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  • It's hard to wrap your head around just how massive and complex DNA is. Almost every

  • cell in your body has about six billion base pairs, these combinations of As and Ts, and

  • Cs and Gs here. To put that in perspective, a credit card is about one millimeter thick.

  • If you stacked six billion credit cards in a straight line you could go from

  • San Francisco to past the North Pole. That's a lot of plastic though, so maybe

  • don't. What I find fascinating though is how good DNA is at making copies of itself.

  • As your cells are growing and replicating, your DNA is copying its genetic data into

  • those new cells. And copying errors do happen, but still, they only happen at about one in

  • ten thousand base pairs.

  • Even then, our genetic machinery has checkpoints in place to make sure those copying

  • errors get tossed out and don't harm us.

  • And all that is so we can make new cellsIn today's final

  • episode of this season of Human, we're going to learn how the cell grows and replicates,

  • including how it copies that DNA, and we'll get into a little stem cell talk.

  • In the previous episode, we talked about how complex DNA is and how it transcripts and translates genetic

  • code into usable proteinsThat's all greatour cells made stuff. I emphasize stuff

  • for a reason. The proteins that we make from DNA aren't alive and don't have any genetic

  • information of their own. They're just kinda nonliving stuffIt becomes a little trickier

  • when it comes to making new cells. And the way that adults make new skin cells or connective

  • tissue cells is different from how they make new sperm or egg cellsThe difference between

  • them comes back to DNA, specifically, how we store DNA in those larger structures called

  • chromosomes. Big picture, the purpose of all cell division is to grow cells and replace the

  • old ones. So our bodies have a few ways of making replacements for those cells. One way

  • to do that is by making identical cells from some kind of parent cell. If we think of the

  • parent cell as a template for its offspring, it should be a super simple copying process,

  • right? Well, that's where we see the first asterisks of the episode. The copying process

  • depends on what we're making and what genetic information we'll copy into the new cell.

  • To understand that, we need to get deeper into chromosomes. Chromosomes are made of

  • chromatin, a combination of DNA and proteins called histonesThe histones are there so

  • we can wrap those long strands of DNA around them and pack a lot of genetic information

  • into a small package. We have twenty three pairs of chromosomes, one from each parent,

  • for a total of forty six chromosomesAll of our cells, except sperm, egg cells, and

  • red blood cells, have twenty three pairs of chromosomes. That makes almost all of our

  • cells diploid, meaning they have two sets of each chromosome. Sperm and egg cells, what

  • we call gametes or germ cells, are an exception because they only have one set of

  • their chromosomesthey're haploid. Now, this wouldn't be a biology series if we

  • didn't go through the stages of mitosis. I'm not going to quiz you on the names of

  • the phases, but this process happens in phases. The first thing your cells do is make identical

  • copies of the genetic material during interphase. DNA replication could fill multiple videos

  • worth of information by itself, so we're dramatically oversimplifying hereBut after

  • DNA replication, you have two identical copies of your DNA. These get assembled into identical

  • sister chromatids. At this point, each sister chromatid gets joined together by a little

  • structure called a centromere.

  • Now the cell needs to organize all those chromatids

  • and prep them for division, so it condenses them them into thick, tight bandsThis is

  • prophase, and it's the first time during the cell cycle that you can see chromosomes with

  • a simple light microscope. This phase is also when important structures are being built

  • just outside of the nucleus that we'll need in the next phases of mitosis. During prometaphase,

  • the nucleus starts to dissolve, which gives those newly built structures access to the

  • chromosomes, allowing them to attach to each one. Now, some of those new structures are

  • called microtubules, and in this next phase, metaphase, those microtubules line the chromosomes

  • up in the middle of the cell. Then during anaphase, the chromosomes are pulled towards

  • opposite ends of the cell. At this point, each chromatid is a brand new chromosome.

