You guessed it: another post about evolution. This time it’s the third of four (?), the other two being about Endogenous Retroviruses and the Liver Lancet Fluke’s Reproductive Cycle. This post is about endosymbiosis, which is like an extreme version of a symbiotic relationship (which itself deserves like a million posts, like trees and fungus, or ants and fungus, or ants and aphids, clownfish and sea anemones, …nope, fuck it, I’m making another post about that). The specific example of endosymbiosis I want to write about is the one that took place during the origins of eukaryotic cells (the cells that make up all multi-cellular organisms; they contain complex structures enclosed within membranes (the nucleus, for example)).
Mitochondria, the organelles that are described as the ‘power-houses’ of our cells, have their own distinct DNA. Yeah, that’s right, their DNA isn’t contained in the nucleus. Furthermore, their DNA doesn’t go through genetic recombination, the stuff our DNA does when the DNA from the sperm and the ovum (egg) exchange information. Further-furthermore, we only inherit mitochondrial DNA from our mothers, because when a sperm fuses with an ovum, it will only insert its DNA, centriole and flagellum, leaving behind everything else, including the mitochondria.
[Aside: I just wanted to point out one cool thing about the fact that we only inherit our DNA from our mothers. This fact is used in human evolutionary genetics, where the rate of random mutational change in mitochondrial DNA can be used to date the most recent common ancestor of humans, i.e. the woman from which all humans came from (in so many words).]
How, then, could this have happened: why is it that our cells have ‘alien’ DNA in them? The theory is that eukaryotes evolved from archaea that formed a symbiotic relationship with prokaryotic cells (mitochondria, or whatever its ancestor was) and eventually enveloped them. Like, swallowed ’em and incorporated them into their BODIES (and by bodies I mean simply their cell’s membrane…) In so many words.
I mean…when you look at it, it seems obvious:
- New mitochondria are formed only through a process similar to binary fission (which is what bacteria use to reproduce/divide).
- They are surrounded by two or more membranes (the outer one being from when the one cell ingested the mitochondria), and the innermost of these shows differences in composition from the other membranes of the cell. They are composed of a peptidoglycan cell wall characteristic of a bacterial cell.
- Mitochondria contain DNA that is different from that of the cell nucleus and that is similar to that of bacteria (in being circular in shape and in its size).
- DNA sequence analysis and phylogenetic estimates suggest that nuclear DNA contains genes that probably came from plastids (Archaea DNA)
- The mitochondrial ribosomes are like those found in bacteria (70S).
- Proteins of mitochondrial origin, like those of bacteria, use N-formylmethionine as the initiating amino acid.
- Mitochondria have several enzymes and transport systems similar to those of bacteria.
- Mitochondria are similar in size to bacteria.
If this doesn’t fill you with awe…I…I just don’t know what would.
Oh, and btw, plants did the same thing again with chloroplasts (that is, …they were engulfed and then incorporated permanently).