Mitochondria.ppt and choloroplast origin

Editor's Notes

• #2 We’ve just learned about many different organelles, including two that have some unique characteristics. Recall that mitochondria are the powerhouses of the cell—the site of cellular respiration where ATP is generated. And chloroplasts are the organelles in plants that are responsible for photosynthesis. They use the energy from sunlight to produce sugars. These organelles have some unique features. First they have a double membrane. That’s two distinct lipid bilayers.
• #3 Second, they each have their own DNA, separate from that in the nucleus of the cell. This DNA carries genes that are distinct from those of the nuclear genome. Mitochondrial genes generally code for proteins associated with cellular respiration and also for tRNA (though this varies by species). Chloroplast genes code for proteins involved with photosynthesis. Furthermore, the genomes of mitochondria and chloroplasts are much shorter than the nuclear genome. This microscope image shows a human cell with the nucleus shown in blue, mitochonria in red, and mitochondrial DNA in green.
• #4 More specifically, this mitochondrial and chloroplast DNA is circular. It is not laid out linearly as our chromosomes are, but in a loop. This diagram is a schematic of a chloroplast genome. The different lines on the circle indicated the location of different chloroplast genes on the loop of DNA. It is shown adjacent to a map of human chromosome 1, which is linear.
• #5 Mitochondria and chloroplasts even have their own protein-making machinery in the form of ribosomes. However, the ribosomes in mitochondria and chloroplasts are a little different from the ribosomes in the rest of the cell. All ribosomes are made of RNA and proteins, and all of them have two subunits. But the mitochondrial and chloroplast ribosomes are smaller than those in the rest of the cell. They have shorter lengths of RNA and incorporate fewer proteins. This image shows a mitochondrial ribosome. The large subunit is in blue, the small subunit is in yellow.
• #6 Finally, unlike other structures in the cell, such as the Golgi apparatus and the lysosome, which develop from other structures in the cell, mitochondria and chloroplasts multiply by splitting in two—through binary fission. Mitochondria are only produced by other mitochondria, and chloroplasts are only produced by other chloroplasts. These microscope images show a mitochondrion and chloroplasts in the process of fission.
• #7 In fact, all the mitochondria in your body today are descended directly from mitochondria that were in the egg cell that you came from. (In humans and most other animals, sperm have mitochondria but they are destroyed inside the egg cell, so it is only mitochondria from the mother that are inherited.) This means that your mitochondria came from your mother, and hers came from her mother, and so on back in time, forming a single, maternal line of descent. In some plant lineages, chloroplasts are inherited through the female line and in others lineages, through the male line (pollen). In fact, in at least one plant lineage (a kiwi), chloroplasts are inherited from the male gamete, while mitochondria are inherited from the female gamete.
• #8 Mitochondria and chloroplasts have many unusual characteristics that other organelles in the cell don’t have. In fact, in a lot of ways, they seem like individual organisms living inside another cell. Why are they so odd?
• #9 Because they once were free-living organisms! Both mitochondria and chloroplasts evolved through endosymbiosis, where one organism takes up residence inside another and stays forever, evolving such a close relationship that the two become a single organism. Both mitochondria and chloroplasts are descended from free-living bacteria that had: a single membrane their own circular DNA their own ribosomes binary fission When these free-living bacteria were engulfed by another bacterium, they brought with them their DNA, ribosomes, and method of reproduction—and their single membrane became double when it was engulfed in the host cell’s membrane. All of the odd characteristics of these organelles make sense in light of their evolutionary history.
• #10 This happened in a series of steps. First, around 1.8 billion years ago, a proteobacterium that could aerobically respirate (probably an ancestor of modern Rickettsia bacteria, which cause typhus) was engulfed by another cell.
• #11 This endosymbiont evolved into mitochondria and the lineage diversified into all eukaryotes. The other lineages present at the time evolved into the Eubacteria and the Archaea.
• #12 Later on, around 1.5 billion years ago, one lineage of the Eubacteria (a cyanobacterium, which had the ability to photosynthesize) invaded a lineage of eukaryotes (which was already carrying its own mitochondria).
• #13 Of course, this group later diversified into the plants. That’s the story for most of life. But for a few lineages, it wasn’t over yet . . .
• #14 Some lineages have undergone additional rounds of endosymbiosis—being invaded by a cell that was already host to permanent endosymbionts. For example, euglenoids have photosynthetic plastids with three membranes—the result of engulfing an alga that already contained double-membrane endosymbiont-derived plastids. And in the dinoflagellates, this has happened yet again. Dinoflagellates evolved when an alga already containing an endosymbiont within an endosymbiont was engulfed by a fourth type of cell! It’s like Russian nesting dolls!