Intro
Among cell organelles, two of them especially stand out – mitochondria and plastids. Unlike the others, they have an additional membrane, divide independently, and have their own DNA. In this topic, we will discuss why this happened, what is the structure of these organelles and how their DNA is structured.
What are endosymbiosis and secondary endosymbiosis?
Endosymbiosis is the situation when one organism is living inside another and it is not as rare as one might think. An example of endosymbiosis is the bacteria in our gastrointestinal tract: they live in us all our lives, feed on our food and in return (most often) secrete substances that are useful to us. This is endosymbiosis at the species level – bacteria do not cease to be bacteria and do not become human or their organs.
However, in the middle of the last century, humanity approached the example of another endosymbiosis. It was found that mitochondria (energy organelles of the cell) and plastids (photosynthetic organelles of plants) are different from other organelles. Mitochondria and plastids are quite independent: they have their own genome and ribosomes and an extra membrane, they can divide inside cells, and outwardly they differ little from ordinary bacteria.
On this basis, it was hypothesized that mitochondria and plastids are an example of endosymbiosis. Once upon a time, their ancestors were free-living bacteria — proteobacteria that became mitochondria and cyanobacteria that became plastids — but they were swallowed up by larger nucleated cells and left as a workforce. Both mitochondria and plastids provide a lot of energy for the cell, which is why they were "left" to live as organelles.
So, we understand what endosymbiosis is. Secondary endosymbiosis occurs when the engulfing process occurs again – only this time, an organism that has already engulfed another organism is itself engulfed.
Mitochondrion and plastid
Mitochondria and plastids are an example of primary endosymbiosis – once their ancestors were free-living bacteria. Mitochondrial DNA is quite small, and most of the proteins important for the functioning of the organelle are encoded in the nuclear genome. However, mitochondrial DNA encodes several essential proteins of the electron transport chain, as well as some tRNAs (pic. 2). Mitochondrial DNA are circular double-stranded molecules 5-30 µm in size, and each mitochondrion appears to have several copies of such circular molecules.
Because of their own genome, mitochondria are a good way to track family ties and evolutionary changes. The fact is that during sexual reproduction, only maternal mitochondria enter the daughter organism – all sperm mitochondria are destroyed. Mutational changes in mitochondrial DNA can lead to hereditary diseases that are transmitted only through the maternal line.
We can consider plastids an example of secondary endosymbiosis. Once upon a time, there was a heterotrophic organism (that is, an organism that feeds on ready-made products and does not synthesize them itself) that absorbed a cyanobacterium capable of synthesizing sugar with the help of sunlight and carbon dioxide. This is how primary endosymbiosis occurred. Then another heterotrophic organism absorbed the first one, with cyanobacteria already included in it – a secondary endosymbiosis occurred. This is how plastids appeared – from a common organism that arose from a heterotrophic cell and cyanobacteria (pic. 2). Interestingly, some algae also have tertiary endosymbiosis, so the idea of using other organisms by making them part of your cell is quite common.
Plastid DNA is similar to that of prokaryotic algae. Like the DNA of mitochondria, it is circular and does not contain all the genes (some of them "migrated" to the nucleus). Unlike mitochondrial DNA, plastid DNA can also be inherited from the paternal organism.
How we can use knowledge about secondary endosymbiosis
But mitochondrial DNA is also an interesting object for research by phylogenetic scientists involved in elucidating relationships between organisms and species. One of the most important mitochondrial proteins that have had a great impact on science, in general, is the enzyme c oxidase I (COX1). Mitochondrial CO1 has become a kind of marker since this gene is to some extent "universal" for all types of animals. Its overall structure is rather conservative. At the same time, the gene is easily mutated and, due to the lack of an advanced repair system in mitochondria, these mutations are well accumulated and transmitted. Relationships can be calculated from such mutations – if two species share the same mutation, we can say for sure that they once had a common ancestor.
In addition, due to the fact that there are usually several copies of the same DNA per mitochondria, and there are many mitochondria in a cell, this gene can be easily obtained in sufficient quantities for research.
Conclusion
Once upon a time, the ancestors of mitochondria and plastids were free-living prokaryotes, which then got into other cells and remained there to live. In the case of plastids, this happened several times, so we can talk about secondary or tertiary endosymbiosis. The genome of mitochondria and plastids is quite small and mutates a lot, which allows some genes to be used as markers of change – this helps phylogeneticists study relationships between species.