There are two ideas on the origin:
Infolding of the plasma membrane
- Part of the plasma membrane infolded until it was inside the cell – now the cell contained a membrane bound organelle
- Would take a very long time
Endosymbiosis
- Mutually benefiting relationship brought about by one prokaryote cell becoming the host of another (one engulfs the other)
- Abrupt change
Mitochondria and chloroplast were once free living Prokaryotes
- Aerobic Prokaryote destined to become mitochondria after being engulfed by another Prokaryote
- Cyanobacteria destined to become chloroplast since it undergoes oxygenic photosynthesis
The model of the origin of Eukaryotes
- Started with an ancestral prokaryote
- The first step was the infolding of the plasma membrane to create the nuclear envelope and the endoplasmic reticulum – this did not make the mitochondria or the chloroplast
- Next, the cell engulfed an aerobic prokaryote which became the mitochondria – if development stopped at this point the cell became an animal cell
- If the cell went on to engulf cyanobacteria which became the chloroplast then it became a plant cell
Evidence for Endosymbiosis
- A billion years ago mitochondria and chloroplast were free living organisms (prokaryotes)
Morphology
o Mitochondria are like bacteria, chloroplasts are similar to cyanobacteria
o What they look like
Formation/Division
o Chloroplast and mitochondria are never made de novo (from nothing) they always come from other chloroplasts or mitochondria – when they replicate they undergo binary fission which is the same way that bacteria replicate
Electron transport
o Chloroplast and mitochondria undergo electron transport similar to bacteria
Possess their own genomes
o Prediction of endosymbiosis, both chloroplast and mitochondria contain single circular chromosomes and lack histones, which is the same as bacteria
o The genes that each contain encode for proteins required for organelle function, just as bacteria contains genes that encode for proteins required for its function
Possess protein synthesizing machinery
Chloroplasts and mitochondria contain ribosomes similar to the ones in bacteria. The protein synthesizing machinery in chloroplast s and mitochondria are sensitive to the same antibiotics that kill bacteria
What Drove The Evolution of Eukaryotes?
- The earliest prokaryotes were anaerobic (no O2)
- After about 2.2 billion year ago cyanobacteria evolved
- Cyanobacteria undergo oxygenic photosynthesis (produce oxygen – changed everything)
- Cyanobacteria evolved two photosystems so that they could use water as an electron donor instead of H2S since water is much more abundant than H2S, which would make them more productive
- This increased the amount of O2 in the atmosphere by a significant amount
- Next evolved prokaryotes that undergo aerobic respiration (O2 is the final electron acceptor) since the [O2] increased in the atmosphere – now huge amounts of ATP can be made
- Large anaerobic organic prokaryotes engulf these smaller aerobic prokaryotes that produce ATP to create eukaryotes
Eukaryotic Cells are Larger
- A eukaryotic cell can have 100’s of mitochondria and therefore a tremendous surface area it can devote to ATP production
- Increased membrane surface area (e.g. lots of mitochondria for ETC)
- This leads to increased energy production and increased size (volume)
- Allowed greater more complex molecules (more genes)
- Increased specialization (why eukaryotes are specialized and prokaryotes aren’t)
- Prokaryotes use a lot of their membrane for ETC so they don’t have room to be specialized
Lateral Gene Transfer
- The mitochondria is the aerobic prokaryotic cell inside the large anaerobic cell
- The nucleus and mitochondria are competing genomes in the cell and for the eukaryotic cell to develop the nucleus genome must be the dominant repository of information to control the processes
- Over millions of years the mitochondria began to send its genes to the nucleus
- The mitochondria still needs these genes to code for its necessary proteins, but instead of making them itself, it transfers control to the nucleus to make these proteins
- The mitochondria copies the gene before it sends it to the nucleus then destroys the copy
- The mitochondrial genome today contains 16500 bp, 13 proteins for electron transport (the rest are in the nucleus) it can undergo transcription and translation and it contains tRNA and rRNA
- Certain mitochondrial genes aren’t in the nucleus because it is important for the mitochondria to synthesize key proteins of the ETC because they need the proteins fast and the proteins break down fast
The Earliest Eukaryotes
- Giardia are diplomands
- They are a very primitive eukaryote and human pathogen
- They contained two haploid nuclei, and lack mitochondria – therefore the is possible to be a eukaryote and not contain mitochondria
- Did Girardia never have mitochondria or did it have them but lost them?
What is Giardia doing with cpn60?
- Cpn60 is a nuclear gene that encodes for a mitochondrial protein
- Giardia contains cpn60 in their nucleus even though they contain no mitochondria, the fact that they contain cpn60 is evidence of lateral gene transfer
- The mitochondria much have transferred cpn60 by lateral gene transfer then giardia lost the mitochondria for reasons unknown
- Other eukaryotes like our own cells contain cpn60 in their nucleus and a mitochondria
Organelle genomes
- The human mitochondrial genome consists of 16500 bp
- The chloroplast DNA consists of 200799 bp
- Therefore the chloroplast tend to be much larger
- Over time the genomes have shrunk due to lateral gene transfer
Secondary Endosymbiosis
- A cyanobacterium was engulfed by other cells
- A secondary endosymbiosis occurred where another cell engulfed the algae
- Therefore the cells have four membranes
- Some genes move around, how?
- A gene originated in the prokaryotes bacterium
- Genes transfer from the plastid to the nucleus
- And are transferred again from the plastid to the nucleus
- The advantages to moving genes to the nucleus are that it allows you to get your genes away from oxygen and DNA is always more stable in the nucleus and can be reproduced in the nucleus
