Giant virus

A giant virus is a very large virus. They are sometimes called a girus. Some of them are larger than typical bacteria. All known giant viruses belong to the phylum Nucleocytoviricota.

Megaviricetes
File:Electron microscopic image of a mimivirus - journal.ppat.1000087.g007 crop.png
Mimivirus
Virus classification
Unrecognized taxon (fix):	Megaviricetes

The genomes of many giant viruses code for unusual genes which are not found in other viruses. These include genes involved in glycolysis and the TCA cycle, fermentation, and the cytoskeleton.

The first giant viruses to be described were discovered in 1981.

Note, in the illustration from an electron microscope, the virus is surrounded by a thick (~100 nm) layer of filamentous protein fibres.

Origin

There are two main hypotheses. Either they evolved from small viruses by picking up DNA from host organisms; or they evolved from very complicated organisms by genome reduction.

Giant Virus Media

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  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.

• Phylogeny of Nucleocytoviricota