Sperm meets egg, which then undergoes multiple cell divisions to make a living organism with trillions of cells all doing their own specialised function. Impressive, but where do the egg and sperm come from? The answer is, as you may have guessed, from the title meiosis.
First, it's good to know that there are two types of cell division processes. The simpler one is mitosis, which produces two identical cells with exactly the same genetic information. You can think of them as clones of each other.
And to find out more, why not look at our videos about mitosis? The other process is called cell division. is meiosis. It's a much more complicated process creating not two but four cells with only half the number of chromosomes and crucially all genetically different from each other.
Both mitosis and meiosis include the same phases. Prophase, metaphase, anaphase and telophase. Except in meiosis they happen twice so they usually refer to as one and two. The easiest way to remember these phases names is to remember IPMAT.
So let's look at meiosis in more detail. As always cellular division starts with a process called DNA replication. This involves making two identical copies of the original DNA molecule. The cell ends up temporarily with double the normal number of chromosomes.
You can learn more about DNA replication in this video. In Prophase 1, the duplicated chromosomes join up with the pair from the other parent. So the mother's pair bind with the father's pair, forming a group of two chromosomes called homologous chromosomes. Now, What happens next is vital for successful meiosis. As each chromosome is lined up next to its partner's pair, one chromatid from each side gets entangled with a corresponding chromatid from the other side.
This is called crossing over. During this brief period the two chromatids swap certain sections of DNA. This is called recombination. The sections that they trade correspond to the same location.
so that each chromatid retains the correct number of genes. Recombination is really important because it creates variety. The new cells aren't identical to the other parents and they also are different to one another as well.
They are new genetic combinations. In fact, that's the whole point of sexual reproduction. It increases genetic variability.
Each chromatid is now different and as each one will end up in a separate gamete it means each sex cell is genetically different from all others. This explains why brothers and sisters are different despite having the same parents. Only identical twins have the same genetic makeup as they both originate from the exact same egg and sperm.
Now back to meiosis. Next comes metaphase 1. As the chromosomes align themselves up in the middle of the cell, and in anaphase 1, the spindle fibers pull the chromosomes apart to opposite ends. And during telophase 1 and cytokinesis, the cell pinches apart in the middle, and the nuclear membrane reforms around the two new daughter cells.
That's the end of meiosis 1. Now for part 2. We start with our recombined daughter cells, each still with 46 chromosomes, but sperm and egg cells only have 23 chromosomes, so we need to cut these cells in half. Here goes a second round of division. The process is exactly the same as before, except that there is no DNA replication. We start straight with prophase 2, with chromatin clumping again to form chromosomes. Then a line in the middle of the cell during metaphase 2 and chromatids are pulled apart during anaphase 2 by the spindle fibers.
Telephase and cytokinesis pinch the cells together, with four new granddaughter cells being formed. The end of meiosis gives us 4 different sick cells, each with only 23 chromosomes, ready for future fertilization. But that's a story for another video.
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