Comparing Genome Sizes: Skills (DP IB Biology) : Revision Note

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

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