Which organisms have the largest and smallest genomes?
Matic BrozComputational chemist
Genome size spans at least 3.2 million-fold across cellular life. The largest measured nuclear genome contains 160.45 billion base pairs. The smallest known non-organellar bacterial genome contains 50,141 base pairs, but belongs to a symbiont that cannot live independently.
Those endpoints use different kinds of evidence. Giant eukaryotic genomes are often measured as haploid nuclear DNA content, while familiar human, plant, animal, fungal, and bacterial figures often describe the span of a particular genome assembly.
Which organism has the largest genome?
The largest genome measured with modern methods belongs to the fork fern Tmesipteris oblanceolata, at 160.45 billion base pairs per haploid nucleus.[1]
Researchers measured the amount of DNA in stained nuclei by flow cytometry. The result is a C-value measurement, not a 160-billion-base-pair sequence assembly. Genomes this large contain extensive repetitive DNA and remain difficult to assemble end to end.
The previous record holder, the Japanese canopy plant Paris japonica, has 152.23 picograms of DNA per haploid nucleus. Using 1 picogram as 0.978 billion base pairs, that is about 148.88 billion base pairs.[3][11]
The largest measured animal genome belongs to the marbled lungfish, Protopterus aethiopicus. Its 132.83-picogram C-value corresponds to about 129.9 billion base pairs, roughly 42 times the span of the current human reference assembly.[4][11]
Which organism has the smallest genome?
The smallest known non-organellar bacterial genome is the 50,141-base-pair genome of Vidania strain VFSACSP1, an obligate symbiont of a planthopper in the genus Saccharosydne.[2]
A 2026 study reported 131 complete Sulcia and Vidania genomes. Its two smallest Vidania genomes were 50,141 and 52,460 base pairs and encoded 68 and 62 identified protein-coding genes, respectively.[2] These bacteria live inside specialized host cells and depend on the insect and its other microbes for functions that autonomous cells perform themselves.
Searches for the “shortest genome” usually mean the genome with the fewest base pairs. Viruses and organelles need separate comparisons because they are not cellular organisms. The 50,141-base-pair record excludes both, but it does not represent an independently living bacterium.
Among eukaryotes, the microsporidian parasite Encephalitozoon intestinalis has a 2,216,898-base-pair reference nuclear assembly.[5][7] Among well-characterized free-living bacteria, Pelagibacter ubique has a 1,308,759-base-pair genome.[8]
How large are genomes across species?
Selected reference assemblies range from 580,076 base pairs in Mycoplasma genitalium to 3,099,441,038 base pairs in humans, a 5,343-fold difference.
| Organism | Group | Reference assembly span |
|---|---|---|
| Human | Animal | 3,099,441,038 bp |
| Maize | Plant | 2,182,075,994 bp |
| Rice | Plant | 385,710,679 bp |
| Fruit fly | Animal | 143,706,478 bp |
| Baker's yeast | Fungus | 12,071,326 bp |
| E. coli | Bacterium | 4,641,652 bp |
| M. genitalium | Bacterium | 580,076 bp |
These are NCBI reference-assembly total sequence lengths, including unplaced and unlocalized nuclear sequences where present.[5] They are comparable assembly statistics, not C-value estimates.
Download the 27-row genome size dataset (CSV). It covers representative animals, plants, fungi, protists, and bacteria. Each row records whether the value is a measured haploid nuclear DNA amount, an NCBI reference-assembly span, or a complete genome sequence reported in a paper.
The dataset also includes assembly accessions, release dates, source links, and calculation notes. It can support cross-species genome statistics, but it should not be treated as a complete census of sequenced species.
Which genome size databases should you use?
The best genome size database depends on whether the target is a measured C-value or an assembled sequence.
The Animal Genome Size Database is the main searchable source for animal C-values and currently covers 6,534 species and subspecies.[9] Its records can differ because studies used different samples, standards, and measurement methods. The database asks users to inspect individual records rather than treating its summary statistics as uniformly precise.
The Plant DNA C-values Database from Royal Botanic Gardens, Kew covers measured nuclear DNA content for 12,273 plant and algal species in release 7.1.[10] Both C-value databases are broader than the 27-row downloadable comparison here.
