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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!