Comparing Genome Sizes
Comparing genome sizes
- Advances in technology have allowed scientists to sequence genomes of many species
- Genome-wide comparisons can now be made between individuals and between species
- Genome sizes can differ in different organisms:
- Viruses and bacteria tend to have very small genomes
- Prokaryotes tend to have smaller genomes than eukaryotes
- The size of plant genomes can vary widely
Comparing the genome size of different organisms table
| Organism | Common name / description | Genome size (million base pairs) |
| Enterobacteria phage T2 | Virus that infects E. coli | 0.17 |
| Escherichia coli | E. coli bacteria | 5 |
| Drosophila melanogaster | Fruit fly | 140 |
| Homo sapiens | Human | 3000 |
| Paris japonica | Japanese canopy plant | 150 000 |
Using a genome size database
- It is possible for anyone to look up the genome size of a wide range of organisms using a genome size database, e.g. the Animal Genome Size Database or the Plant DNA C-Values Database
- Databases present genome sizes using a measure known as a C-value
- The C-value is the haploid nuclear DNA content of an organism
- C-value units can be given in mass: picograms (pg) where 1 pg = 10-12 g
- C-value units can be in number of bases: megabases (Mb) where 1 Mb = 106 bases
- 1 pg = 978 Mb
Genome size and organism complexity
- We might expect there to be a clear relationship between genome size and organism complexity, but there are plenty of examples that do not fit with this pattern, e.g.
- Humans = 3 100 Mb
- Hagfish = 4 200 Mb
- Common wheat = 17 000 Mb
- In some cases similar species may have very different genome sizes
- E.g. common wheat above has a genome size of around 17 000 Mb, while red wild einkorn wheat has a genome size of around 5 000 Mb
- There are several factors that should be considered when thinking about the reason for these unexpected numbers:
- The genome contains all of an organism's DNA, not just the DNA that codes for proteins
- Plants can have polyploidy, meaning that their cells can contain many sets of chromosomes, giving them very large genomes
- Our view of 'complexity' may not be correct; we generally associate complexity with brain function, but there are other ways in which an organism can be complex
