Shadowing Practice: Mitosis: Splitting Up is Complicated - Crash Course Biology #12 - Learn English Speaking with Video
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Now check this out.
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Ha, cool huh?
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I bet you wish you could do this.
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Have a clone, clean up around the apartment for you, go to class, maybe take your mom out to dinner on her birthday.
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Well, you can't do that, and actually there are some really good reasons why you can't do that.
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We're gonna talk about those in the next episode.
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But you know what can clone themselves?
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Your cells.
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Like almost every single one of them, and in fact, they're doing it right now.
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For any creature bigger than a single -celled organism, all of life stems from cells' ability to reproduce themselves
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because that's what allows organisms to develop and grow and heal and keep from dying for as long as possible.
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This particular kind of cell division is called mitosis and it's responsible for a whole lot of your body's key functions.
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If you get it cut, your body needs to make new cells.
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Mitosis.
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Have too much to drink?
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Damage your liver?
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You gotta replace those cells.
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Mitosis.
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Tumor growing in your spine?
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Unfortunately, again, mitosis.
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While you go from a seven pound baby to a 70 pound child, it's not your cells that are increasing in mass,
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you're just getting more of them over and over and over again.
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That's mitosis.
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This process is so central to your life that it will take place in your body, over your lifetime, about 10 quadrillion times.
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That's 10 ,000 billion times.
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Like all split ups, it's not easy.
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It's gonna maybe be a little bit messy.
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There's a lot of drama and it can take a surprisingly long amount of time
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But trust me after we're done with it.
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We'll all be better off
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So you are made of trillions of cells just like giraffes
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and redwood trees and remember that inside each cell there's a nucleus
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that stores your DNA which contains all of the instructions on how to build you.
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That DNA is organized into chromosomes, and as we've mentioned before, in your body cells, or somatic cells, you have 46 chromosomes grouped into 23 pairs.
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One in each pair is from your mom, and the other one is from your dad.
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Cells with all 46 chromosomes are called diploid cells because they have two sets each, and that's what we're focusing on today.
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You also have haploid cells that have half as many chromosomes as 23.
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And those are your sex cells, they're produced in an equally fantastic process called meiosis, which we'll be talking about in the next episode.
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But for now, the main thing to remember about mitosis is
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that it allows one cell with 46 chromosomes to split into two cells that are genetically identical,
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each with 46 chromosomes, all in order to keep the party of life going.
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Now the nucleus in your cell controls everything that goes on in the cell, it has all the instructions necessary for making the cell survive,
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so you don't need to duplicate the whole cell.
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All you need to do is duplicate the DNA, get it wrapped up, and then if you have two separate pockets of DNA,
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that's all you need to have two new cells.
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Mitosis takes place in a series of discrete stages called prophase, metaphase, anaphase, and telophase.
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And you could just say that over and over again and let it sink into your head.
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And part of what's, you know, really amazing about this whole process is that while we know what these stages are,
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we don't always know the underlying mechanisms that make all of them happen.
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And this is part of science.
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Science isn't like all the stuff we know, it's how we're trying to figure all this stuff out.
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Consider it job security, if you ever want to be a biologist, there is a lot of stuff that future biologists have to still figure out.
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And this is one of them.
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Alright, let's get our clone on.
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So most of their lives, cells hang out in this limbo period called interphase, which means they're in between episodes of mitosis.
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Mostly growing and working and doing all the stuff that makes them useful to us.
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During interphase, the long strings of DNA are loosely coiled and messy, like that dust bunny of dog fur and laundry lint under your bed.
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That mess of DNA is called chromatin.
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But as the mitosis process begins to gear up, lots of things start happening in the cell to get ready for the big division.
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One of the more important things that happens is
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that this little set of protein cylinders next to the nucleus called the centrosome duplicates itself.
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We're gonna have to move a lot of stuff around in the nucleus, and that's gonna be regulated by these centrosomes.