  • The last phase is telophase, where these new chromosomes get wrapped in a new nucleus,

  • giving us two nuclei in one cell. Finally, the cell splits into two identical daughter

  • cells. Almost every cell in your body is the result of mitosis. That single cell that would

  • become the trillions in your body replicated and grew and eventually, your body became

  • a thing because of mitosis. So then why do we have a separate process for egg and sperm

  • cells? We need a separate way to make these cells, in this case, meiosis, because our

  • goal is to end up with a cell that has half of the genetic material as its parent. Then

  • when a sperm and egg cell combine, their single sets of chromosomes combine into a brand new

  • genome, making a new diploid cell. We're not trying to make a totally perfect clone,

  • we're trying to make something intended to be combined with another thing. In a lot

  • of ways, mitosis and meiosis are really similar. Namely, the parent needs to make copies of

  • its genetic information. But it differs in a few waysin meiosis, there are two rounds

  • of cell division instead of one, and the chromosomes swap genetic material with each other to create

  • a totally unique cell. That recombination and first division happens during the first

  • branch of meiosis, or Meiosis 1. This step is crucial, it's what gives your offspring

  • some kind of genetic variation. At this point, you now have haploid cells. Then in its sequel,

  • or Meiosis 2: Meiosis Strikes Back, they divide again really similarly to mitosis. By the

  • end of meiosis, we end up with either four sperm cells or one egg cell. The process is

  • different for each type of cell, but both undergo meiosis.

  • The cells that eventually become egg

  • cells develop while you're still in the womb, but stop growing for a period of time

  • until puberty. Of the thousands of cells that could become mature egg cells, about four

  • hundred do. Speaking of how long cells can grow for, we need to talk about a special

  • type of cell. In the last few years, scientists have been researching different cells that

  • have their own interesting replication process, stem cells. We've mentioned them a few times

  • throughout this series, but stem cells, depending on specifics, can differentiate into other

  • types of cells, or it can keep making copies of stem cells. For context, your body started

  • from a single fertilized egg cell and through mitosis, grew into exponentially more cells.

  • But at some point, those cells had to differentiate into the many specialized cells you have now.

  • Those specialized cells came from stem cells. They can become any type of cell in the body

  • and have the ability to keep replicating throughout a person's life. Because of that, scientists

  • are interested in studying them for certain treatments, like growing some nerve cells

  • from stem cells and using them for degenerative diseases like Parkinson's. In the past,

  • scientists were extra interested in embryonic stem cells, or stem cells that came from embryos.

  • Using these types of cells comes with some ethical concerns which I'm not going

  • to talk about since it would ignite a dumpster fire in the comments section and nobody would

  • be happy. Luckily, these days we have less controversial ways of making stem cells, like

  • turning mature cells into stem cells. Back in 2006, scientists transformed mature fibroblast

  • cells from a mouse to create a stem-cell-like state by activating certain genes. They called them

  • induced pluripotent stem cells, or iPSCs, since they could induce the cell's ability

  • to become stem cellsAnd they're pluripotent because they can become any cell type in the

  • body. And these things are so useful, not only because it gets around some of the ethical

  • concerns, but because they come from someone's mature cells, scientists could develop new

  • tissues that are compatible with that person's immune system and are less likely to get rejected.

  • Plus, we can culture them to study different diseases as well. Many scientists do animal

  • studies on rodents as a way to test different treatments. But obviously there are some differences

  • between us and mice, so treatments don't always translate a hundred percent. By taking

  • someone's skin cells or fat cells and reprogramming them into stem cells, we could research how

  • real human tissue reacts to whatever we're trying to test. This technology has only been

  • around since earlier this century, and in the time since, we've already seen clinical

  • trials aiming to improve human lives. For instance, a few different research groups

  • have used iPSCs in treatment trials of age-related macular degeneration, or AMD, a common cause

  • of vision loss for the elderly. They take a few of the patient's cells, turn them

  • into stem cells, then turn them into the right kind of retinal cell, and implant them back

  • into the patient's eye.

  • Sometimes, it works and it restores the patient's vision! Sometimes

  • it doesn't work out though, and that's something to keep in mindThis is a cool

  • and new science, but it's still science, not a magic cure-allAnd that's one of

  • the things I love about this field of study. We are constantly learning new things that

  • our bodies can do and how they interact with the world, both the big macro world and the

  • tiny chemical world. We hope you've enjoyed the season, I know I've loved writing and

  • hosting it. For more body related content, check out our ongoing series Sick, and follow

  • us on all our social media for even more content. We're @seeker on everything.

  • I'm Patrick Kelly, thanks for learning with us.

It's hard to wrap your head around just how massive and complex DNA is. Almost every

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你的身體是如何製造新細胞的? (This is How Your Body Makes New Cells)

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    Summer 發佈於 2021 年 01 月 14 日
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