Describe how genetic analysis has led to the suggestion of the three…

The idea

Classification groups organisms by shared features. The five-kingdom method sorts organisms into large groups based mainly on observable features such as cell type and how they obtain food. Genetic analysis, by comparing the DNA (and RNA) base sequences of organisms, allows scientists to measure how closely related they are: the more similar the sequences, the more recently they shared a common ancestor. When scientists compared genetic sequences, some groups that looked similar in the five-kingdom system turned out to be very different genetically. In particular, the prokaryotes were found to fall into two distinct genetic groups, the bacteria and the archaea, alongside the eukaryotes. This led to the suggestion of three domains: Bacteria, Archaea and Eukaryota. Genetic evidence therefore replaced a system based on appearance with one based on evolutionary relationships.

What earns the marks

• States that genetic analysis involves comparing DNA (or RNA) base sequences between organisms.
• Explains that more similar genetic sequences indicate a closer evolutionary relationship and a more recent common ancestor.
• Describes that genetic comparisons showed the prokaryotes fall into two distinct groups, the bacteria and the archaea.
• States that this led to the three-domain system: Bacteria, Archaea and Eukaryota.
• Explains that the three-domain system is based on genetic and evolutionary relationships rather than mainly observable features used in the five-kingdom method.

Common mistakes and how to put them right

• Thinking the three domains are simply a renaming of the five kingdoms.

  Put it right by explaining the domains are based on genetic comparisons and group organisms differently, especially splitting the prokaryotes.

• Believing genetic analysis compares physical appearance.

  Put it right by stating it compares DNA or RNA base sequences.

• Assuming more different DNA means a closer relationship.

  Put it right by explaining that more similar sequences indicate a closer relationship and more recent common ancestor.

Exam tips

• State clearly that genetic analysis compares DNA base sequences to judge how closely organisms are related.
• Name the three domains accurately: Bacteria, Archaea and Eukaryota.
• Explain the key change: genetic evidence split the prokaryotes into two groups, which the five-kingdom method did not do.