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Both the number of base pairs and the number of genes vary widely from one species to another, and there is little connection between the two (an observation known as the C-value paradox ). At present, the highest known number of genes is around 60,000, for the protozoan causing trichomoniasis (see List of sequenced eukaryotic genomes ), almost three times as many as in the human genome .
Note that a genome does not capture the genetic diversity or the genetic polymorphism of a species. For example, the human genome sequence in principle could be determined from just half the information on the DNA of one cell from one individual. To learn what variations in genetic information underlie particular traits or diseases requires comparisons across individuals. This point explains the common usage of "genome" (which parallels a common usage of "gene") to refer not to the information in any particular DNA sequence, but to a whole family of sequences that share a biological context.
Although this concept may seem counter intuitive, it is the same concept that says there is no particular shape that is the shape of a cheetah . Cheetahs vary, and so do the sequences of their genomes. Yet both the individual animals and their sequences share commonalities, so one can learn something about cheetahs and "cheetah-ness" from a single example of either.
| Organism | Genome size ( base pairs ) | Note |
| Virus , Bacteriophage MS2 | 3,569 | First sequenced RNA-genome [3] |
| Virus , SV40 | 5,224 | [4] |
| Virus , Phage Φ-X174; | 5,386 | First sequenced DNA-genome [5] |
| Virus , Phage λ | 48,502 | |
| Bacterium , Haemophilus influenzae | 1,830,000 | First genome of living organism, July 1995 [6] |
| Bacterium , Carsonella ruddii | 160,000 | Smallest non-viral genome. [7] |
| Bacterium , Buchnera aphidicola | 600,000 | |
| Bacterium , Wigglesworthia glossinidia | 700,000 | |
| Bacterium , Escherichia coli | 4,600,000 | [8] |
| Amoeba , Amoeba dubia | 670,000,000,000 | Largest known genome. [9] |
| Plant , Arabidopsis thaliana | 157,000,000 | First plant genome sequenced, Dec 2000. [10] |
| Plant , Genlisea margaretae | 63,400,000 | Smallest recorded flowering plant genome, 2006. [10] |
| Plant , Fritillaria assyrica | 130,000,000,000 | |
| Plant , Populus trichocarpa | 480,000,000 | First tree genome, Sept 2006 |
| moss , Physcomitrella patens | 480,000,000 | First genome of a bryophyte , January 2008 [11] |
| Yeast , Saccharomyces cerevisiae | 12,100,000 | [12] |
| Fungus , Aspergillus nidulans | 30,000,000 | |
| Nematode , Caenorhabditis elegans | 98,000,000 | First multicellular animal genome, December 1998 [13] |
| Insect , Drosophila melanogaster aka Fruit Fly | 130,000,000 | [14] |
| Insect , Bombyx mori aka Silk Moth | 530,000,000 | |
| Insect , Apis mellifera aka Honey Bee | 1,770,000,000 | |
| Fish , Tetraodon nigroviridis , type of Puffer fish | 385,000,000 | Smallest vertebrate genome known |
| Mammal , Homo sapiens | 3,200,000,000 | |
| Fish , Protopterus aethiopicus aka Marbled lungfish | 130,000,000,000 | Largest vertebrate genome known |
All organisms store genetic information in the same molecules - DNA or RNA. Written in the genetic code of these molecules is compelling evidence of the shared ancestry of all living things. Evolution of higher life forms requires the development of new genes to support different body plans and types of nutrition. Even so, complex organisms retain many genes that govern core metabolic functions carried over from their primitive past.
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