In 2002, the organism with the smallest known number of genes was Nanoarchaeum equitans,1 a microbe discovered living on the surfaces of another microbe, Igniococcus. Both are members of the Archea domain. They were found in gravel taken from the ocean floor north of Iceland, in an area heated by volcanic activity. N. equitans is extremely small, roughly a sphere 400 nanometers in diameter. As of this writing, the microbe's genome had not been mapped completely, but was estimated at a few hundred genes, well below the size of the previous record holder.
The previous record holder (from 1995) was the bacterium Mycoplasma genitalium, with about 470 genes.2 From this and other studies two researchers3 estimated the smallest possible genome at about 256 genes.
1.
Harald Huber, Michael J. Hohn, Reinhard Rachel, Tanja Fuchs, Verna C. Wimmer and
Karl O. Stetter.
A new phylum of Archea represented by a nanosized hyperthermophilic symbiont.
Nature, volume 417, pages 27-28, 2 May 2002.
A letter to the editor.
2.
Claire M. Fraser, Jeannine D. Gocayne, Owen White, Mark D.
Adams, Rebecca A. Clayton, Robert D. Fleischmann, Carol J. Bult, Anthony
R. Kerlavage, Granger Sutton, Jenny M. Kelley, Janice L. Fritchman, Janice
F. Weidman, Keith V. Small, Mina Sandusky, Joyce Fuhrmann, David Nguyen,
Teresa R. Utterback, Deborah M. Saudek, Cheryl A. Phillips, Joseph M.
Merrick, Jean-Francois Tomb, Brian A. Dougherty, Kenneth F. Bott,
Ping-Chuan Hu, and Thomas S. Lucier.
The minimal gene complement of Mycoplasma genitalium.
Science, volume 270, pages 397-404 (20 October 1995).
3.
Arcady R. Mushegian and Eugene V. Koonin.
A minimal gene set for cellular life derived by comparison of bacterial
genomes.
Proceedings of the National Academy of Sciences, volume 93, no. 19, pages
10268–10273 (September 17 1996).
For the genomes, see www.ncbi.nlm.nih.gov/Complete_Genomes/.
Jack Maniloff.
The minimal cell genome: 'On being the right size.'
Proceedings of the National Academy of Sciences, volume 93, no. 19, pages
10004–10006 (September 17 1996).
Fugu rubripes, a Japanese puffer fish. Although its genome has about 30,000 genes, it is small because it includes very little "junk". See:
www.lbl.gov/Science-Articles/Archive/fugu-decoded.html
www.lbl.gov/Science-Articles/Archive/fugu-facts.html
| Organism | Number of base pairs in millions |
Number of genes |
When first sequenced |
By whom, notes |
|---|---|---|---|---|
| Haemophilus influenzae (bacteria) |
1.8 | 1,740 | 1995 |
Fleischmann R. D., Adams M. D., White O., Clayton R. A., Kirkness E. F., Kerlavage
A. R.,
Bult C. J., Tomb J. F., Dougherty B. A., Merrick J. M., et al. |
| Saccharomyces cerevisa (yeast) |
12.1 | 6,034 | 1996 | |
| Caenorhabditis elegans (roundworm) |
97 | 19,099 | 1998 |
First animal genome to be sequenced. |
| Arabidopsis thaliana (thale cress) |
125 | 25,500 | 2000 | First plant genome to be sequenced.
The Arabidopsis Genome Initiative. Other articles in the same issue treat the sequences of chromosomes 1, 3 and 5. |
| Drosophila melanogaster (fruit fly) |
185 | 13,061 | 2000 | Adams, M. D. et al. The genome sequence of Drosophila melanogaster. Science, vol. 287, page 2185. (24 March 2000) |
| Mus musculus (laboratory mouse) |
3000 | 50,000 | ||
| Homo sapiens (human) |
3120 | 30,000 | 2001 | Human Genome Project and Celera
Venter, J. C. et al. International Human Genome Sequencing Consortium. |
| Agrobacterium tumefaciens (crown gall bacterium) |
5.67 | ? | 2001 | www.agrobacterium.org |
| Oryza sativa (rice) |
389 | 37,544 | 2005 | Second plant genome to be sequenced. |
The National Center for Biotechnology Information maintains a fascinating, though advanced, repository of information at www.ncbi.nlm.nih.gov/Complete_Genomes/.
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Last revised: 15 November 2001.