Source: Image of Karyotype, Public Domain http://en.wikipedia.org/wiki/File:NHGRI_human_male_karyotype.png
Welcome to this lesson today on changes to chromosome structure. Today, we will be discussing different ways in which the structure of a chromosome can change and the effects that that can have.
A gene mutation is the change in the nucleotides that make up a gene. Sometimes, chromosome structure can be altered during cell division, which will lead to a certain type of gene mutation. The gene mutations we're going to be discussing today are deletions, translocations, and duplications. These are all ways in which the structure of a chromosome could change. And again, that change in the nucleotides that make up that gene is called a gene mutation.
Let's start by talking about deletions. A deletion is the removal of one or more genes on the chromosome. Let's take a look at an example.
Let's say our original chromosome looks something like this. And then during cell division, maybe a deletion occurs, where one or more of those genes are removed. Let's say that this gene right here, which we just have label as C, is removed. So our end result would look something like this. This is an example of a deletion-- the removal of one or more genes on a chromosome.
An example of a deletion is cri-du-chat. Cri-du-chat is a disorder that is caused by a deletion on chromosome number five. With this type of disorder, what happens is that a baby's larynx will not properly develop, so that when the baby cries, it will sound like a meowing cat. Their cry will sound like a cat, because this mutation causes an abnormally shaped larynx and can also be associated with other abnormal mental development as well. That is what a deletion is.
Translocations are when part of one chromosome will switch place with a corresponding part of another non-homologous chromosome. This type of gene mutation can lead to certain types of cancers. Let's take a look at an example.
Let's say our original chromosome, we have A, B, C, D, E, F. And then our non-homologous chromosome is going to be G, H, I, J, K, L. In a translocation, part of one chromosome will switch places with a corresponding part of the non-homologous chromosome. So in this case, our F and our L could switch places.
So their corresponding parts are non-homologous chromosomes that are switching places. And again, this can lead, as I mentioned, to certain types of cancers. So that is a translocation.
And then a duplication is when a nucleotide sequence is repeated. So let's say we have A, B, C, D, E, F. If a duplication occurs, we might end up with A, B, C, B, C, D, E, F. So right here is our duplication. We have the B, C portion was repeated.
Duplications are generally not harmful. And they actually have not even been linked to any particular genetic disorder, unlike deletions and translocations.
Let's take a look at this page here. We are going to be discussing karyotypes. Karyotypes are the arrangement of a person's complete set of chromosomes by length, shape, and the location of the centromere.
These pictures are taken well in metaphase of mitosis, because the chromosomes at that point are most easy to identify. These chromosomes will be photographed through the microscope, cut out, and arranged. They're arranged from the largest all the way down to the smallest.
We have, as humans, 23 pairs of chromosomes. You can go through and count, we have 23 pairs of chromosomes, which is a total of 46 chromosomes. We have 46 chromosomes. Half of them are from our mom, and half of them are from our dad.
In each pair we look at here, one chromosome would be from your mom, one would be from your dad. So it adds up to 46 total chromosomes.
Basically, these karyotypes allow you to see a picture of a person's chromosome. And what that can do is help to identify any abnormalities. If we take a look at this karyotype right here, the last pair of chromosomes in a karyotype is always the sex chromosomes.
We can tell by looking at this picture right here that it's a male, because the chromosomes are different. One is an X chromosome, and one is the Y chromosome. So this is a karyotype of a male.
Trisomy 21 is a genetic disorder in which a person has three copies of chromosome number 21. By looking at a karyotype, it can allow you to see any abnormalities. This is chromosome pair number 21, so they just have the regular two copies. They do not have trisomy 21, also known as Down Syndrome.
But let's say that we did have an extra copy right here. We would then know, by looking at this karyotype, that this individual had three copies of chromosome 21 and, therefore, had Down Syndrome. So you can see how karyotypes could be useful for identifying abnormalities.
This lesson has been an overview on changes to chromosome structure and how changes to chromosome structure can affect an individual.