3.2 Genome size

Given that eukaryotes are more complex, it is no question that eukaryotic genomes are greater in size than those of prokaryotes. While true, eukaryotic organisms can also have small- to large-sized genomes; the smallest eukaryotic genome can be around 10 Mb (megabase pairs) or less, while the largest can reach to approximately 100,000 Mb or longer. As touched upon in the earlier segments of the course, the length of the genome can be tied to its complexity, up to a certain extent. The simplest eukaryotes such as fungi have the smallest genomes, while more complex organisms such as vertebrates and flowering plants have larger genomes 1. Over time, genome size in eukaryotes has continued to increase. One study has shown that the tempo of genome-size evolution is directly correlated to the genome size of eukaryotic organisms 2. Some of the mechanisms that are mentioned below contribute in part to the change in genome size of both prokaryotes and eukaryotes.

Ranges of genome sizes across varying taxonomic groups (in base pairs).

Despite having mentioned that eukaryotic genomes are generally much larger than prokaryotic genomes, there can still be some overlap between the genome sizes for the largest prokaryotes and the smallest eukaryotes. However, as a whole, prokaryotic genomes are typically smaller. To illustrate, the genome of Escherichia coli K12 is found to be 4,639 kb (kilobase pairs) in length, which is around two-fifths of that of the yeast genome, containing around 4,405 genes.

  1. Genome Anatomies – Genomes – NCBI Bookshelf, (n.d.). https://www.ncbi.nlm.nih.gov/books/NBK21120/  (accessed September 22, 2022).
  2. M.J. Oliver, D. Petrov, D. Ackerly, P. Falkowski, O.M. Schofield, The mode and tempo of genome size evolution in eukaryotes, Genome Res. 17 (2007) 594. https://doi.org/10.1101/GR.6096207
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