BIOL113 M2 L4

• Before Carl Woese started to use 16S rRNA sequence divergence as a measure of divergence what tool was commonly used to categorise single celled prokaryotes?

  Bergey's Manual of Systematic Bacteriology, a system for classifying bacteria based on their shapes, straining properties, and chemical characteristics. 
• The 16s rRNA was an excellent choice for building the "big tree" of life because ...

  The 16s rRNA and its Eukaryotic equivalent, the 18s rRNA are universally conserved for life as we know it. 

  
  

  The patterns that Woese was able to see in the gels that he used to determine the sequence at the time had recognisable patterns. 

  
  

  The rRNAs are more slowly evolving than protein coding regions. This is because of the interdependent molecular binding of the molecule that makes its 3 dimensional shape.  
• The Bergey's manual classification system oversimplifies complex relationships and is ambiguous
• Carl Woese's approach


  1. Studied the genetic code

  2. Discoveries made by Crick and Watson of the 'frozen accident theory of the genetic code'

  3. Knew that because the genetic code was conserved, there must have been a single recombining organism in the beginning responsible for this

  4. Discovered that RNA subunits are very different between prokaryotes and eukaryotes and believed they could be used as a 'tape measure' for the time of divergence
• Before Woese there were two known domains of life, Bacteria and Eucarya
• 16s rRNA (and the 18s rRNA eukaryotic equivalent)


  Universally conserved across all living organisms, evolves more slowly than other protein encoding regions due to interdependent molecular bonding
• How Woese built the 'big tree' of life

  1. Sequenced 16s rRNA of all kinds of prokaryotic organisms

  2. Discovered bacteria that behaved differently, giving rise to the 3rd domain of life, the archaea

  3. Found the missing linkage between bacteria and eukarya, and could pinpoint the time of divergence of eukaryotes from prokaryotes
• Carl Woese uncovered the third domain of life and was finally able to build the tree of life as we know it
• What was the first archaea discovered by Carl Woese?

  Methanobacterium thermoautotrophicum
• What were the bugs in Bergey's manual ?

  it made assumptions about trait inheritance without knowledge of horizontal gene transfer, leading to inaccuracies. Microorganisms with similar traits were often placed in different groups, early editions classified bacteria as fission fungi, this made it hard to determine relationships between prokaryotes, let alone the relationships between prokaryotes and eukaryotes.
• Why is the 16s rRNA slow evolving ?

  The 16S rRNA evolves slowly primarily due to its conserved structure and function. The molecule's three-dimensional shape relies on interdependent molecular bonding, which is crucial for its role in the ribosome and protein synthesis. Any mutations in the 16S rRNA could disrupt these bonds, leading to alterations in its structure and function, which would likely be harmful to the cell. Therefore, natural selection favors the preservation of the 16S rRNA sequence, resulting in rare mutations or evolution.
• why is is 16s rRNA described as recognisable ?

  The 16S rRNA is described as recognisable because it shows consistent patterns when studied. Carl Woese was able to see these patterns in the gels that he used to sequence the genes . This consistency makes it easy to identify and compare the 16S rRNA sequences from different organisms. The 16S rRNA has regions that stay the same across organisms, making them easy to spot. Even though some parts vary between species, they still follow certain patterns within groups.