Lesson Video: Chromosomal Abnormalities Biology

In this video, we will learn how to identify chromosomal abnormalities from a karyotype and explain the effects of these abnormalities.

15:19

Video Transcript

In this video, we’ll learn more about chromosomal abnormalities. We’ll cover some common abnormalities, such as Klinefelter syndrome, Turner syndrome, and Down syndrome. We’ll learn more about a karyotype and how do we interpret this to identify these kinds of chromosomal abnormalities.

We can find our DNA in the nucleus of most of our cells. We have a lot of it. If we were to take out all the DNA from the nucleus and line it up, we would get about two meters worth of DNA in a single human cell. This DNA isn’t one continuous molecule. Instead, we have 46 molecules of DNA inside our cells that we call chromosomes. Let’s take a closer look and see what these look like. Our chromosomes are numbered one to 22, with chromosome one being the largest and 22 being the smallest. We call chromosomes one to 22 autosomes.

We also have chromosomes X and Y, which we call the sex chromosomes because they determine our biological sex. Having one copy of the X chromosome and one copy of the Y chromosome produces a biological male, while two copies of the X chromosome produce a biological female. We have two copies of each chromosome that we call a homologous pair. We get one copy from our biological father and the other copy from a biological mother. So, we really have 23 pairs of chromosomes. To give a total of 46 chromosomes. We can actually see how many chromosomes our cells have by performing a karyotype.

A karyotype, like the one shown on the right, is an image of an organism’s chromosomes that can be used to look for chromosomal abnormalities. The problem is most cells don’t have their chromosomes in its compacted and X-shaped structure that we see here. A typical cell will have its DNA all mixed up, like we see here. This big mess has 46 individual chromosomes, but it’s not possible to see them separately. In real life, they’re not colored blue and pink as we’re showing here. So, to perform a karyotype, we have to pause the cell during a critical point of the cell’s division where they take on this condensed and X-shaped structure. Let’s look at this in more detail.

You’ll recall that the cell cycle is a series of phases that dividing cells go through in order to divide. It’s separated into interphase and mitosis. During interphase, the cell copies its DNA so it can be separated into a new cell during mitosis. In interphase, the cell’s chromosomes are all mixed up as you see here. To make things a little easier to see, let’s just look at one homologous pair. So now, we see the two copies of a single chromosome, which are long molecules of loosely compacted DNA. During interphase, these chromosomes replicate to make a copy of themselves. Then, as a cell enters mitosis, these replicated chromosomes compact or condense to eventually form this familiar X-shaped structure, which you can see over here.

These duplicated and condensed chromosomes are what we want in a karyotype because it makes it much easier to identify each chromosome because they’re condensed and not long strings of DNA anymore and they’re all separated from each other. If we were to let the cell continue through mitosis, then the duplicated chromosomes would separate as the cell prepares to divide. And this is what normally happens during mitosis. But we don’t want this to happen because these duplicated and condensed chromosomes are exactly what we’re looking for for our karyotype. So to prevent mitosis from continuing on, we use a chemical called the mitotic inhibitor to prevent mitosis from continuing. Mitosis is therefore paused just at this perfect moment where the duplicated and condensed chromosomes are.

Remember, we’re just showing one homologous pair here. But in reality, there’s 23 pairs or 46 chromosomes in total. It’s not quite 46, but it gives you an idea of what this might look like. So now that mitosis is paused at just the right time, we take a picture and use a computer to organize the duplicated and condensed chromosomes to give the karyotype that we see here. This makes it very easy to see if there’s any differences in the chromosome number and structure.

So now that we know how to do a karyotype, we found that in certain individuals there are different chromosomal abnormalities. In some cases, structural changes to chromosomes can occur where parts of a single chromosome can be deleted or duplicated. And in other cases, individuals may have an extra copy of a chromosome, so now there’s three copies. And in other cases, one of the two copies of a particular chromosome might be missing altogether. This can happen because of errors during the formation of the sex cells or gametes, such as the egg or sperm cells.

Each gamete has half the number of chromosomes needed to form the embryo. So instead of having 23 pairs, the sperm cell just has one chromosome for each of the 23 different chromosomes, as shown here in blue. And the same goes for the egg cell; it just has one copy of each of these 23 chromosomes, shown here in pink. This way when a sperm fertilizes the egg, the 23 chromosomes from the sperm come together with the 23 chromosomes in the egg to make the 46 chromosomes in the zygote. This can then go on to develop into an embryo and a baby with the proper number of chromosomes.

This sperm and egg came from the biological father and biological mother, both of whom had 46 chromosomes. These 46 chromosomes are split in half to make gametes that contain 23 chromosomes during a special type of cell division called meiosis. Although only two gametes are shown here from each parent, meiosis actually produces four gametes from a single cell, but two are shown here for simplicity. Sometimes errors in meiosis don’t reduce the chromosome number properly and an egg or a sperm cell can have an extra chromosome or one might be missing a chromosome.

So, when the egg and sperm come together during fertilization, the resulting zygote will have a chromosome number that isn’t 46 and is therefore abnormal. These abnormal chromosome numbers can cause a variety of syndromes, and we’ll talk about that in a moment. But first, why is this so important to have 46 chromosomes and not any more or less? Each chromosome has a lot of information. Chromosome one is about 249 million base pairs long and has about 2000 protein-coding genes. If we look at all of our chromosomes, we have about 20,000 protein-coding genes in all. And we have two copies of each chromosome.

