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Sizing up genomes: Amoeba is king
Reasons for the size of a genome and the number of genes are not clear
Edward R. Winstead

Biologists have been estimating the size of genomes for decades. In 1971, for example, researchers reported that a species of Italian bat, Miniopterus, has a genome half the size of the human genome. Similarly, the genome of Muntiacus, a species of Asian barking deer, was estimated at about 70 percent of that of humans. As reported this week, the human genome contains about 3 billion chemical units of DNA, or base pairs.

In the animal kingdom, the relationship between genome size and evolutionary status is not clear. One of the largest genomes belongs to a very small creature, Amoeba dubia. This protozoan genome has 670 billion units of DNA, or base pairs. The genome of a cousin, Amoeba proteus, has a mere 290 billion base pairs, making it 100 times larger than the human genome.

A Sample of Species and Genome Size (in base pairs)

Amoeba dubia
Amoeba proteus
Courtesy of Yuuji Tsukii
Bufo bufo
Homo sapiens
Copyright 2001, Mary S. Gibbs, GNN
Muntiacus muntjak vaginalis
Boa constrictor
Rhinolophus ferrumequinum
Plasmodium falciparum
Courtesy of NLM
Human immunodeficiency virus type 1
Copyright 2001, Mary S. Gibbs, GNN
Source: Database of Genome Sizes (Center for Biological Sequence Analysis)

Among the organisms whose genomes are sequenced, genome size does not correlate with the number of genes.

Species Size of genome Number of genes
2.9 billion base pairs 30,000
Fruit fly (Drosophila melanogaster)
120 million base pairs 13,601
Baker's yeast (Saccharomyces cerevisiae)
12 million base pairs 6, 275
Worm (Caenorhabditis elegans)
97 million base pairs 19,000
E. coli
Courtesy of Agricultural Research Service, USDA
4.1 million base pairs 4,800
Arabidopsis (Arabidopsis thaliana)
125 million base pairs 25,000


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