Chromosome Numbers During Division: Demystified!

Closed captioning is on. To turn off, click CC button at bottom right. Follow the amoebas on Facebook and Twitter (@AmoebaSisters)! If you have ever studied mitosis or meiosis,
chances are, the whole chromosome number thing, it’s crossed your mind. How do you start out
with a cell with 46 chromosomes, And then it does mitosis, and then you end with two cells that each have 46 chromosomes? Or in meiosis you end op with 4 cells that each have 23 chromosomes. “Here’s a chromosome,
there’s a chromosome, everywhere a chromosome chromosome…” but how? This video is going to explain the chromosome numbers in these processes, but please watch our mitosis and meiosis videos first, so that you can understand the actual proccess first. And also out videos are about chromosome numbers in human cells, so please assume that for the entire video. And just a reminder: human body cells have 46 chromosomes, and human sex cells, also called gametes, have 23 chromosomes. Chromosomes, as you may know, are made up
of DNA and protein. The DNA is highly condensed—-tightly wound—that’s important because it means
that when cells split and they need to share that DNA, it’s easy to do that when the
DNA has condensed into chromosomes. It makes DNA so portable and organized. Which is great
in processes like mitosis and meiosis, because when you make new cells, you also need to
get DNA into those cells. This is a chromosome. This is also a chromosome. The difference is that the first one only
has one chromatid, or copy. The second one has two chromatids, or copies. The area in the middle of the chromosomes are called the centromere And the centromere is where the spindle is going to attach That is kind of a big deal because that is how you are going to move the chromosomes during division. Generally, when counting chromosomes, you can count the number of centromeres present. Interphase is a step that happens one time
before mitosis and meiosis. In this stage, we duplicate chromosomes. So we start with
46 of these chromosomes. which is how many chromosomes are found in human cells and as you can see in our little picture,
they look like little sticks. And after duplicating—interphase—we still
have 46 chromosomes technically, but they have two chromatids. What we call sister chromatids.
Held together at one centromere. As long as the chromatids are still attached at the centromere,
we still count 46 chromosomes. but they are duplicated and if they weren’t attached, you could say there are 92 chromosomes. But right now in this pictures, there are 46 chromosomes and 92 chromatids. So here’s a little chart here because we
love charts. So is there a time in mitosis when there are 96 chromosomes? Technically,
maybe you could argue that at anaphase when the sister chromatids are separating and no
longer attached, you would have 92 chromosomes as they aren’t attached at the centromere
during that phase. That’s why our chart has a little asterisk here. Although we usually
still refer to them as chromatids during the actual process of cell division. At the end
of mitosis, those chromatids separate and you end up with 2 cells after mitosis that
each have 46 chromosomes. Those “chromosomes” look just like sticks. The two daughter cells are identical to the starting cells in mitosis. Now, let’s take a look at meiosis. We had 46 chromosomes but they are
only made up of one chromatid. They duplicate at interphase. We still have 46 chromosomes
but now there are 92 sister chromatids! At the end of meiosis I, you have 2 daughter cells and they each contain 23 chromosomes with 46 chromatids. At the end of meiosis II, you have 4 cells with each 23 chromosomes and 23 chromatids. Just a reminder that in mitosis the cells
are identical so they have the same number of chromosomes as the original cell.
So the two daughter cells after mitosis have 46 chromosomes and are identical.
But in meiosis, it says reduction devision. The 4 daughters cells only have half of the amount of
chromosomes as the original cell. That is why each daughter cell after meiosis have 23 chromosomes So we hope that clears it up a little bit for you.
Just remember that chromosome counting, it has a lot to do with the way we count chromosomes
and this definition of chromatid. That’s it for the amoeba sisters and we
remind you to stay curious!

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