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  • I'm gonna make a bold prediction. You could watch this video 5 seconds after it's posted

  • and someone will already have commented the mitochondria is the powerhouse of the cell.

  • It's pretty clear how these little organelles became a meme. It's got to be the most repeated

  • line in biology and has been firmly inserted into our middle school textbooks for years.

  • There's just something catchy to it. Sidenote, do powerhouses say they're the mitochondria

  • of wherever? If you're in charge of the PR for a powerhouse anywhere in the world,

  • DM usThat whole powerhouse nickname came from the mitochondria's energy production

  • capabilities, but it's so much more than that. Today we'll go over how this little

  • powerhouse powers our cells, where it came from, and some new research that you probably

  • didn't hear about back in your middle school textbook.

  • You may remember some variation

  • of this diagram, a classic animal cell with a little jellybean-shaped organelle called

  • the mitochondrion. Mitochondrion for singular, mitochondria for plural. That diagram is fine

  • when you're learning the structures for the first time, but of course, a real life

  • cell is more complexThat little jellybean shape is only one of the possible shapes a

  • mitochondrion can take. What it looks like can be different from cell to cellPlus,

  • you don't just have a handful of mitochondria per cell, you have hundreds to thousands of

  • them floating in your cells. And even that number depends on what type of tissue we're

  • talking about. Like a skeletal muscle cell might be 3 to 8 percent mitochondria by volume

  • but a liver cell could be about 20 percent. Meanwhile, heart muscle cells are laughing

  • at those numbers because they're about 35 - 40 percent mitochondria by volume. They

  • win by a long shot. Now, all mitochondria do have some structural things in commonThey

  • each have two membranesone outer layer, one inner layer, and some space in between

  • them.That outer membrane works like a protective but permeable layer, letting different compounds

  • in or out of the mitochondrion. Meanwhile the inner membrane is where some important

  • biology happens to manufacture ATP. This is the molecule that fuels our major biological

  • processes, so it's often called energy currency. We're going to get into more depth on that

  • whole ATP thing in the next video, but this inner layer, as well as the matrix within

  • the mitochondria, is where the cells generate most of their ATP. Zooming back out to the

  • mitochondrion as a whole, it looks almost like a separate cell in its own right. That's

  • because at one point, it was. The most widely accepted theory of how we got these little

  • guys is the endosymbiosis theoryEndo- meaning into, -symbiosis meaning living togetherthis

  • word means that one cell engulfed another cell and it resulted in a mutually beneficial

  • relationship. About 3.8 billion years ago, earth's atmosphere didn't have oxygen

  • in it, and the only things living on our planet were single celled organisms that were anaerobic,

  • meaning they didn't need oxygen to surviveFast forward about six hundred million years and

  • photosynthetic bacteria were everywhere, taking sunlight and a few other ingredients and cranking

  • out oxygen as a byproduct. A few hundred million years later, those photosynthetic bacteria

  • had produced so much oxygen that it fundamentally changed the composition of Earth's atmosphereHere's

  • the thing, oxygen was actually toxic to those anaerobic cells. It's so bizarre that something we

  • need on a daily basis was so deadly back thenIt's like if I found out my ancestors were allergic

  • to tacos. That meant that these anaerobic bacteria were at a huge disadvantage once

  • the atmosphere was made of, what was to them, poison gas. By two and a half billion years

  • ago, a new type of bacteria started showing up in the fossil recordThese bacteria were

  • aerobic, meaning they could use oxygen, and it even helped them create energyThat is

  • an excellent evolutionary advantage when the atmosphere is made of a gas you can useThe

  • theory suggests that eventually, one of those anaerobic single celled organisms consumed

  • an aerobic purple bacteria that survived being eaten and they kicked off a symbiotic relationship.

  • That was the first mitochondria. That purple bacteria could consume and metabolize oxygen,

  • which provided energy for the host cell. And in return, the host cell protected the bacteria.

  • We still don't totally know the conditions around that moment of symbiosis, but we have

  • fossil evidence of it starting about one and a half billion years agoThe result of this

  • ancient endosymbiosis is today's powerhouse of the cell. We kept mitochondria around to

  • make energy for our cells. And since they were once their own separate organisms, they

  • retained certain features of their past selves, one of which was their genetic information.

