Q.Describe the three-domain system of classification of life. How does it differ from earlier classification systems that recognised only two major groups (prokaryotes and eukaryotes)?

[!TLDR] Woese's three-domain system (Bacteria, Archaea, Eukarya) split the old prokaryote group into two genetically distinct domains. [!ANSWER] The three-domain system recognises Bacteria, Archaea and Eukarya as the three most fundamental divisions of life, whereas the older two-group system lumped Bacteria and Archaea together as a single 'prokaryote' category based only on the absence of a true nucleus.

For much of the twentieth century, life was broadly divided into just two groups based on cell structure: prokaryotes, whose cells lack a true, membrane-bound nucleus (such as bacteria), and eukaryotes, whose cells possess one (such as plants, animals, fungi and protists). This two-group system relied primarily on visible cell structure.

In 1977, Carl Woese compared ribosomal RNA sequences — a molecule present in all cellular life, useful for judging deep evolutionary relationships — and discovered that the organisms grouped together as 'prokaryotes' were not, in fact, a single natural group. A distinct set of prokaryote-like organisms, many found in extreme environments such as hot springs and salt lakes (though later shown to be widespread generally), turned out to be genetically as different from ordinary bacteria as bacteria are from eukaryotes. Woese proposed elevating this group to its own domain, the Archaea, giving rise to the three-domain system: Domain Bacteria (true bacteria), Domain Archaea (genetically distinct prokaryote-like organisms), and Domain Eukarya (all organisms with a true nucleus).

The key difference from the older two-group system is therefore this: the two-group (prokaryote/eukaryote) system was based purely on a single visible structural feature (presence or absence of a nucleus), while the three-domain system is based on deeper molecular/genetic evidence, and it recognises that what looked like one uniform 'prokaryote' group under a microscope actually contains two profoundly different evolutionary lineages (Bacteria and Archaea). The three-domain system therefore sits above the kingdom level, representing the most fundamental, most ancient splits in the history of life. [!ANSWER] The three-domain system (Bacteria, Archaea, Eukarya), based on molecular evidence, replaced the older two-group prokaryote/eukaryote system by splitting the single structural 'prokaryote' category into two genetically distinct domains, Bacteria and Archaea.

First describe the old two-group system and its basis (nucleus presence/absence), then explain Woese's molecular evidence and how it led to splitting the prokaryotes into two domains, and finally state the difference explicitly.

Students often describe the three domains correctly but forget to explicitly answer the second part of the question — how it differs from the older two-group system — losing marks for an incomplete comparison.