What is a smallest organism?

Answer 1

The smallest organism in the world is a virus, although 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 synthesise 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. The smallest RNA viruses in terms of genome size are small retroviruses such as rous sarcoma virus with genomes of 3.5 kb and particle diameters of 80 nm. The smallest DNA viruses are the hepadnaviruses such as Hepatitis B, at 3.2 kb and 42 nm; parvoviruses have smaller capsids, at 18-26 nm, but larger genomes, at 5 kb. The smallest DNA bacteriophage is the Phi-X174 phage, thought to be larger than Hepatitis B, at about 4 kb. It is important to consider other self replicating genetic elements such as satelliviruses, Viroids and Ribozymes. The Porcine circovirus 2 is a single stranded DNA virus having a capsid diameter of 17 nm. In which that case, the smallest living organism in the world would be a nanobe. Nanobes are thought by some to be the smallest known organism, about one tenth the size of the smallest known bacteria.