فهم الانقسام المنصف: العملية التي تخلق التنوع الوراثي

Captions are on! Click the CC button at bottom right to turn off. Follow us on Twitter (@AmoebaSisters) and Facebook! Have you ever wondered how two siblings can have the same mom and dad and still look so

different? Well, today we’re going to talk about a process that makes that possible----a process called meiosis ----not to be confused with mitosis, which sounds unfortunately similar.

Mitosis makes identical body cells like your skin cells and stomach cells. Recall from our mitosis clip that since it makes identical body cells, mitosis is important for growth and for repair of damage or to replace worn out cells.

But NOT meiosis. Meiosis is a process that contributes to genetic variety. Meiosis also doesn’t make body cells.

It makes sperm and egg cells; otherwise known as gametes, the fancier word. You might recall that humans have 46 chromosomes. That’s how many chromosomes most body cells in your body have.

But there are some human cells that don’t have 46 chromosomes. Human sperm cells and egg cells have 23 chromosomes. Why the number difference?

Well, if a sperm cell has 23 chromosomes and an egg cell has 23 chromosomes, when they come together that makes 46 chromosomes. That will allow a newly formed fertilized egg to develop into a human.

Meiosis is what we call a reduction division because you have a starting cell that has 46 chromosomes and your ending cells---the sperm and egg cells---have only has 23 chromosomes. Before we start getting into the stages of meiosis to make gametes, we have to remember

what happens before meiosis can start. Actually, this also happens before mitosis. It’s the stage known as interphase.

If you remember interphase, it’s when the cell is growing, it’s replicating its DNA, it’s carrying out cell processes. Just like mitosis, interphase happens before meiosis is going to start.

So the starting cell has 46 chromosomes, and you have to duplicate those chromosomes in interphase before meiosis starts. That basically means you’re duplicating your DNA, since chromosomes are made of DNA

and protein. Ready for the tricky part? Because we tend to count chromosomes by the number of centromeres present, when the 46

chromosomes duplicate, we still say there are 46 chromosomes because the sister chromatids are still attached and we’re counting by centromeres. So 46 chromosomes here, they replicate in interphase, and you still have 46 chromosomes

in this picture. But you went from 46 to 92 chromatids. Little tricky there.

We have a detailed video that explains these chromosome numbers before and after replicating in interphase that can be useful for understanding meiosis. Ok so interphase checklist done---now we can move into meiosis.

You might remember the mitosis stages PMAT; the ‘p’ was for prophase, the ‘m’ for metaphase, the ‘a’ for anaphase, and the ‘t’ for telophase. The good news is that in meiosis, you still use those terms, but because meiosis is actually

a reduction division, you’re going from 46 chromosomes to 23. Which means you actually divide twice. So instead of mitosis where you divide one time and do PMAT one time, in meiosis, you’re

going to divide twice and therefore do PMAT twice. Because of this, in meiosis, you put numbers after the phases to indicate whether you’re in the first division or the second division.

So let’s dive right in. So let’s start with the very first step – prophase I. One thing I like to remember about prophase is “pro,” this actually means “before.”

It kind of helps you remember that it comes before all the other stages start. This is where the chromosomes are going to condense and thicken. They are actually going to line up with their homologous pairs.

The word homologous means that the chromosomes are approximately the same size and that they contain the same types of genes in the same locations. They are going to match up.

It is during this prophase 1 that this amazing process occurs called crossing over. I know crossing over probably sounds like something very different, but this is a really awesome process because when these chromosomes are lined up in homologous pairs, they have

a way that they can transfer their genetic information and exchange it between each other. It’s kind of like these chromosomes flop over each other and they do a little genetic information exchange here.

It makes for what we call---recombinant chromosomes---which can eventually contribute to the variety that we were mentioning that siblings can have even when they have the same parents. More about that later.

Now we move into metaphase I. In metaphase I, think if the M as standing for middle. The chromosomes are going to be in the middle of the cell.

It’s a little different, though, from mitosis because they’re still going to be in pairs in the middle of the cell so it’s not a single file line; they are in pairs in the middle.

During anaphase I, think A for away because the chromosomes are going to be pulled away by the spindle fibers.Then, we end with telophase I, where you have two newly formed nuclei and it becomes obvious you will end meiosis 1 with two new cells.

Cytokinesis follows with splitting the cytoplasm. But we’re not done yet. On to meiosis 2!

The very first step in meiosis II is prophase II. It’s not going to be nearly as eventful as it was in prophase I because they are not going to have homologous pairs.

They also are not going to have that amazing process called crossing-over. That doesn’t happen again in prophase II. You have your chromosomes and the spindles starting to form like in prophase I but prophase

II is not nearly as eventful of having that process of crossing over. In metaphase II, remember think m for middle, the chromosomes are going to line up in the middle.

This time, though, they are in a single file line. They are not in pairs like they were in metaphase I. Anaphase II, remember A for away, but this time it’s the chromatids that are getting pulled away by the spindle fibers.

Chromatids are getting pulled away to opposite sides of the cell. In telophase II, nuclei reform and the 2 cells are each going to divide so you can see here that 4 cells are going to be formed.

Cytokinesis will follow to completely split the cytoplasm. Now keep in mind that meiosis in males produces sperm cells and in females, it produces egg cells.

Because of independent assortment and also crossing over, you’re going to have variety. For example, in a male the four sperms cells that are produced each time, they are all different from each other.

They are also different from the starting cell because the starting cell had 46 chromosomes and the ending cells only have 23. So they are not identical to the original and they are not identical to each other.

This is going to lead to variety. A reason why two siblings with the same parents can look different from each other---they still developed from an unique egg and unique sperm cell that came together.

One last thing to think about. Scientists are often looking into the process of meiosis because sometimes the chromosomes don’t separate correctly.

It’s called nondisjunction when a cell can receive too many or too few chromosomes in the separation. This contributes to some genetic disorders, which is something scientists continue to

study. Well that’s it for the Amoeba Sisters, and we remind you to stay curious.