Monster Viruses – Mimivirus and Pandoravirus (Comparative Understanding)

Mimivirus, the largest DNA virus isolated, capable to infect Acanthamoeba polyphaga. Since then larget viruses, often called monster viruses have been discovered and in the year 2013 even larger family of giant amoeba-infecting viruses was discovered named Pandoravirus. Mimivirus is short for “mimicking microbe” due to its large size and Gram-staining properties. Discovery of Pandora virus shattered the notions that viruses could not be seen through light microscope. These two big viruses are physically and genetically unlike, therefore it would be nice to know about them more in details with comparisons.


Discovery of mimi virus was accidental in 1992, identified within Acanthamoeba polyphaga. It was mistaken to be a gram-positive bacterium, when observed under gram staining technique. Bradfordcoccus was named due to consequence, as the amoeba was sourced from Bardford, England. Later in 2003, a research published in Science journal, identifying Bradfordcoccus to be a virus. Comparatively, the discovery of pandoravirus was done by French researchers Jean-Marie Claverie and Chantal Abergal from mud of an Australian pond and also from the coast line of Chile. Pandora virus discovery overturned the concept of mimivirus, since pandoravirus’ genome twice as long than the former.

Pandoravirus (left) and infected amoeba (right)

Pandoravirus (left) and infected amoeba (right)


Overlooking the structures of the viruses, since both are already having a moster size. But it is remarkable to understand both of its genome. Mimivirus genome is a liner, double-stranded DNA molecule with 1181Kbs in length with 979 protein coding genes, which exceeds the 4genes required by smaller viruses to exist. Comparing with the Pandora viruses which is also double stranded DNA molecule but varying in length from 2.77Mbs to 2.47Mbs. They varying in length is due to the tandem repeats (repeated sequences at the end of the DNA). Both mimiviruses and pandoraviruses are proved to be bigger than well known intercellular bacteria Tremblaya (138Kbs) and Rickettsia (1111Kbs).



Mimivirus genome analysis reveals the presence of genes, which is not common in the smaller viruses, like aminoacyl tRNA synthases, and other genes thought to be encoded by cellular organisms. Along with genes for sugar, lipid and amino acid metabolism, there are additional metabolic genes which are absent in any other viruses. Approximately 90% genome has coding capacity, leaving behind junk DNA with 10%.

The pretty amazing feature of Pandoravirus salinus open reading frames (ORF) encode very new proteins. Among 2556 putative proteins producted by Pandoaraviral genome, 93% is very new. Like the members of megaviridae (including mimivirus) they encode DNA polymerase, DNA-dependent RNA polymerase and several aminoacyl t-RNA synthases. The intervening sequences that are present in coding regions, are removed by RNA splicing.

Replication strategy:

Least known about mimiviruses’ replication strategy, though it is understood that the virus attaches to the chemical receptor present on the surface of the amoeba and is taken into the cell. The eclipse phase begins where the virus disappears, and after 8hours of infection virions are clearly visible within the cell. After around 24hours, cells burst open to release new mimivirus virions.

Comparatively, Pandoraviruses have atypical replication cycle. The virion is taken inside amoeba by phagocytic vacuoles, and the viral contents is released into the cytoplasm leaking through a pore on the virion apex. It takes 2-4hours to recognize the cell’s nucleus, and 8-10hours to identify new particles to appear. Finally by 10-15hours new virions release by bursting out of the infected cell.


Mimivirus would be a causative agent of some forms of pneumonia, since there are some idirect evidences in the form of antibodies to the virus discovered in pneumonia patients. In comparison, the effects of Pandoravirus is still unknown.


Coming to the conclusion of the total aspect of giant viruses, both of these virus is still new, although some of the genetic studies have been performed and there are least number of paper published based on their pathogenic properties of both of these viruses. Still, it is understandable although both are giant DNA viruses but have classic differences between them.

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About Saumyadip

Science Communicator and Biologist. Keep interests in host-pathogen interaction research. Specifically bacterial infection mechanism, host infection evasion and immune susceptibility of host. PhD student at Academia Sinica Molecular and Cell Biology, Taiwan
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One Response to Monster Viruses – Mimivirus and Pandoravirus (Comparative Understanding)

  1. Varun C N says:

    The 2 viruses (or rather megavirus), named as Mimi virus and the more recent Pandora virus have huge genomes. The genome is more than what u had expect for small bacteria. Both discovered to infect Amoeba. But Amoeba too have huge genomes…. To me it sounds the “red queen hypothesis”. One more point… By analysis, more than 3/4th of genome codes for proteins, for which we have no data match, and nobody knows what it is. There is a very good discussion in TWiV podcast. Link-

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