mitochondria (singular :mitochondrion) :
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Answer:
mitochondrion is a double membrane-bound organelle found in most eukaryotic organisms. Some cells in some multicellular organisms lack mitochondria. A number of unicellular organisms, such as microsporidia, parabasalids, and diplomonads, have reduced or transformed their mitochondria into other structures.
Mitochondria: To most people, they’re little more than a ghostly memory fragment from middle school biology. However, these tiny “powerhouse(s) of the cell” are much more than they seem.
Mitochondria live in every cell in your body and are essential for human life. As University of California post doc Samantha Lewis pointed out to me: “There’s mitochondrial involvement in almost every disease.”
Yet, we rarely hear of or think about our cells’ powerhouses.
Here are seven facts you probably haven’t heard about mitochondria:
1: Mitochondria are interconnected shape-shifters.
[A bone cancer cell with stringy mitochondria highlighted in yellow. Photo by NICHD via Flickr & CC 2.0 License.]
We say “Mitochondria is the powerhouse of the cell” as if mitochondria is a singular word, but actually it’s plural. (The singular of mitochondria is mitochondrion.) However, in most cells mitochondria act as a collective, passing electrons and genetic information from mitochondrion to mitochondrion.
“They’re [descended from] bacteria that divide in a binary fashion,” explained UC Davis cell biologist and mitochondria specialist Jodi Nunnari. “During the course of evolution [the mitochondrial] genome has been greatly reduced. As a consequence of that and the fact that they were reproducing in a new environment, a few of those do mitochondrial fusion.” Mitochondria’s habit of merging sets them apart from all known bacteria. “Bacteria divide, but they don’t fuse,” Nunnari added.
In fact, mitochondria are so tightly connected that many scientists think of them as a membrane network rather than a series of jelly-bean shaped organelles.
They can also swell to become round or stretch out to become skinnier. Looking at the shapes of mitochondria can give researchers clues about the health of cancer cells.
2: Scientists have recently learned how to kinda control their shape-shifting.
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[Mitochondria (in red) surround a cell nucleus (blue). Image by NIHCD via Flickr & CC 2.0 license.]
Until recently, mitochondrial motives for fusing and dividing have remained murky. However, one team of scientists at Washington University at St. Louis have discovered one molecule that exerts an outsize influence on mitochondrial fusion.
“They have kind of a mob mentality,” Gerald Dorn, a cardiologist from Washington University in St. Louis said of mitochondria. “They do a lot of things that are out of our control.”
However, Dorn and his team recently identified a peptide that is mounted on the ends of individual mitochondria, which opens and closes “like a diaper pin”.
When the diaper pin peptide is “open”, mitochondria stick to each other like Velcro and fuse. When the diaper pin peptide is closed, the mitochondria go on their solitary way.
By adding drugs that open or close the peptide, Dorn and his colleagues were able to mostly control the rate of mitochondrial fusion in the cell. For cell biologists, that’s a new one. The study ran in the prestigious journal Nature. They’re hopeful that someday, the ability to manipulate mitochondrial merging and dividing will lead to treatments for killers like heart disease.
3: They swap genes amongst themselves.
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[Depiction of mitochondrial DNA (blue) inside a textbook mitochondria. Image via National Human Genome Project on Flickr & CC 2.0 License.]
The cell’s nucleus is a hub of genetic information, but mitochondria have kept a handful of their essential genes all to themselves.
Mitochondria store their genetic info on little globs of DNA called nucleoids, which are spread throughout the cell’s mitochondria. Although nucleoids have their own name, they can be thought of as chromosomes for mitochondria. “I do call them mitochondrial chromosomes,” Nunnari admitted.
Nucleoids get shuttled from one mitochondrial compartment to another, and only a small fraction of them are copied to make new mitochondria.
4: Mitochondria are BFFs with the Endoplasmic Reticulum.
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[An artist’s rendition of a macrophage with an endoplasmic reticulum by Liz Hirst. Photo by NIMR London via Flickr & CC 2.0 license.]
Samantha Lewis, a postdoc in Nunnari’s lab, recently captured images of strategic bonding between two of cells’ oldest organelles–the powerhouse mitochondria and the molecule-delivering endoplasmic reticulum.
In fact, the spots where the ER touched mitochondria were exactly where mitochondria made copies of their nucleoids.
“There’s all these nucleoids, and then there’s some spark; there’s some event that sets off the program of replication,” said Lewis. “I like to use the word ‘fire’ because it’s irreversible.”
“It’s not random,” added Nunnari. “It happens at positions where this other really ancient endomembrane system called the endoplasmic reticulum is in contact with the mitochondria.”
In its role as the cell’s transport system, the ER is in contact with many different organelles. It’s also one of the few organelles thought to be as old as mitochondria.