It’s meiosis that gives us the male and female reproductive cells known as the egg and sperm. In meiosis, two cell divisions take place, which produces four genetically unique cells with half the number of chromosomes as the parent cell.
I have a brother. He has brown hair and brown eyes. I don’t look much like him because I have blonde hair and blue eyes. We share the same parents, yet we don’t share all of the same genes.
For instance, my brother got the gene for brown eyes, and I got the one for blue eyes. While there are a number of things to consider, it’s basically meiosis that gave me different genes than my brother.Meiosis is a type of cell division that produces gametes. The term gametes is just the scientific way of saying sperm and eggs.
During meiosis, one parent cell divides into two cells and then divides again to make a total of four cells. During all of this dividing, genes get shuffled around, and the number of chromosomes gets cut in half. So the result of meiosis is four genetically unique cells, or gametes, with half the number of chromosomes as the parent cell. Now that we kind of have an overview, let’s take a closer look at how this works.Before we go too far, I want to point out something. Meiosis is different than mitosis, which you might have learned about in a different lesson. In mitosis, we see body cells divide to form identical body cells.
For example, skin cells make more skin cells. Mitosis is mainly about growing new cells and repairing or replacing damaged cells, whereas meiosis is all about reproduction. After all, it’s meiosis that gives us the sperm and the egg.Okay, now that we have that straight, we need to understand that the sperm and egg are different than other cells in your body.
They are different because they only have half the number of chromosomes, or genetic material. A basic human body cell contains 46 chromosomes, but a gamete only contains 23 chromosomes. That might seem strange, but it works out really nicely when an egg and a sperm come together, which is a process called fertilization.
When fertilization occurs, each gamete contributes 23 chromosomes. Add 23 and 23 together, and you get 46. In other words, you create the first full body cell of the new human.
Meiosis is a big event in a cells life, but I should point out that most of a cell’s life is spent in interphase. This is a phase of a cell’s life in which the cell is growing, replicating DNA and carrying out general activities. It’s during interphase that a cell gets ready for meiosis.When we learn the steps of meiosis, we can use a mnemonic that you might have learned if you studied mitosis, which is PMAT. This stands for prophase, metaphase, anaphase and telophase. This is helpful, but it gets a little tricky.
You see, in meiosis we need to go through these steps twice. The first trip through, we get two new cells with equal numbers of chromosomes. The second trip is what reduces the number of chromosomes in half. Want some good news? The steps that happen during the first and second division keep the same names and simply add numbers. So, the first division, called meiosis I uses prophase I, and the second division, called meiosis II, uses prophase II and so on.
Let’s start at the beginning with prophase I. This happens before any other step. You can remember this name if you recall that the prefix ‘pro-‘ can mean ‘before,’ just like the prologue of a book is that part that appears before the story. In prophase I, we see changes start to happen with the chromosomes or genetic material of the cell. Specifically, the chromosomes condense and meet up to form homologous pairs.Homologous is a big word, but it really just means matching.
Basically, each chromosome finds another chromosome that is about the same size and structure. This matching pair of chromosomes contains the same types of genes, which allows some shuffling of genetic information to take place. We call this crossing over, and it forms new hybrid or unique chromosomes.
This is what eventually is going to lead to the gametes carrying unique genetic information and why people look different, even if they are siblings.Now we are ready to move into metaphase I, where the homologous pairs of chromosomes line up in the middle of the cell. Remember that metaphase and middle both start with ‘m,’ and it might help you recall what is happening in this step.Next we have anaphase I.
I like to think of the ‘a’ as standing for ‘away’ or ‘apart,’ because in anaphase, the homologous pairs pull apart and move away from each other. Their goal is to reach opposite ends or poles of the cell.When the chromosomes reach the opposite poles we call it telophase I, and the parent cell can now become two cells.
At this point meiosis I is done, but we have to keep going. You see a cell that reproduces by meiosis starts out with twice as much DNA as it needs, so it’s must go through meiosis II to reduce that amount.
What’s the first step? You guessed it, prophase II! Now we no longer have homologous pairs and there is no crossing over, like we saw with prophase I, so prophase II is a bit boring. We do see the chromosomes condense making them more visible, so that’s at least something.In metaphase II the chromosomes line up in the middle of the cell so we can still use our ‘m’s (metaphase and middle) to recall what is going on here.
In anaphase II we can still use the ‘a’s that stand for ‘away’ and ‘apart’ to recall what is going on. But, what happens is the chromosomes split, the chromosome pieces are pulled apart and move away from each other. This splits the genetic material so each new cell will get one-half of the genetic information.All that is left now is telophase II. In telophase II the chromosomes reach the opposite poles, but remember that we had two cells to divide.
At the end of telophase II, we get four unique cells that contain half of the genetic information. In other words, these are the gametes made through the process of meiosis.
Let’s review. Meiosis is a type of cell division that produces gametes, which we commonly call the sperm and egg.The steps of meiosis can be remembered using the mnemonic PMAT, which stands for prophase, metaphase, anaphase and telophase. But we learned that we have to go through these steps twice because there are two divisions taking place during meiosis.
We call the divisions meiosis I and meiosis II.Meiosis I starts with prophase I, and we see the chromosomes condense and meet up to form homologous pairs. This connection allows for crossing over, which is the shuffling of genetic information. Then the cell moves into metaphase I, where the homologous pairs of chromosomes line up in the middle of the cell. Next we have anaphase I, where the homologous pairs pull apart and move away from each other, followed by telophase I, which gives us two cells as the chromosomes reach the opposite poles.Meiosis II involves prophase II at which time the chromosomes condense followed by metaphase II, which is when the chromosomes line up in the middle of the cell.
Next comes anaphase II, where we see the chromosomes split and the chromosome pieces are pulled apart and move away from each other. This leads to the final step, which is telophase II, when the chromosomes reach the opposite poles. The result of meiosis is four genetically unique cells, or gametes, with half the number of chromosomes as the parent cell.
When you are finished with this lesson, you should be able to:
- Describe meiosis
- Recall what gametes are and what makes them different from other human body cells
- Name and describe the steps of meiosis
- Explain what occurs during meiosis I and II