Illustration of crop of rice, fluit fly and yeast

Measuring up

How does the human genome compare with the genomes of other organisms?

Gene number and genome size by organism
Organism Gene number (protein-coding) Genome size (base pairs)
Loblolly pine 50,200 20.1bn
Human 20,800 3.3bn
Chimpanzee 18,800 3.0bn
Mouse 23,100 3.5bn
Rice (Japonica subspecies) 35,700 374m
Fruit fly 14,000 169m
Yeast (Saccharomyces cerevisiae) 6,700 12m
E. coli (K12) 4,100 4.6m
Influenza virus (A, Hong Kong) 14 13,500 (rounded)
Ebola (Zaire) 7 19,000 (rounded)

All figures from Ensemble, except pine, which is from Genome Biology, and Ebola, from MicrobeWiki.

Now we have whole genomes from a range of organisms, comparing them is a powerful way to investigate what makes each creature distinctive. In the simplest cases, this means counting genes and checking which ones are there. For example, the first non-human vertebrate animal to have its genome sequenced was the tiger pufferfish (Fugu rubripes). Counting and checking shows that it has many genes in common with us. Even animals without backbones, such as fruit flies and nematode worms, share very many genes with humans. However, around a quarter of human genes have no equivalents in the fish, and we have almost ten times as much DNA as they do.

The chimpanzee genome was first published in draft in 2005. Initial comparisons with the human genome – made by simply lining the two genome sequences up together – suggested the chimp genome was 98.8 per cent identical to our own. This seemed to confirm our long-held belief that the chimp is our closest living relative. However, in 2012 the publication of the genome sequence of the bonobo – which, like the chimpanzee, is an African ape – showed that bonobos are just as close a relative to us as chimpanzees are.

For very closely related organisms, like chimps and humans, we can align most of one organism’s genome with that of the other. This reveals obvious differences and allows us to calculate a simple percentage similarity. Such a comparison is less meaningful when it involves two organisms that are less closely related.

But even for our closest relatives, the measure of 98.8 per cent similarity conceals important differences. In addition to the precise sequence of genes, there are some differences in the overall set of genes. Comparing genome sequences also doesn’t give us information on when particular genes are switched on and off during development – especially ones that affect the brain. These subtle differences in gene regulation and gene expression are likely to underlie the most important differences between closely related species.

Lead image:

Illustration © Glen McBeth

References

Further reading

About this resource

This resource was first published in ‘Genes, Genomes and Health’ in January 2010 and reviewed and updated in December 2014.

Topics:
Statistics and maths, Genetics and genomics
Issue:
Genes, Genomes and Health
Education levels:
16–19, Continuing professional development