Adrianne has a master's degree in cancer biology and has taught high school and college biology.
Stop to think for a moment about your immediate family. Do you have some physical characteristics that look more like one of your parents than the other? Are there some characteristics that are the same among you and your siblings? I bet you can readily tell the differences between you and your siblings. Is there a member in your family that doesn't look anything like the rest of you guys at all? How does that happen when you guys all came from the same two people?
One of the major reasons for the similarities and differences is the process of meiosis. Meiosis is the division of genetic material and multiplication of cells to produce reproductive cells. Chromosomes are the coiled form of the genetic material that is visible when cells are ready to multiply. Meiosis creates reproductive cells that have 23 chromosomes. When we are created, a reproductive cell from each parent joins together and contributes its 23 chromosomes. All of our cells end up with a set of 23 chromosomes that contains one copy of each chromosome from our moms and one from our dads.
Well, so far this sounds like you and your sisters and brothers should look just alike. That would be the case if certain events didn't take place when each reproductive cell is created. Let's see how this seemingly simple process leads to the differences within your family.
The beginning of the process is usually the best place to start anything, so let's do that here. Meiosis starts with one cell that contains a set of 23 chromosomes, giving a total of 46 chromosomes. Remember that each of the 23 chromosomes is different and there are two copies for each chromosome. Just before the start of meiosis, the chromosomes duplicated, so there are two copies of the mom's chromosomes and two copies of the dad's chromosomes. The two copies of each chromosome are referred to as homologous chromosomes.
The homologous chromosomes may or may not be in the same place in the cell when it is time for meiosis to start. It is imperative that the homologous chromosomes get together prior to the cell dividing in meiosis. The coming together and pairing of homologous chromosomes is referred to as synapsis. Synapsis takes place during prophase I, or the first stage of meiosis. It helps to ensure that each cell created when the cell divides has a full set of chromosomes.
Now we're getting to the fun part where we all become a little different. When synapsis happens, the homologous chromosomes end up next to each other. Each set of homologous chromosomes contains the same genes on them in the same location. There are two identical strands of genetic material that make up each chromosome called sister chromatids. The set of homologous chromosomes will be composed of four sister chromatids called a tetrad.
One of the sister chromatids from each chromosome will have the chance to exchange genetic material with a sister chromatid from the homologous chromosome. This exchange of material is called crossing over. The name tells you what you need to know. The sister chromatids cross over each other and everything from the point of cross over down to the end of the chromatid is switched between the chromatids.
Crossing over makes the sister chromatids in the same chromosome different now. Any change in the original form of the genetic material is called a mutation. One of the sister chromatids will contain the same exact genetic material as one parent. The other sister chromatid in the chromosome will have a mixture of genetic material from both parents. It is even likely that there may be more than one crossing over that occurs between the same two chromatids. This really changes the genetic material.
So, you may still not get exactly why this creates differences between you and your siblings. You have to bear in mind that meiosis continues until 4 cells are produced that contain one sister chromatid for each of the 23 chromosomes. One reproductive cell will have a sister chromatid that is like the mom, two reproductive cells will have mixed sister chromatids, and the last reproductive cell will have a sister chromatid that is like the dad. That means the genetic material in each reproductive cell is different. To make things even more interesting, each of the 23 sets of chromosomes goes through this independently of each other, so the combinations of sister chromatids in the reproductive cells varies every time the process of meiosis occurs.
Boy did that get complicated fast! Let's recap what we went over just now. First, we discussed that meiosis is the division of genetic material and multiplication of cells to produce reproductive cells. We then covered that chromosomes are the coiled form of the genetic material. We also learned that the two copies of each chromosome are called homologous chromosomes. We know there are two identical strands of genetic material that make up each chromosome called sister chromatids. The set of homologous chromosomes will be composed of four sister chromatids called a tetrad.
The next thing we went over is the coming together and pairing of homologous chromosomes referred to as synapsis which takes place during prophase I , the first stage of meiosis. You should now know that synapsis makes crossing over, or the exchange genetic material with a sister chromatid from the homologous chromosome, possible. The other thing to note is that any change in the original form of the genetic material is called a mutation.
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