| Base pairs | Genes | Notes | |
|---|---|---|---|
| Phi-X 174 | 5,386 | 10 | virus of E. coli |
| Human mitochondrion | 16,569 | 37 | |
| Epstein-Barr virus (EBV) | 172,282 | 80 | causes mononucleosis |
| Mycoplasma genitalium | 580,073 | 467 | two of the smallest true organisms |
| Mycoplasma pneumoniae | 816,394 | 679 | |
| Chlamydia trachomatis | 1,042,519 | 938 | most common sexually-transmitted disease (STD) bacterium in the U.S. |
| Rickettsia prowazekii | 1,111,523 | 867 | bacterium that causes epidemic typhus |
| Treponema pallidum | 1,138,011 | 1091 | bacterium that causes syphilis |
| Borrelia burgdorferi | 1.4 x 106 | >1,283 | bacterium that causes Lyme disease [note] |
| Aquifex aeolicus | 1,551,335 | 1,522 | bacterium isolated from a hot spring in Yellowstone National Park |
| Campylobacter jejuni | 1.64 x 106 | frequent cause of food poisoning | |
| Helicobacter pylori | 1,667,867 | 1,590 | chief cause of stomach ulcers (not stress and diet) |
| Methanococcus jannaschii | 1,664,970 | 1,738 | These unicellular prokaryotes look like typical bacteria but their genes are so different from those of either bacteria or eukaryotes that they are classified in a third kingdom: Archaea. |
| Pyrococcus horikoshii | 1,738,505 | 2022 | |
| Methanobacterium thermoautotrophicum | 1,751,377 | 1,869 | |
| Hemophilus influenzae | 1,830,138 | 1,743 | bacterium that causes middle ear infections |
| Thermotoga maritima | 1,860,725 | 1,877 | marine bacterium |
| Archaeoglobus fulgidus | 2,178,400 | 2,436 | another member of the Archaea |
| Synechocystis | 3,573,470 | 3,169 | a marine prokaryote, one of the cyanobacteria ("blue-green algae") |
| Mycobacterium tuberculosis | 4,411,529 | 3,922 | causes tuberculosis |
| Bacillus subtilis | 4,214,814 | 4,100 | another prokaryote |
| E. coli | 4,639,221 | 4,377 | 4290 of these genes encode proteins; the rest RNAs |
| Saccharomyces cerevisiae | 12 x 106 | 5,885 | Budding yeast. A eukaryote. Includes 275 tRNA and 140 rRNA genes. |
| Caenorhabditis elegans | 95.5 x 106 | 19,820 | The first multicellular eukaryote to be sequenced. |
| The genomes above have been completely sequenced. Those below have not so their figures are estimates. | |||
| Arabidopsis thaliana | 1.2 x 108 | ~20,000 | flowering plant (angiosperm) |
| Drosophila melanogaster | 1.65 x 108 | ~12,000 | the "fruit fly" |
| Humans and other mammals | 3.3 x 109 | ~70,000 | |
| Wheat | 16 x 109 | ~30,000 | |
| Some amphibians | 9 x 109 | ? | |
| Psilotum nudum | 2.5 x 1011 | ? | Note |
Note: The gene total for Borrelia burgdorferi is based on 853 genes on its single chromosome (of 910,724 base pairs) plus 430 genes on 11 of the 17 plasmids it contains.
Arabidopsis thaliana is a plant (in the mustard family) that has the smallest genome known in the plant kingdom and for this reason has become a favorite of plant molecular biologists. Even though Psilotum nudum (sometimes called the "whisk fern") is a far simpler plant than Arabidopsis (it has no true leaves, flowers, or fruit), it has 3000 times as much DNA. No one knows why, but 80% or more of it is repetitive DNA containing no genetic information. This is also the case for some amphibians, which contain 30 times as much DNA as we do but certainly are not 30 times as complex.
The total amount of DNA in the haploid genome is called its C value. The lack of a consistent relationship between the C value and the complexity of an organism (e.g., amphibians vs. mammals) is called the C value paradox.
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