A3.2 Domains and rRNA - student reading
Classification uses the information available to group organisms together. In the time of Carl Linneus often this evidence was taken from morphological features. This can align with evolutionary pathways if the right features are used, but DNA sequences or amino acid sequences have improved the evidence available and has supported the development of cladistics. Carl Woese was one of the first to use DNA sequences and revolutionised the tree of life.
Single celled organisms
Single celled organisms have been classification into Eukaryotes (with a membrane bound nucleus) and Prokaryotic Cells (without a nucleus). For a long time the shape of prokaryotes was used to classify them.
Image source: Janice Haney Carr, Dr. Richard Facklam, Centers for Disease Control and Prevention
A prokaryote is a simple, single-celled (unicellular) organism that lacks an organized nucleus or any other membrane-bound organelle. Prokaryotic DNA is found in a central part of the cell: the nucleoid.
With today's DNA sequencing technology it seems like a logical idea to try to use part of this DNA to make comparisons between different species of bacteria. Carl Woese was working in the 1970's thirty years before the human genome project and the development of rapid DNA sequencing techniques. He wanted to find a small gene which was present in all species,to achieve this task.
All prokaryote cells have ribosomes. Ribosomes are made of ribosomal RNA. RNA is a molecule known to exist in the earliest evolution of life on Earth. Could RNA or ribosomes provide a tool to compare all of life?
Ribosome structure
Ribosomes are composed of ribosomal RNA (rRNA) and protein. Prokaryotic cells have three types of rRNA: 16S rRNA, 23S rRNA, and 5S rRNA. The single strands of rRNAs use Hydrogen-bonding between complementary nucleotide bases to form complex folded structures.
Ribosomes are composed of two subunits a large subunit and a small subunit. Called 50S and 30S ("S" refers to a unit of density called the Svedberg unit). The two subunits combine during protein synthesis to form a complete 70S ribosome. The 30S subunit contains 16S rRNA which is 1540 nucleotides long.
Carl Woese decide to explore differences in the gene which codes from this 16S rRNA molecule which is found in the ribosome small-subunit. It was a good choice, as it is just the right length. Long enough to show differences between groups, but short enough to be seqenced using the early technology available.
Woese’s approach was revolutionary because comparisons of physical features are insufficient to differentiate between the prokaryotes that appear fairly similar in observations. The comparison of DNA and RNA sequences provided Woese with a sensitive tool that revealed the extensive variability of prokaryotes.
The RNA, which is transcribed from DNA, varies between organisms. This allowed Woese to suggest a new classification using rRNA base sequences.
The pioneering work of American microbiologist Carl Woese in the early 1970s has shown, however, that life on Earth has evolved along three lineages, now called domains—Bacteria, Archaea, and Eukarya. Woese defined Archaea as a new domain, and this resulted in a new taxonomic tree. Many organisms belonging to the Archaea domain live under extreme conditions and are called extremophiles. To construct his tree, Woese used genetic relationships rather than similarities based on morphology (shape).
Three Domains classification system
Classification of all organisms into three domains used evidence from rRNA base sequences.
This reclassification adding the extra taxonomic level 'domain' above kingdoms was proposed in 1977.
Key points
- Carl Woese defined Archaea as a new domain and constructed the phylogentic tree of life which shows separation of all living organisms.
- The phylogenetic tree of life was constructed by Carl Woese using sequencing data of ribosomal RNA genes. Therefore, genetics classification surpassed morphological cataloguing, which was the traditional way of organizing living beings.
Nature of Science questions
Archaea
- The group Archaea was recognised in 1977 by Carl Woese as a distinct domain different from bacteria.
This division was not immediately accepted by scientists. What are the benefits of this 'conservatism'?
Three Domains classification system
- Consider this quote, about the struggle to get a theory accepted,
"A theory is a species of thinking, and its right to exist [depends on] its power of resisting extinction by its rivals".— Thomas Henry Huxley, Science and Culture, and Other Essays (1890),
Test yourself with this quiz
Which two Biologists that helped us make advances in classification are mentioned in the article?
Thomas Huxley gave us a nice quite about theories, but he didn't work on classification.
Which type of classification group has Archae become?
The three domain system is now widely accepted.
Which of the following are names of shapes of bacteria?
Shapes are Coccus, Bacillus, Vibrio, and Spirillum.
Which gene did Carl Woese sequence to classify the organisms?
The small subunit of ribosomes contains a 16S rRNA molecules, coded for by a DNA gene.
Why did Thomas Huxley suggest that a theory was like a species?
"A theory is a species of thinking, and its right to exist [depends on] its power of resisting extinction by its rivals".
This idea of a new species appearing and causing the extinction of an extablished species in ecology is nicely illustrated by the three domain theory of classification. It has largely replaced the five kingdom system.
This page uses information from the page titled 1.4: Carl Woese and the Phylogenetic Tree is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.