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The other thing that happens is that all of the DNA begins to replicate itself too, giving the cell two copies of every strand of DNA.
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To brush up on how DNA replicates itself like this, check out this episode and then come on back.
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Now the cell enters the first phase, or the prophase, when that mess of chromatin condenses
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and coils up on itself to produce thick strands of DNA wrapped around proteins.
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Those, my friends, are your chromosomes.
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Instead of dust bunnies, the DNA is starting to look a little bit more like dreadlocks.
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And the duplicates that have been made don't just float around freely.
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They stay attached to the original, and together they look like little X's.
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These are called the chromatids, and one copy is the left legging arm of the X, and the other copy is the right legging arm.
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Where they meet in the middle is called the centromere.
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Just so you know, these X's are also called chromosomes, sometimes double chromosomes or double -stranded chromosomes,
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and when the chromatids separate.. they're considered individual chromosomes too.
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Now while the chromosomes are forming, the nuclear envelope gets out of the way by like, completely disintegrating.
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And the centrosomes then peel away from the nucleus, start heading to the opposite ends of the cell.
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As they go, they leave behind a wide trail of protein ropes called microtubules, running from one centrosome to the other.
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You might recall from our anatomy of the animal cell, that microtubules help provide a kind of structure to the cell.
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This is exactly what they're doing here.
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Now we reach the metaphase, which literally means after phase, and it's the longest phase of mitosis that can take up to 20 minutes.
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During the metaphase, the chromosomes attach to those ropey microtubules right in the middle at their centromeres.
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The chromosomes then begin to be moved around, and this seems to be being done by molecules called motor proteins.
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And while we don't know too much about how these motors work, we do know, for instance,
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that there are two of them on each side of the centromere.
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These are called centromere -associated protein E.
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So these motor proteins attached to the microtubule ropes basically serve to spool up the tubule slack.
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At the same time, another protein called dynion is pulling up the slack from other ends of the ropes near the cell membranes.
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After being pulled in this direction and that, the chromosomes line up right down the middle of the cell.
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And that brings us to the latest installment of Biolography.
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So how do chromosomes line up like that?
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We know that there are motor proteins involved, but like, how?
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What are they doing?
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Well, remember when I said earlier that there were a lot of things that we don't totally understand about mitosis?
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It's sort of weird that we don't, because we can literally watch mitosis happening under microscopes.
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But chromosome alignment is a good example of a small detail that has only very recently been figured out.
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And it was a revelation like 130 years in the making.
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Mitosis was first observed by a German biologist by the name of Walter Fleming, who in 1878 was studying the tissue of salamander,
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gills, and fins when he saw cells' nuclei split in two and migrate away from each other to form two new cells.
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He called this process mitosis, after the Greek word for thread because of the messy jumble of chromatin,
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a term he also coined that he saw in the nuclei.
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But Fleming didn't pick up on the implications of this discovery for genetics, which was still a young discipline.
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And over the next century, generations of scientists started piecing together the mitosis puzzle by determining the role of microtubules, say, or identifying motor proteins.
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Now the most recent contribution to this research was made by a postdoctoral student named Tomomi Kiyomitsu at MIT.
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He watched the same process that Fleming watched
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and figured out how at least one of the motor proteins helped snap the chromosomes into line.
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He was studying a motor protein called dynion, which sits on the inside of the membrane.
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Think of the microtubules as tug -of -war ropes, with the chromosomes as the flag in the middle.
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What Kiyomitsu discovered was that the Dainian plays tug -of -war with itself.
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Dainian grabs onto one end of the microtubules and pulls the tubules and chromosomes toward one end of the cell.
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When the ends of the microtubules come too close to the cell membrane,
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they release a chemical signal that punts the Dainian to the other side of the cell.
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There, it grabs onto the other end of the microtubules and starts pulling until until smack, it gets punted back again.
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All of this ensures that the chromosomes will line up exactly in the middle, so that they will be split evenly.
