Video Transcript: Meiosis

Meiosis 

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. We recalled 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 cell, the sperm and egg cells have only 23 chromosomes. Before we can start getting into the stages of meiosis to make gametes, we have to remember what happens before meiosis can even start. Actually, this also happens before mitosis. It's the stage known as interphase. And if you remember, interphase, is when the cell is growing, it's replicating its DNA. It's carrying out cell processes. And just like mitosis, interphase happens before meiosis is even 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 are duplicating your DNA since chromosomes are made of DNA and protein, ready for the tricky part. What 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 actually 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 an interface that can be useful for meiosis. Okay, so interface check was done.

Now we can move into meiosis, you might remember the mitosis stages P met, the P was for prophase, the M was for metaphase, the A for anaphase. And the T for telophase. And 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 p mat, one time in meiosis, you're going to divide twice, and therefore do p mat twice. And 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.

Llet's start with the very first step: prophase. One thing I like to remember about prophase is Pro. This actually means before, and 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 they contain the same types of genes in the same locations, they're going to match up, it is during this prophase. One thing that happens is this amazing process occurs called crossing over. Now I know crossing over probably sounds like something very different. But this is actually really awesome process because this is when the chromosomes, they're lined up in homologous pairs. And they have this way that they can transfer their genetic information and exchange it between each other. It's kind of like these chromosomes pop over each other. And they do a little genetic information exchanged 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, one in metaphase. One can think of the M is standing for middle, the chromosomes are now going to be in the middle of the cell. It's a little bit different, though, from mitosis, because these chromosomes are going to be in pairs in the middle of the cell. So it's not a single file line. They're in pairs in the middle. Now during anaphase, one, think of a four away, because the chromosomes are going to be pulled away by the spindle fibers, then we end with telophase, one where you have two newly formed nuclei. And it becomes obvious that you're going to end meiosis one with two new cells. Cytokinesis follows with splitting the cytoplasm.

But we're not done yet. Onto meiosis two, the very first step in meiosis two is prophase. Two, it's not going to be nearly as eventful as it was in prophase. One though, because they're not going to have homologous pairs of chromosomes. They also are not going to have this amazing process called crossing over. That doesn't happen again and prophase two, you have your chromosomes, and the spindles are starting to form like they did in prophase one, but prophase two is just not as eventful as having that process of crossing over. In metaphase, to remember, think M four 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 one; anaphase two, remember a far away, but this time, it's the chromatids that are going to be pulled away by the spindle fibers. In telophase. Two, you can see the nuclei reforming, and you can also see that the two cells have divided there's going to be four cells forming cytokinesis will follow to completely split the cytoplasm.

Now keep in mind that meiosis and 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 sperm cells that are produced each time, they are all different from each other. They're also different from the starting cell because the starting cell had 46 chromosomes, and the ending cells only had 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 a unique egg and a 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