# Chromosome Numbers Before and After Mitosis and Meiosis (UPDATED) ## Метаданные - **Канал:** Amoeba Sisters - **YouTube:** https://www.youtube.com/watch?v=Al7FueXTCKs ## Содержание ### [0:00](https://www.youtube.com/watch?v=Al7FueXTCKs) Segment 1 (00:00 - 05:00) If you’ve ever studied mitosis or meiosis, chances are, the whole chromosome number thing has crossed your mind. What do I mean by that? Well, consider starting with a cell that has 46 chromosomes. The cell divides into two and yet – each daughter cell still has 46 chromosomes. On paper, the math doesn’t seem to math. Or in meiosis, you can start with a cell that has 46 chromosomes and by the end of meiosis, you end with four cells that each have 23 chromosomes, which mathematically also seems wild. And that confusion makes the process confusing and so, with this video, we’d like to clear this up a bit. Chromosomes, as you know, are made up of DNA and protein. The DNA in a chromosome is highly condensed, tightly wound – which is really important, it makes it more transportable. And this matters when a cell divides and each daughter cell gets a copy of the DNA from the original. In fact, the DNA of a cell is not always condensed, but it tends to be before division. What can be confusing is this a chromosome. And so is this. But this one has two sister chromatids, or copies. You could say this is a chromosome made up of two sister chromatids. They’re attached together. And as long as they are attached, this entire unit itself is a chromosome. You could say that this chromosome here is made up of one chromatid. BUT here is where it is confusing – when it doesn’t have sister chromatids – we can call a single chromatid a chromosome. This unit is a chromosome. That’s why in a lot of textbooks, when you have the mitosis step of anaphase where sister chromatids are pulled apart, many textbooks will state something along the lines of “the chromatid is now called a chromosome. ” But that can be a little confusing- it isn’t that it morphed into some totally new structure – it’s actually more of a naming thing – whenever it is its own unit and stands alone, the chromatid can be called a chromosome. To add to this: some textbooks only like to reserve the term “chromatid” for when you are referring to two sister chromatids; so if you’re referring to one of two. And some textbooks don’t. Ultimately, the concept we want to really emphasize: this is a chromosome. It’s just this one has copies – it has two sister chromatids attached. And you could say this one consists of just a single chromatid but when it stands alone, this can be called a chromosome. Ok, so let’s get to counting. Interphase is a step that happens one time before mitosis and meiosis. In interphase, we duplicate DNA so you’re going to get duplicated chromosomes. So, if starting with 46 of these chromosomes, we’re representing them like single sticks for simplicity but technically - keep in mind- in interphase they wouldn’t be so nicely condensed like this yet. And after duplicating, in interphase, we still have 46 chromosomes, but each one of these has two identical chromatids. They’re copies of each other. Sister chromatids. Held together at the centromere region. As long as the chromatids are still attached, we still count 46 chromosomes. Generally, when counting chromosomes, you can count the number of these centromere regions to see the chromosome number. But now, we need to talk about the centromere. Chromosomes have centromeres and this is something we’ve made some posts and YouTube Shorts about because semantics will come back heavy here. You can recognize the term centromere as a traditional, physical definition of a constricted region – as you’ve likely seen in diagrams like this, a region of where the sister chromatids would be tightly connected and you’d see a narrowing. But, also, at the molecular level, a centromere can be defined as a specific area of DNA and the proteins that bind there, and with that definition, each of these sister chromatids technically has a centromere. Which makes sense, I mean, these sister chromatids are identical copies. There’s a lot more detail you can explore about centromeres: for example, exploring kinetochores, which assemble on centromeres and are important for the spindle. So, we have videos on mitosis and meiosis but if you saw the title of this video, you are probably curious about how the number of chromosomes change in mitosis and meiosis. Let’s start with mitosis and let’s bring out our fabulous table at the same time because we love tables. In most cases, humans have 46 chromosomes in most of their body cells. Now, before mitosis begins, this body cell does something called interphase - where it duplicates its chromosomes. So now, it looks like this. So, after interphase, you still have 46 chromosomes but each chromosome has two sister chromatids. Two sister chromatids, identical copies. You start mitosis like that. During mitosis, the chromatids separate. And remember the terminology: as soon as the sister chromatids are separated, those chromatids can now be called chromosomes. And so after mitosis, you end with two daughter cells that each have ### [5:00](https://www.youtube.com/watch?v=Al7FueXTCKs&t=300s) Segment 2 (05:00 - 07:00) 46 chromosomes that look just like what you started with before interphase began. What about meiosis in humans? Ok, so similar thing here: let’s consider the starting cell in meiosis. In most cases, these cells have 46 chromosomes. This is before interphase. Then after interphase, chromosomes are duplicated; you see each chromosome has two sister chromatids attached. A cell doing meiosis I will make two cells that each have 23 chromosomes - each of those still have two sister chromatids. In meiosis II, the two daughter cells start dividing. The sister chromatids in them are separated so you can see 23 chromatids in each cell - but remember once chromatids are separated - they can now be called chromosomes. You can see the daughter cells do not look like the original starting cell before meiosis began. Ultimately in females, the daughter cells will be egg cells and in males, they will be sperm cells. Which brings us to some important vocabulary words. Haploid and diploid. Diploid cells have two chromosome sets. Haploid cells have half of that- just one chromosome set. That vocabulary is confusing for a LOT of students. When we say two chromosome sets, we mean having two versions of each chromosome type. In most cases, if you saw a display of a person’s chromosomes, you may see 46 chromosomes but you would notice there are 23 TYPES of chromosomes. One set of 23 chromosomes from one parent and the other parent. So a diploid cell has two versions of every chromosome type — one version from each parent — which together make up two chromosome sets. However, a haploid cell has only one version of each chromosome type, so it has just one chromosome set. And this is true whether a chromosome is duplicated or not. Duplication makes chromatids, but it doesn’t change whether a cell is haploid or diploid. So, I could add another row to the table now about haploid and diploid. Here is meiosis with a new row added to show whether cells are haploid or diploid. Oh and we get asked this a lot - after meiosis I, these daughter cells are haploid because each one has just one version of every chromosome type. Meaning each daughter cell after meiosis I only has one set of chromosomes. And the cells would also be haploid after meiosis II. We can add the row for haploid and diploid to our mitosis table, too, although it’s not quite as exciting because it’s always diploid from start to end. Ah, beautiful, beautiful tables. Tables that we hope will help with understanding how chromosome numbers can change in cell division. Well, that’s it for the Amoeba Sisters, and we remind you to stay curious. --- *Источник: https://ekstraktznaniy.ru/video/33399*