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That discovery was published in February of 2012, a couple of weeks before I sat down in this chair,
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and 134 years after mitosis was first observed.
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If you want to join the ranks of scientists who are answering the many questions left about mitosis
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and lots of other things about our lives, maybe someday I'll do a biolography about you.
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Now so far we've gone through the interphase, where the centrosomes in DNA replicate themselves and get ready for the split.
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The prophase, where the chromosomes form and the centrosomes start to spread apart.
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And the metaphase, where the chromosomes align in the middle of the cell.
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And now it's time to separate the chromosomes from their copies.
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This time, motor proteins start pulling so hard on the ropes
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that the X -shaped chromosomes split back into their individual single chromosomes.
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Once they're detached from each other, they're dragged toward either end of the cell.
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Since the prefix ana means back, that may help you remember the name of this phase called anaphase.
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After this, it's just a matter of using all that genetic material to rebuild,
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so that the copied genetic material has all of the accoutrements of home.
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In the last phase, telophase, each of the new cell structures are reconstructed.
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First, the nuclear membrane reforms, the nucleoli form within them, and the chromosomes relax back into chromatin.
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Then a little crease forms between the two new cells, which marks the beginning of the final split.
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That division between the two new cells is called cleavage, and all that's left is to make a clean break.
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This is done by cytokinesis, literally cell movement, by which the two new nuclei move apart from each other, and the cells separate.
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We now have two new cells, each with a full set of 46 chromosomes.
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These clones are called the daughter cells of the original cell,
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and like identical twins, they are genetic copies of each other and also of their parents.
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But of course, that's obviously not the case for you, even if you are an identical twin.
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Shout out identical twins!
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See me in the comments.
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While you kind of are a clone of your sibling, you are not a clone of your parents.
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Instead, half of your DNA in each of your cells is from your mom and half is from your dad.
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To understand why that is, we have to understand how eggs and sperm are formed, and that is meiosis.
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And that's what we're going to be talking about next week on Crash Course.
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Until then, you can just watch this video over and over again,
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or you can just watch the bits that you want to rewatch using our table of contents,
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which is also available in the description for people who are using iPhones and can't click annotations.
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If you have any questions, you can reach us on Facebook or on Twitter, or of course, in the comments below.
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Vocabulary and speaking notes for this lesson
This speaking lesson is built on the video “Mitosis: Splitting Up is Complicated - Crash Course Biology”. The speaker keeps coming back to these words: cell, chromosome, mitosis, Dna, protein. This video has 146 sentences and 2110 words to shadow. The speech runs for 10:38. The speaker talks fast, about 199 words per minute, so expect linked and reduced sounds. 83% of the words are among the 3,000 most common in English; the rest is worth studying before you start.
