Smallest organism

The smallest organisms found on Earth can be determined according to various aspects of organism size; including volume, mass, height, length, or genome size.

Given the incomplete nature of scientific knowledge, it is possible that the smallest organism is undiscovered. Furthermore, there is some debate over the definition of life, and what entities qualify as organisms; consequently the smallest known organism (microorganism) is debatable.

Many biologists consider viruses to be non-living because they lack a cellular structure and cannot metabolize by themselves, requiring a host cell to replicate and synthesize new products. A minority of scientists hold that, because viruses do have genetic material and can employ the metabolism of their host, they can be considered organisms. As well, an emerging idea that is gaining traction among some virologists is the concept of the virocell, in which the actual phenotype of a virus is the infected cell, and the virus particle is simply a reproductive or dispersal stage, much like pollen or a spore.

The smallest viruses in terms of genome size are single-stranded DNA (ssDNA) viruses. Perhaps the most famous is the bacteriophage Phi-X174 with a genome size of 5386 nucleotides. However, some ssDNA viruses can be even smaller. For example, Porcine circovirus type 1 has a genome of only 1759 nucleotides and a capsid diameter of only 17 nm. As a whole, the viral family geminiviridae is only about 30 nm in length. However, the two capsids making up the virus are fused; divided, the capsids would be 15 nm in length. Other environmentally characterized ssDNA viruses such as CRESS DNA viruses as well as others can have genomes that are considerably less that 2,000 nucleotides.

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