The big idea: Compare genome sizes across groups and a pattern appears, but it is not a simple link with complexity.
Genomes vary enormously in size between species. But — and this surprises many students — a bigger genome does not mean a more complex organism.
The points to remember
- Prokaryotes tend to have smaller genomes than eukaryotes: evidence for a link.
- Much of a eukaryote genome is non-coding, and eukaryote genes contain introns.
- Within eukaryotes there is no relationship with complexity: evidence against.
- Some plants and amphibians have genomes far larger than the human genome.
Real example: plants and amphibians: Some plants and amphibians have genomes larger than a human's, yet are not more complex. Much of the extra DNA does not code for proteins.
| Organism | Genome size | Number of genes |
|---|---|---|
| Bacterium (E. coli) | very small | few thousand |
| Fruit fly | small | ~14 000 |
| Human | large | ~20 000 |
| Some plants & amphibians | huge (bigger than human) | not many more |
Real example: a lungfish and you: The marbled lungfish of African swamps has a genome of about 43 billion base pairs, roughly 14 times the human genome. Most of it is repeated, non-coding DNA.
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The big idea: Genome databases list the genome size of thousands of species, so groups can be compared.
The points to remember
- A genome database (such as the one at the US NCBI) gives each species' genome size.
- Sizes are given in base pairs: Mbp (million) or Gbp (billion).
- Pick species from different groups (bacteria, fungi, plants, animals) and list their sizes.
- Then compare: do the sizes rise with complexity? Look for exceptions.
Complexity is not measured: There is no number for complexity, so say how you judged it and that it is subjective.
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How it is tested: Paper 1B gives a table of genome sizes and asks you to discuss a link with complexity, using the data.
IB-style questionDiscuss[2 marks]
Using the table (baker's yeast 12, thale cress 135, house mouse 2700, bread wheat 17 000 Mbp), discuss whether genome size positively correlates with organism complexity.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
Quote the numbers: Name the organisms and their sizes from the table, and give a point on each side.