Three-domain model of classification (Edexcel A-level Biology B)

This lesson describes the evidence that led to the three-domain model of classification as an alternative to the five-kingdom model. The detailed PowerPoint and accompanying resources have been designed to cover point 3.1 (vii) of the Edexcel A-level Biology B specification and focuses on Carl Woese’s detailed study of the ribosomal RNA gene and the need for this evidence to be validated by the scientific community

The lesson begins with an introduction of Woese and goes on to describe how he is most famous for his definition of the Archaea as a new domain of life. Students were introduced to domains and the other classification taxa in a lesson at the start of this topic, so their recall of this knowledge is continually tested and built upon as details are added. Students will discover the key differences between Archaea and Bacteria that led to the splitting of the prokaryotae kingdom and the addition of this higher classification rank and will understand that it wasn’t until 13 years after the discovery that it was adopted. Moving forwards, the rest of the lesson describes how molecular phylogeny uses other molecules that can be compared between species for classification purposes. One of these is a protein called cytochrome which is involved in respiration and can be compared in terms of primary structure to determine relationships. At this point in the lesson, the students are also tested on their knowledge of the nature of the genetic code (as covered in topic 1) and have to explain how mutations to DNA can also be used for comparative purposes.