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Endosymbiotic Theory
Recall our unit on cell transport? Remember endocytosis? It may sound strange to return to this information
while discussing genetics, but it will make sense in a minute!
But first! Draw a diagram of endocytosis. You may need several people to recall what this looked like.
Next, get online and go to: http://www.youtube.com/watch?v=pvOz4V699gk
Did you watch it? Good, now watch it again! And be sure to look at all the interesting details!
Now, look at these diagrams of a chloroplast and a mitochondrion.
It is time for you to determine what is meant by the “endosymbiotic theory”
Break it down into its components to find out what it means
Endo –
Symbiosis –
How could the organelles above be related to this theory of endosymbiosis?
Draw a cell that has taken a bacterium into a vesicle. How many membranes does it have?
Hopefully you have made the connection between organelles like the mitochondria and chloroplast and
endocytosis! It is believed that the origin of these organelles is a story of endosymbiosis. That a single-celled
organism was taken into another larger celled organism. Instead of digesting that organism, though, it retained
it without harming it, perhaps because of some benefit both organisms could obtain from the relationship.
Evidence for endosymbiotic theory
Compare prokaryote DNA to human mtDNA. To do this, first go to www.tinyurl.com/genomesizes
How many base pairs are in the bacterial genome? Scan the list of species and provide a range of sizes.
How many base pairs are in the human mtDNA?
How many genes do prokaryotes have? Again, provide a range.
How many genes does the human mtDNA have?
There are obviously far fewer genes in the mtDNA compared to the bacteria. Why is this? Write your answer in
full sentences below…you may have to brainstorm!!
Inheritance and Utility
DNA is passed down from parents to their children. You already know how chromosomes are packaged into
sperm and egg, and are combined to create a full genome in the child. Mitochondrial DNA is separate from
nuclear DNA (that found in the nucleus). Think about where in the cell mitochondria are found.
mtDNA is passed down from a mother to her children. Propose a theory for why only the mother gives mtDNA
to the children. Hint: think about sperm and egg.
Because mtDNA is passed from mother to child and is highly conserved over time, it is often used for
determining relatedness between two or more individuals. There are also many more copies of mtDNA in each
cell than nuclear DNA. So in ancient specimens, nuclear DNA may be too deteriorated to use, but mtDNA should
be much more prevalent.
Example inheritance:
So as long as daughters are being produced and have children, the maternal mtDNA will continue to be passed
on.