For bacterial genome size database searches, NCBI Datasets is the more useful source when the question concerns assemblies. NCBI reports the total sequence length, ungapped length, contig count, assembly level, and accession for each deposited genome.[5] GenBank also publishes expected species ranges based on ungapped assemblies to flag submissions that are unusually large or small.[6]
What is the difference between genome size and assembly size?
Genome size is the amount of DNA in one complete haploid nuclear genome, while assembly size is the span reconstructed and submitted for a particular sample.
C-values are often measured in picograms. A commonly used conversion is 1 picogram of double-stranded DNA to 978 million base pairs.[11] Measurements can include repetitive regions that a sequence assembly collapses or fails to recover.
NCBI's total sequence length is the length of the submitted nuclear assembly, including unplaced and unlocalized sequences and assembly gaps. Its ungapped length excludes stretches of 10 or more unknown bases.[5] Organellar genomes are reported separately.
This distinction explains why human genome statistics often cite roughly 3.2 billion base pairs, while the GRCh38.p14 RefSeq assembly spans 3,099,441,038 base pairs. An assembly is a versioned scientific record, not a direct measurement of all DNA in every member of a species. MUMmer4 can align assembled genomes, but it does not turn an assembly span into a C-value measurement.
Why do genome sizes vary so much?
Genome size varies mainly because lineages gain and lose non-coding and repetitive DNA, undergo whole-genome duplication, or discard genes after becoming dependent on a host.
Large plant genomes accumulate transposable elements and other repeats, and polyploid plants may carry several sets of chromosomes. This is why a small fern can contain far more DNA than a human without having proportionally more genes.[1][3] The amount of non-coding DNA matters more to these size records than protein-coding gene count.
At the opposite extreme, obligate symbionts lose genes for metabolism and cellular maintenance because the host environment supplies or complements those functions. The smallest Vidania genomes retain little beyond core information-processing genes and one amino-acid biosynthesis pathway.[2] Free-living cells need a broader set of functions, which is why their genomes do not approach the 50-kilobase record.
Genome size also correlates with some cellular traits, including cell size and division rate, but it does not measure biological complexity. Cross-species cell-size records, human gene spans, and proteome sizes each answer a different question.
Sources▼
- A 160 Gbp fork fern genome shatters size record for eukaryotes iScience · 2024. https://www.cell.com/iscience/fulltext/S2589-0042(24)01111-8
- Convergent extreme reductive evolution in ancient planthopper symbioses Nature Communications · 2026. https://www.nature.com/articles/s41467-026-69238-x
- The largest eukaryotic genome of them all? Botanical Journal of the Linnean Society · 2010. https://doi.org/10.1111/j.1095-8339.2010.01072.x
- DNA content, ribosomal gene multiplicity, and cell size in fish Journal of Experimental Zoology · 1971. https://doi.org/10.1002/jez.1401770108
- Genome assembly report NCBI Datasets · August 10, 2026. https://www.ncbi.nlm.nih.gov/datasets/docs/v2/reference-docs/data-reports/genome-assembly/
- Genome Size Check NCBI GenBank · August 10, 2026. https://www.ncbi.nlm.nih.gov/genbank/genome-size-check/
- The complete sequence of the smallest known nuclear genome from the microsporidian Encephalitozoon intestinalis Nature Communications · 2010. https://doi.org/10.1038/ncomms1082
- Genome streamlining in a cosmopolitan oceanic bacterium Science · 2005. https://doi.org/10.1126/science.1114057
- Animal Genome Size Database Gregory TR · August 10, 2026. https://www.genomesize.com/
- Plant DNA C-values Database, release 7.1 Royal Botanic Gardens, Kew · August 10, 2026. https://cvalues.science.kew.org/releases
- Nuclear DNA content and genome size of trout and human Cytometry Part A · 2003. https://doi.org/10.1002/cyto.a.10022
Founder and computational chemist, ProteinIQ
Dr. Matic Broz is the founder of ProteinIQ and a computational chemist. He completed a PhD focused on protein structure, molecular dynamics, and neural networks, and writes about structural biology and scientific software.