Key vocabulary in this video
15 words from the video worth learning, with pronunciation and meaning:
| Word | Pronunciation | Meaning |
|---|---|---|
| nucleus noun | /ˈnuː.kli.əs/ | The core, central part of something, around which other elements are assembled. |
| clone noun | /kloʊn/ | A living organism (originally a plant) produced asexually from a single ancestor, to which it is genetically identical. |
| rope noun | /ɹoʊp/ | Thick strings, yarn, monofilaments, metal wires, or strands of other cordage that are twisted together to form a stronger line. |
| identical adjective | /ɪˈdɛntɪkl̩/ | Bearing full likeness by having precisely the same set of characteristics; indistinguishable. |
| duplicate verb | /ˈd(j)uː.plɪˌkeɪt/ | To make a copy of. |
| genetic adjective | /d͡ʒəˈnɛt.ɪk/ | Relating to genetics or genes. |
| twin noun | /twɪn/ | Either of two people (or, less commonly, animals) who shared the same uterus at the same time; one who was born at the same birth as a sibling. |
| messy adjective | /ˈmɛsi/ | In a disorderly state; chaotic; disorderly. |
| membrane noun | /ˈmɛm.bɹeɪn/ | A mechanical, thin, flat flexible part that can deform or vibrate when excited by an external force. |
| attach verb | /əˈtæt͡ʃ/ | To fasten, to join to (literally and figuratively). |
| strand noun | /ˈstɹænd/ | Each of the strings which, twisted together, make up a yarn, rope or cord. |
| replicate verb | /ˈɹɛplɪkeɪt/ | To make a copy (replica) of. |
| discovery noun | /dɪˈskʌv.ɹi/ | Something discovered. |
| dust noun | /dʌst/ | Fine, dry particles of matter found in the air and covering the surface of objects, typically consisting of soil lifted up by the wind, pollen, hair, fiber… |
| pound noun | /ˈpaʊ̯nd/ | A unit of weight in various measurement systems. |
Phrasal verbs you will hear
| Word | Pronunciation | Meaning |
|---|---|---|
| figure out verb | To come to understand; to discover or find a solution; to deduce. | |
| line up verb | /laɪnˈʌp/ | To align; to put in alignment; to put in correct adjustment for smooth running. |
| check out verb | To record the departure or withdrawal of someone or something (such as guests, employees, books, etc.). | |
| clean up verb | To make an area or a thing clean; to pick up a mess; to tidy. | |
| get out of verb | To leave, exit or escape from; to become free of. | |
| go through verb | To travel from one end of something to the other. | |
| leave behind verb | To abandon. | |
| move around verb | To relocate to new homes repeatedly; to not live in any one place for long. |
Sentences worth repeating
Short, complete lines from the video that you can reuse in everyday conversation:
- We're gonna talk about those in the next episode.
- Tumor growing in your spine?
- Alright, let's get our clone on.
- We know that there are motor proteins involved, but like, how?
Grammar in this video
The structures the speaker uses most, with the exact words from the video:
| Structure | In the video |
|---|---|
| Passive voice be + past participle — the focus is on what happens, not who does it | is called · are made · is organized |
| Present perfect have/has + past participle — a past action that still matters now | we've mentioned · have been made · we've gone |
| Relative clauses who / which + clause — extra information about a person or thing | interphase, which means · genetics, which was · people who are |
Pronunciation to watch
The speaker uses 35 contractions and reduced forms, such as they're, don't, we're. Say them the short way, as you hear them.
- The “th” sounds: thick /θɪk/, thread /ˈθɹɛd/
- The “sh” and “zh” sounds: instruction /ɪnˈstɹʌkʃən/, contribution /ˌkɑntɹɪˈbjuʃən/, tissue /ˈtɪʃ.(j)u/, brush /bɹʌʃ/, shout /ʃaʊt/
- Long words — get the stress right: organism /ˈɔɹ.ɡəˌnɪz.əm/, contribution /ˌkɑntɹɪˈbjuʃən/, mechanism /ˈmɛk.ə.nɪ.zəm/, surprisingly /səˈpɹaɪzɪŋli/, revelation /ˌɹɛv.əˈleɪ.ʃən/
How to practise with this video
- Listen to the whole video once without speaking and note the words you do not know.
- Start at 0.75× speed, shadow it sentence by sentence, then go back to normal speed once it feels easy.
- Record yourself and compare with the original, paying attention to words like nucleus, clone, rope.
What is the Shadowing Technique?
Shadowing is a science-backed language learning technique originally developed for professional interpreter training and popularized by polyglot Dr. Alexander Arguelles. The method is simple but powerful: you listen to native English audio and immediately repeat it out loud — like a shadow following the speaker with just a 1–2 second delay. Unlike passive listening or grammar drills, shadowing forces your brain and mouth muscles to simultaneously process and reproduce real speech patterns. Research shows it significantly improves pronunciation accuracy, intonation, rhythm, connected speech, listening comprehension, and speaking fluency — making it one of the most effective methods for IELTS Speaking preparation and real-world English communication.