Three Domains of Life Classification

Phylogeny refers to the evolutionary relationships between organisms. The Three Domain System, proposed by Woese and others, is an evolutionary model of phylogeny based on differences in the sequences of nucleotides in the cell’s ribosomal RNAs (rRNA), as well as the cell’s membrane lipid structure and its sensitivity to antibiotics. Comparing rRNA structure is especially useful in understanding phylogeny, because rRNA molecules throughout nature carry out the same function, their structure changes very little over time. Therefore similarities and dissimilarities in rRNA nucleotide sequences are a good indication of how related or unrelated different cells and organisms are.

It is generally thought that all cells came from common ancestor cell named as last universal common ancestor(LUCA). It is believed that these LUCA evolved into three different cell types each representing a domain namely archaea, bacteria and eukarya.

Three microbiologists (Woose, Kandler and Wheelis) introduced new classification scheme based on the above domains during 1990. They proposed that all living organisms be divided into three major groups called domains, the eucarya(contains all eucaryotes), bacteria(contain most familiar prokaryotes) and the archaea(orginally called as archaebacteria contains prokaryotes that live in extreme environments).

Each Domain is mapped to one of more kingdoms of Five Kingdom classification. Kingdom Monera is made obsolete because domains bacteria and archaea split the kingdom into two domains. Both these domains represents earliest organisms whereas domain eucarya contains organisms from protista, fungi, plantia and animalia kingdom which developed much later.

Organisms in bacteria(also known as eubacteria or “true bacteria”) domain are the good bacteria which live in stomach, skin and yogurts whereas organisms in archaea domains live in extreme environments. The cells of bacteria unlike archae and eukarya contain peptidoglycan.

The Archaea have membranes composed of branched hydrocarbon chains. Bacteria and Eukarya, have membranes composed of unbranched fatty acid chains attached to glycerol by ether linkages or ester linkages.

Archaea are not sensitive to some antibiotics that affect the Bacteria, but are sensitive to some antibiotics that affect the Eukarya. Bacteria are sensitive to traditional antibacterial antibiotics but are resistant to most antibiotics that affect Eukarya. Eukarya are resistant to traditional antibacterial antibiotics but are sensitive to most antibiotics that affect eukaryotic cells.

Archaea contain rRNA that is unique to the Archaea as indicated by the presence molecular regions distinctly different from the rRNA of Bacteria and Eukarya.

Archaea are Bacteria are prokaryotic cells whereas eukarya are eukaryotic cells.

todo: DRAW 3 SET DIAGRAM AND EXPLAIN IN VIDEO. WHAT IS COMMON AND WHAT IS DIFFERENCE.