  • Just like our larger cells, mitochondria need certain proteins to do their jobs, so they

  • need genes to tell them what proteins to makeOur DNA, which makes up our genes, is kept in

  • our cell's nucleus, what I'll call nuclear DNA for the rest of the episode. Some of our

  • nuclear DNA makes proteins for the mitochondria, then ships them out for it to use. But the

  • mitochondria also has its own DNA, separate from the DNA in your cell's nucleusPlus,

  • it has the cellular machinery to make new mitochondrial proteins, again, separate from

  • the rest of your cell. This is the mitochondrial genome, or the entirety of its genetic information,

  • and it's much smaller than the genome in the cell's nucleus. It's a small circle

  • with only about sixteen thousand base pairs while the nuclear genome has billions of

  • base pairs. Now, the vast majority of proteins that get used by the mitochondria come from

  • nuclear DNA, but that mitochondrial DNA lets us make some cool observations. Thanks to

  • sexual reproduction, humans are genetic mishmashes of our parents, so you might expect that our

  • mitochondrial DNA comes from our parents tooAs a matter of fact, for a few reasons, we only

  • inherit mitochondrial DNA from our mothers. When you were first developing in utero, most

  • of the chromosomes from your biological parents recombined to form your chromosomes. This

  • is part of what makes you physically different than your parentsBut mitochondrial DNA,

  • as well as the Y chromosome from your father, don't recombine so they get used to study

  • lineage. This kind of DNA does mutate, but it's otherwise well conserved, so the information

  • in our mitochondria's genes are similar to our maternal ancestors way way back in

  • the pastSequencing that mitochondrial DNA and comparing genomes has allowed researchers

  • to trace people back to a single female ancestor in Africa thousands of years ago, and follow

  • human migration. Now, why does mitochondrial DNA only come from your mother? Good question!

  • The first is that egg cells hold way more mitochondrial DNA than sperm cells, it's

  • around two hundred thousand molecules in an egg cell and like, single digits, in sperm

  • cellsSome estimates are a little higher, but the point remainsegg cells outnumber

  • sperm by a lot when it comes to mitochondrial DNA. Plus, sperm store most of their mitochondria

  • in their metabolically active tailsIt does take a lot of energy to swim, after all. Now,

  • aside from helping your cells make energy and providing clues about our ancestry, ongoing

  • research is showing us some new features of our mitochondria. For example, research by

  • scientists at the Salk Institute showed that mitochondria can kick off a series of events

  • that signal the rest of the cell that it's under stressthe kind of chemical stress

  • that can damage DNA. This phenomenon caught their attention when they observed how defective

  • mitochondrial DNA caused the cell to eject the damaged mitochondria and actually send

  • out a chemical warning signal that strengthens the cell's defenses. So they investigated

  • what would happen if any of that DNA spilled out of the mitochondria and into the liquid

  • around it. When they did, they saw that a certain set of genes were activated that usually activate

  • when there's an invading virus. Awesome, that's exactly what we want our immune system to

  • do, attack a virus when it detects oneNow, that same set of genes is also activated by

  • chemotherapy-resistant cancer cells. Specifically, cancer that's resistant to doxorubicin,

  • a chemotherapy drug that attacks nuclear DNA. When they studied this drug more closely,

  • they found that it caused the release of mitochondrial DNA from the mitochondria, which activated

  • a subset of those protective genes, which then protected the nuclear DNAThe point

  • of this drug was to attack nuclear DNA, but when these genes were activated, it set up

  • a pathway to defend the nuclear DNA, which explains why some cancers were resistant to

  • the drug. The researchers took cancer cells and induced stress on their mitochondrial

  • DNA, and as expected, they activated more of those genes and developed a resistance

  • to doxorubicin afterwardsThis research doesn't show that doxorubicin is a useless

  • chemotherapy drug, it just explains why some cancers develop resistance to the drug. They

  • think the purpose of that response is to protect the DNA in the cell's nucleus, making the

  • mitochondria a warning signal that something bad is happeningThey hope that if they

  • can find a way to protect the mitochondrial DNA, they'll prevent that immune response

  • within the cell and find more effective chemo treatments. So not only is our powerhouse

  • of the cell effective in generating energy, it's got a fascinating backstory, with clues

  • to our past and to our future medical treatments. If you're wondering why we kind of glossed

  • over the energy generating aspects of the mitochondria, it's because we're saving

  • it for later. Check out the next episode in the series to learn about how our cells generate

  • energyYep, I'm gonna try to teach ya'll how ATP works without making you fall asleep

  • from boredom. Wish me luck.

  • I'm Patrick Kelly, thanks for watching this episode of Seeker Human.

I'm gonna make a bold prediction. You could watch this video 5 seconds after it's posted

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Mitochondria Aren't Just the Powerhouse of the Cell

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    Summer posted on 2020/04/23
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