Eukaryotic cells are believed to have evolved via endosymbiosis – whereby one cell was engulfed by another and became assimilated into its cellular structure

• Current evidence suggests that all eukaryotes evolved from a common unicellular prokaryote that had a nucleus and reproduced sexually

• This prokaryotic ancestor engulfed another aerobic bacterium, which over time lost its independent utility and developed into the mitochondria

• A subsequent internalisation of a photosynthetic cyanobacterium likely contributed to the development of the chloroplast in plant cells

  Endosymbiosis

  

  

  

  

  
  

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  1. Ancestral Prokaryote

  

  2. Nucleus Formation

  

  3. Endosymbiosis

  

  4. Eukaryotic Cell

Evidence for Endosymbiosis

Mitochondria and chloroplasts are both organelles suggested to have arisen via endosymbiosis

• These organelles are therefore expected to share characteristics with bacteria as a consequence of their prokaryotic origins

Evidence that supports the extracellular origins of these organelles include:

• Membrane: They both have a double membrane (the outer membrane may have been vesicular in origin)
  

• Antibiotics: Both organelles display susceptibility to certain antibiotics (antibiotics target prokaryotic features)
  

• DNA: Mitochondria and chloroplast possess their own DNA, which is naked and circular (like in prokaryotes)
  

• Division: Both organelles divide by a process similar to binary fission (the bacterial reproduction method)
  

• Ribosomes: They both have 70S ribosomes, which are also found in prokaryotic cells