The Three-Domain System Revision Notes | GCSE OCR Foundation Biology

What this lesson covers

Explore the modern three-domain classification system developed by Carl Woese, covering Archaea (primitive bacteria in extreme environments), Bacteria (...

• Explain how advances in microscopy and biochemical analysis led to new classification models
• Describe the three domains in Carl Woese's classification system
• Compare the characteristics of Archaea, Bacteria, and Eukaryota
• Evaluate why the three-domain system replaced earlier classification methods

Key ideas to keep in view

Use this lesson to stay inside B5.2 Natural selection and evolution while connecting the detail back to the wider B5: Genes, inheritance and selection topic.

• B5: Genes, inheritance and selection
• B5.2 Natural selection and evolution
• GCSE OCR Foundation Biology

Lesson notes preview

Read the core explanations from The Three-Domain System before testing yourself from memory.

• For centuries, scientists grouped living things into just two kingdoms: plants and animals. This seemed logical - plants don't move, animals do. But as microscopes improved and scientists could peer deeper into cells, this simple system started falling apart.
• The breakthrough came when scientists could analyse the actual chemicals inside cells - their DNA, proteins, and metabolic processes. What they found was shocking: some 'bacteria' were as different from other bacteria as they were from humans. This evidence demanded a completely new way of thinking about life on Earth.
• This is where it gets interesting - the story of how one scientist's radical idea revolutionised biology forever.
• In the 1970s, American microbiologist Carl Woese was studying something most scientists ignored: the RNA inside ribosomes. Ribosomes are the protein-making factories found in all cells, and Woese had a hunch that their RNA sequences could reveal evolutionary relationships.
• What he discovered changed everything. Some organisms that looked like bacteria under the microscope had ribosomal RNA completely different from true bacteria. These weren't just different species - they were as fundamentally different as bacteria are from humans.
• A classification system developed by Carl Woese that divides all life into three domains based on cellular structure and biochemical analysis: Archaea, Bacteria, and Eukaryota.
• Woese's evidence came from chemical analysis, not just looking at organisms. This molecular approach revealed that life has three fundamentally different branches, not two. Let's examine each domain and see what makes them unique.
• Single-celled organisms without a nucleus (prokaryotic) that are genetically and biochemically distinct from bacteria. Often called 'primitive bacteria' because they can survive in extreme environments similar to early Earth conditions.
• Archaea look like bacteria under a microscope - they're single cells without a nucleus. But their chemistry tells a different story. They have unique cell wall compositions, different membrane lipids, and RNA sequences unlike any other life form. Many live in extreme environments: boiling hot springs, highly acidic...
• Single-celled organisms without a nucleus (prokaryotic) that represent 'true bacteria'. They have distinctive cell walls containing peptidoglycan and are found in virtually every environment on Earth.

Continue through the topic

Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.