Carl Woese's Three Domain System

Carl Woese, along with George Fox, proposed a system of classifying life into three domains: Bacteria, Archaea, and Eukarya. This system was a significant change from the previous five-kingdom system and was based on molecular evidence, rather than just morphological characteristics.

Basis of Domain Classification

The primary basis for Carl Woese's three domain classification was the analysis of ribosomal RNA (rRNA) sequences. Ribosomal RNA is a key component of ribosomes, the cellular machinery responsible for protein synthesis. Specifically, Woese and Fox studied the sequences of 16S rRNA in prokaryotes and 18S rRNA in eukaryotes.

Why was rRNA used?

• rRNA is found in all known forms of life, making it a universal molecule for comparison.
• rRNA genes evolve relatively slowly, meaning that the sequences can be used to determine relationships between even distantly related organisms.
• The function of rRNA is conserved across different organisms, which implies that similarities and differences in sequence reflect evolutionary history.

By comparing the differences in the rRNA sequences, Woese and his colleagues were able to establish a phylogenetic tree that showed three distinct lineages, which they named domains:

• Bacteria: This domain includes most of the prokaryotes traditionally known as bacteria. They have peptidoglycan in their cell walls (though some exceptions exist) and their rRNA sequences are distinct.
• Archaea: This domain includes prokaryotes that often live in extreme environments (extremophiles), although many also live in common environments. They lack peptidoglycan in their cell walls and have unique membrane lipids and rRNA sequences, which are actually more similar to eukaryotes than to bacteria in certain aspects.
• Eukarya: This domain includes all organisms whose cells contain a nucleus and other membrane-bound organelles, such as protists, fungi, plants, and animals. Their 18S rRNA sequences are characteristic of this group.

While protein sequences are also used in phylogenetic studies today, ribosomal RNA sequencing was the groundbreaking method used by Carl Woese to establish the three domain system, demonstrating fundamental molecular differences between Bacteria, Archaea, and Eukarya. The question asks about the basis of the classification proposed by Carl Woese, and his initial work focused heavily on rRNA, though subsequent support also came from protein studies.

Analyzing Other Options

• mitochondrial DNA and membrane structures: Mitochondrial DNA is primarily found in eukaryotes and endosymbiotic bacteria, and membrane structures are too variable and not the sole basis for this high-level classification.
• mitochondrial DNA and protein sequences: Again, mitochondrial DNA is limited to eukaryotes and related bacteria. Protein sequences are used for classification but rRNA was the primary basis for Woese's initial proposal of the three domains.
• presence of single or double membranes: Membrane presence and structure are important cellular features, but they do not define the three distinct domains as clearly or fundamentally as the molecular differences seen in rRNA sequences. Both bacteria and archaea are prokaryotes lacking internal membrane-bound organelles (though some exceptions exist), and both are distinct from eukaryotes which have a nucleus and other organelles with membranes.

Therefore, the classification proposed by Carl Woese is primarily based on the differences observed in ribosomal rRNA sequences, supported by other molecular data including some protein sequences.