The 3 Domains

Who proposed the Three Domain system?

• Carl Woese.

What are the three Domains?

• Bacteria (Eubacteria)

• Archaea (Archaebacteria)

• Eukarya (Eukaryotes)

What was the primary molecular evidence used to separate Archaea from Bacteria?

• The sequence of nucleotides in Ribosomal RNA (rRNA).

State the key difference in the cell wall composition between Bacteria and Archaea.

• Bacteria: Cell wall contains Peptidoglycan (murein).

• Archaea: Cell wall does NOT contain peptidoglycan.

How do the membrane lipids of Archaea differ from Bacteria and Eukarya? Back:

• Archaea: Have Ether links between glycerol and fatty acids (often branched chains).

• Bacteria/Eukarya: Have Ester links. (Ether bonds are stronger, helping Archaea survive in extreme environments).

Which Kingdoms from the 5-Kingdom system are found in the Domain Eukarya?

• Protoctista, Fungi, Plantae, and Animalia.

Why are Archaea often called extremophiles?

• Because many are found in extreme environments, such as thermal vents (high temp), salt lakes (high salinity), or anaerobic conditions.

In terms of protein synthesis and rRNA structure, which domain are Archaea most similar to?

• Eukarya. (Archaea are more closely related to Eukaryotes than to Bacteria).

Explain why the definition of a domain is considered to be higher in the taxonomic hierarchy than a kingdom. (2 marks)

• Domains are broader groups (1).

• A domain contains one or more kingdoms (e.g., Eukarya contains four kingdoms) (1).

State two differences between the organisms in the domain Bacteria and the domain Eukarya. (2 marks)

• Nucleus: Bacteria do not have a nucleus; Eukaryotes do (1).

• Ribosomes: Bacteria have 70S ribosomes; Eukaryotes have 80S ribosomes (1).

• Organelles: Bacteria lack membrane-bound organelles (like mitochondria); Eukaryotes have them (1).

• Cell Wall: Bacterial walls are always peptidoglycan; Eukaryotic walls (if present) are cellulose or chitin (1). (Any 2)

Scientists originally classified Archaea and Bacteria in the same Kingdom (Prokaryotae). Explain how molecular evidence led to them being separated into different domains. (3 marks)

• Scientists analysed the sequence of Ribosomal RNA (rRNA) (1).

• They found the rRNA sequence of Archaea was different from Bacteria (1).

• Archaea were found to be more closely related to Eukarya (or have unique membrane lipids/no peptidoglycan), justifying a separate group (1).