The big idea: All organisms are classified into three domains: Bacteria, Archaea and Eukarya. The evidence came from ribosomal RNA.
The points to remember
- Before 1977, life was split into prokaryotes (no nucleus) and eukaryotes.
- In 1977 Carl Woese and George Fox compared the base sequences of rRNA from many organisms.
- The prokaryotes fell into two very different groups: Bacteria and Archaea.
- Archaea turned out to be more closely related to eukaryotes than to bacteria.
- The domain became a new level above the kingdoms: a revolutionary reclassification.
Remember it as: Bacteria, Archaea, Eukarya: B-A-E.
Real example: methane makers: Woese's first archaea were methanogens: microbes that make methane, living in places such as marshes and the stomachs of cattle. They looked like bacteria, but their rRNA was very different.
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The big idea: rRNA works across all of life because every organism has ribosomes, and its base sequence changes very slowly.
The points to remember
- Every living cell makes ribosomes, so every organism has rRNA to compare.
- The rRNA base sequence changes very slowly, so even very distant groups can be lined up.
- Differences in rRNA show how long ago groups split, so they show the deepest branches of life.
- Archaea have no nucleus, like bacteria, but share some features of eukaryotes.
- Many archaea live in extreme places such as hot springs and salt lakes; many live in soil and oceans too.
Remember it as: Every cell has ribosomes; rRNA changes slowly.
Watch the wording:
Not 'DNA' in general: The evidence for the three domains is the base sequence of ribosomal RNA, not just 'DNA' or how cells look.
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How it is tested: Paper 1A: which evidence established the three domains, or which cladogram shows them. Learn the shape: Bacteria branch first; Archaea and Eukarya are sisters.
IB-style questionIdentify[1 mark]
Which evidence was used to classify all organisms into three domains? A cell wall structure; B the presence of a nucleus; C base sequences of ribosomal RNA; D the type of nutrition.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
Reading the trees: In a three-domain cladogram, the node joining Archaea and Eukarya is more recent than the one joining them to Bacteria.