucleus of the host cell.
Another viral enzyme, integrase, inserts newly formed viral DNA into the host cell's genome. The host cell's RNA polymerase can make new copies of the virus's single-stranded RNA genome. Descendant viruses assemble and exit the cell to start the process over. These transportable genetic elements make up an astonishing 42% of the human genome and can move through the genome via an RNA intermediary.
With these enzymes, these elements can be transcribed into RNA, back-transcribed into DNA, and then integrated into a new location in the genome. It can be assumed that the acquisition of a few structural proteins could allow the element to leave a cell and enter a new cell, thus becoming an infectious agent. Indeed, the genetic structures of retroviruses and virus-like retrotransposons show remarkable similarities.
The regressive hypothesis
Unlike the progressive process just described, viruses can arise from a regressive or reductive process. The viruses of a particular group, the large nucleocytoplasmic DNA viruses, best illustrate this hypothesis. These viruses, which include the smallpox virus and the recently discovered giant of all viruses, Mimivirus, are much larger than most viruses. Conversely, spherical influenza virus particles can be as small as 80 nm in diameter and poliovirus particles as small as 30 nm, about 10,000 times smaller than a grain of salt.
NCLDVs also have large genomes. In addition to their large size, NCLDVs exhibit greater complexity than other viruses and are less dependent on their host for replication than other viruses. Poxvirus particles, for example, include a large number of viral enzymes and related factors that allow the virus to produce functional messenger RNA in the cytoplasm of the host cell. Due to the size and complexity of NCLDVs, some virologists have speculated that these viruses may be descendants of more complex ancestors.
According to proponents of this hypothesis, autonomous organisms initially developed a symbiotic relationship. Eventually, it was no longer able to reproduce independently and became an obligate intracellular parasite, a virus. The analysis of the giant Mimivirus could support this hypothesis.
The Virus First Hypothesis
A schematic diagram would show hypothetical evolutionary relationships between viruses and the three domains of life if viruses existed before cells. The ancient virosphere is represented by a green oval in the center of the diagram and contains the ancestors of modern viruses and the last universal common ancestor of all life. Blue cones contain bacteria and bacteria viruses. Red cones contain Archaea and Archaea virus.
Yellow cones contain Eukarya and eukaryotic viruses. A question mark above the arrow suggests that there is uncertainty regarding the evolutionary relationship between modern bacterial and archaeal viruses. The progressive and backward hypotheses both assume that cells existed before viruses. Recently, several researchers have proposed that viruses may have been the first entities to replicate.
Koonin and Martin postulated that viruses existed in a precellular world as self-replicating units. Perhaps single replicating RNA molecules, existing before the first cell was formed, evolved the ability to infect early cells. Both groups possibly postulate that the current nucleus of eukaryotic cells arose from an endosymbiotic-like event in which a complex, enveloped DNA virus became a permanent resident of an emerging eukaryotic cell.
No single assumption can be correct
Where viruses come from is not a simple question to answer. It can be said quite convincingly that some viruses, such as retroviruses, arose through a gradual process. Ultimately, the idea that viruses gave rise to life as we know it presents some very intriguing possibilities. Perhaps today's viruses have appeared multiple times, through multiple mechanisms.
Perhaps all viruses arose via some yet-to-be-discovered mechanism.
To date, no clear explanation of the origin of viruses exists. Viruses can originate from mobile genetic elements that have acquired the ability to move between cells. They may be descendants of formerly free-living organisms that have adopted a parasitic replication strategy. Perhaps viruses existed before and led to the evolution of cell life.