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How big is a Mimivirus?

750 nanómetros!

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Mimivirus

Mimivirus is a giant virus that infects amoebae, which are single-celled organisms. Including the coat of hair-like fibers that covers it, a Mimivirus particle is about 750 nanometers (0.75 micrometers) across. A nanometer is a billionth of a meter, and a micrometer is a thousand nanometers. Mimivirus is large enough to be seen with an ordinary light microscope, which most viruses are not.

Size

A Mimivirus particle has two main layers. Its inner shell, the capsid, has 20 faces, like a slightly uneven 20-sided die, and is about 500 nanometers across. A dense layer of fibers, each about 125 nanometers long, covers the capsid. The fibers bring the whole particle to about 750 nanometers across.

These sizes come from cryo-electron microscopy, in which particles are frozen quickly and photographed with an electron microscope. Mimivirus's relatives have the same build, a layer of fibers around a capsid about 0.5 micrometers across. A 2014 study describes particles in this family as up to about 0.7 micrometers across.

Inside the capsid, the virus's DNA is packed in a core wrapped in membranes. The genome is about 1.2 million base pairs long, a base pair being one rung of the DNA ladder. It holds most of the same genes found in small bacteria. One corner of the capsid carries a starfish-shaped seal, which may control how the DNA is released into a host.

Size comparisons

  • A Mimivirus particle is about five times as wide as an HIV particle, which averages 145 nanometers across.
  • An E. coli bacterium is about 1 micrometer wide, so a Mimivirus particle is three-quarters as wide as the bacterium.
  • It is about as wide as one wavelength of red light, around 700 nanometers.
  • Ten Mimivirus particles side by side would span a red blood cell, which is about 7.5 micrometers across.
  • If a Mimivirus particle were enlarged to the size of a basketball, 24 centimeters (9.4 inches) across, an HIV particle would be about 4.6 centimeters (1.8 inches) across. A red blood cell would be about 2.4 meters (7.9 feet) across.

A virus mistaken for a bacterium

Mimivirus was first collected in 1992 from a hospital cooling tower in Bradford, England, and was suspected of causing an outbreak of pneumonia there. It infected amoebae called Acanthamoeba polyphaga. Its size, and the way its outer coat took up a stain used to identify bacteria, made it look like a bacterium. It was named Bradfordcoccus.

Didier Raoult and colleagues at Aix-Marseille University in France identified it as a virus in 2003. Raoult first said the name Mimivirus meant "mimicking microbe". He later linked it to "Mimi the amoeba", a character in stories his father made up to explain evolution.

Larger viruses found since

For years after its discovery, Mimivirus was the largest virus known, and a 2009 study still described it that way. Its discovery showed that some viruses are large enough to see under a light microscope. Since then, even larger viruses have been found in amoebae. Pandoraviruses, reported in 2013, have jar-shaped particles about 1 micrometer long.

In 2014, scientists described Pithovirus sibericum, which has particles about 1.5 micrometers long, twice as long as Mimivirus is wide. It was revived from a sample of Siberian permafrost more than 30,000 years old. Its genome, about 600,000 base pairs, is only half as long as Mimivirus's.

Sources

La Escala del Universo > Mimivirus

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