Revision as of 05:05, 29 September 2021 by Macoipedia(talk | contribs)(i added more info on giant viruses and explained about how they hunt and what hunts them.)
Agiant virus, sometimes referred to as a girus, is a very large virus, some of which are larger than typical bacteria.[1][2] They have extremely large genomes compared to other viruses and contain many unique genes not found in life forms. All known giant viruses belong to the phylum Nucleocytoviricota.[3]
Description
While the exact criteria as defined in the scientific literature vary, giant viruses are generally described as viruses having large, pseudo-icosahedral capsids (200 to 400 nanometers)[4] that may be surrounded by a thick (approximately 100 nm) layer of filamentous protein fibers. The viruses' large, double-stranded DNA genomes (300 to 1000 kilobasepairs or larger) encode a large contingent of genes (of the order of 1000 genes).[3][5] While few giant viruses have been characterized in detail, the most notable examples are the phylogenetically related mimivirus and megavirus—both belonging to the Mimiviridae (aka Megaviridae) family, due to their having the largest capsid diameters of all known viruses.[3][5]Giant viruses from the deep ocean, terrestrial sources, and human patients contain genes encoding cytochrome P450 (CYP; P450) enzymes. The origin of these P450 genes in giant viruses remains unknown but may have been acquired from an ancient host.[6]
Confusing Genes
What is confusing is the fact that there are so many genes inside of a girus. Normally, viruses are nothing more than an outer membrane and a little DNA that is just enough to replicate itself. That's it. No will, no ambition, no emotions, no real life. But giruses have a lot more genes than a normal virus. Some genes scientists don't even know what they do! Others are needed to get oxygen, to photosynthesize, to move or to just be alive. Some giruses are even seen to seem to have a certain amount of self-propulsion, like the pandoravirus. And still they don't seem to be alive. This is one of the mysteries that science just can't solve.
Cryo-EM images of the giant viruses CroV and APMV. (A) Cryo-electron micrograph of four CroV particles. (B) Single CroV particle with concave core depression (white arrow). (C) Single APMV particle. Scale bars in (A–C) represent 2,000 Å.
Hunt For Prey
Giant viruses hunt protists, bacteria and amoebas. They use the host cell's defense systems to get inside. They then large spheroidal viroplasm or virus factories located within the cytoplasm of the infected host cell in which they replicate. They force the host cell to use all it's resources to make more giruses inside the viroplasm. They are even able to make a defensive membrane around the virus factory. When there are a lot of giruses already, they force the host to explode and release the giruses that it created. This is similar to the replication mechanism used by Poxviridae.
Virophages
Even though giruses are very big, there are still viruses that hunt them. Virophagesatellite viruses,such as Sputnik 1, ride with the giruses as they enter a victim amoeba. Inside the amoeba, the virophages implement their DNA to the girus's viroplasm, making the girus make only a few copies of itself, which are mostly incomplete. Instead, the viroplasm makes more virophages. In the end, the victim still dies, but the new virophages go out to find more giruses to hijack.
Defense Against Giruses
The victims of giruses still are not undefended though. Some protists have learned to defend themselves by adding the DNA of virophages to their DNA. When a girus infects this protist, the protist makes some virophages that use the viroplasm to their own uses. The protist still dies, but instead of making more giruses to kill it's companions, virophages go on the hunt for giruses.
History
The first giant virus, then named BV-PW1, was isolated and described in 1995,[7] but was not recognized as such until its sequenced genome was released as Cafeteria roenbergensis virus (CroV) in 2010.[8] Subsequently, the Giant Virus Acanthamoeba polyphaga Mimivirus was characterized,[9] (which had been mistaken as a bacterium in 1993),[10] and then sequenced.[11] The term "girus" was coined to refer to the group in 2006.[12]
Genetics and evolution
The genomes of giant viruses are the largest known for viruses, and contain genes that encode for important elements of translation machinery, a characteristic that had previously been believed to be indicative of cellular organisms. These genes include multiple genes encoding a number of aminoacyl tRNA synthetases, enzymes that catalyze the esterification of specific amino acids or their precursors to their corresponding cognate tRNAs to form an aminoacyl tRNA that is then used during translation.[5] The presence of four aminoacyl tRNA synthetase encoding genes in mimivirus and mamavirus genomes, both species within the Mimiviridae family, as well as the discovery of seven aminoacyl tRNA synthetase genes, including the four genes present in Mimiviridae, in the megavirus genome provide evidence for a possible scenario in which these large DNA viruses evolved from a shared ancestral cellular genome by means of genome reduction.[5]
Their discovery and subsequent characterization has triggered some debate concerning the evolutionary origins of giant viruses. The two main hypotheses for their origin are that either they evolved from small viruses, picking up DNA from host organisms, or that they evolved from very complicated organisms into the current form which is not self-sufficient for reproduction.[13] What sort of complicated organism giant viruses might have diverged from is also a topic of debate. One proposal is that the origin point actually represents a fourth domain of life,[5] but this has been largely discounted.[14][15]
Comparison of largest known giant viruses
Largest giant viruses with complete sequenced genomes as of March 2015[update]
1Mutator S (MutS) and its homologs are a family of DNA mismatch repair proteins involved in the mismatch repair system that acts to correct point mutations or small insertion/deletion loops produced during DNA replication, increasing the fidelity of replication.
2A stargate is a five-pronged star structure present on the viral capsid forming the portal through which the internal core of the particle is delivered to the host's cytoplasm.
^La Scola B, Audic S, Robert C, Jungang L, de Lamballerie X, Drancourt M, Birtles R, Claverie JM, Raoult D (2003). "A giant virus in amoebae". Science. 299 (5615): 2033. doi:10.1126/science.1081867. PMID12663918. S2CID39606235.
^"Giant Viruses". American Scientist. 2017-02-06. Retrieved 2021-09-02.