So here’s chromosome one with its two copies. And let’s say that there’s three genes being expressed that we’ll call A, B, and C. And on the bottom, we can see that these genes are being expressed at different levels. Gene B is being expressed twice as much as gene A, for instance. Inside the cell, these expression levels are carefully coordinated and balanced to give us these amounts. So, what happens if we add another chromosome? Since we now have three copies of chromosome one and therefore another copy of genes A, B, and C, the expression levels of each gene increase, and this might not be good for the cell.

Imagine baking a cake and using too much flour or sugar or some other ingredient. The results might not be very good. Likewise, our cells are optimized to use two copies of each gene in most cases. And by changing these amounts, the cell can suffer. This concept is called gene dosage and is why having an extra copy of a chromosome can cause problems. While having an extra chromosome can be tolerated in some cases, having only one copy of a chromosome is often lethal. In this case, the expression of these genes is too low and many cells can’t function normally. So having a single copy of a chromosome can be lethal, often very early as an embryo.

Now that we understand karyotypes, how we get abnormal chromosome numbers, and why this matters, let’s look at some real-life examples and see what the impact is. The first example we’ll look at is with Down syndrome. Down syndrome was first identified in a group of patients by the physician Dr. John Langdon Down.

In Down syndrome, we have an extra copy of chromosome 21. So, these individuals have a total of 47 chromosomes rather than 46. Having an extra copy of chromosome 21 often causes different physical and mental traits associated with the syndrome. These include delays in language development, heart defects, a flattened facial profile. Individuals with Down syndrome are generally shorter in height, have short, broad hands, and have issues with learning.

With appropriate support, most adults with Down syndrome are able to have a high quality of life and live independently. Down syndrome is a good example of what happens when there’s an extra autosome. But what about the sex chromosomes? Klinefelter syndrome was first described by Dr. Harry Klinefelter in a group of men. Klinefelter syndrome is characterized by an additional X chromosome. So, they have a total of three sex chromosomes and 47 chromosomes in all.

Some of the characteristics of Klinefelter syndrome are that they are tall in height, they have infertility problems, reduced development of facial hair, and more pronounced breast development. Some of these traits are caused by decreases in the hormone testosterone. Even though there’s an extra X chromosome in these individuals, they still have the Y chromosome as well, which allows them to develop male reproductive organs normally. Because the traits of this syndrome can be mild, many males with Klinefelter syndrome are never diagnosed and have a high quality of life.

Turner syndrome was first described by doctor Henry Turner who was studying a group of females. Normally, females contain two copies of the X chromosome as we can see here. But in Turner syndrome, females only have a single copy of the X chromosome, so they have 45 chromosomes in total instead of 46. Turner syndrome is actually the only example where having a single copy of a chromosome is tolerated. As mentioned earlier, loss of a chromosome is often lethal for the embryo.

Some characteristics of Turner syndrome include a webbed neck, where they have extra folds of skin in the neck area, defects in the heart and kidneys, infertility problems, and they’re generally short in height. Because the X chromosome is missing which contains the genes for female hormones, like estrogen, there is a reduction in reproduction. This can lead to underdevelopment of the ovaries and infertility. Hormone therapy can be used to support these individuals who can then go on to have a high quality of life.

Now, let’s take a look at a practice question to apply what we’ve learned in this video.

The passage provided outlines how chromosomal abnormalities can be caused. Chromosomal abnormalities can be caused by changes in the blank of chromosomes or a complete loss or blank of entire chromosomes. Which word would be most appropriate to replace the first blank? (A) Appearance, (B) bonding, (C) replication, or (D) structure.

This question is asking us about chromosomes and chromosomal abnormalities. So let’s take a moment to review what a chromosome is. Inside most of our cells, we have a nucleus which contains our DNA. This DNA is organized into 46 chromosomes that we could see here. Chromosomes are numbered from one to 22, with chromosome one being the largest and chromosome 22 being the smallest, while chromosomes X and Y are sometimes called our sex chromosomes because they determine our biological sex.

We have two copies of each chromosome. One comes from our biological father and the other comes from our biological mother. So, in total, we have 23 pairs of chromosomes for a total of 46 chromosomes. If we take a closer look at one of these chromosomes, we can see that there are many genes within it. These genes are needed for the cell and for us to properly function. Chromosomal abnormalities exist in some individuals where parts of a chromosome are missing or duplicated. These kinds of structural changes in a chromosome can impact the genetic information on it and cause problems for the cell. Therefore, the word that is most appropriate to replace in the first blank in this passage will be “structure.”

Now, let’s look at the second part of this question.

Which word would be most appropriate to replace the second blank? (A) Fusion, (B) specialization, (C) segmentation, or (D) gain.

Besides structural changes in a chromosome, entire chromosomes can be missing or duplicated. Some syndromes, such as Down syndrome, are caused by an additional copy or a gain of chromosome 21. Because there’s now three copies of this chromosome, there’s now additional copies of the genes that it contains. This additional genetic information can cause problems for the cell that are manifested as Down syndrome. Therefore, the word that’s most appropriate to replace the second blank is “gain.”

Therefore the passage now correctly reads “Chromosomal abnormalities can be caused by changes in the structure of chromosomes or complete loss or gain of entire chromosomes.”

Now let’s go over the key points that we covered in this video. Humans have a total of 46 chromosomes or 23 pairs of chromosomes. A karyotype is an image of an organism’s chromosomes and is what we use to identify any chromosomal abnormalities. Errors in forming gametes, like the egg or sperm cell, can cause a gain or loss of chromosomes. Down syndrome is caused by three copies of chromosome 21 instead of the normal two copies. Klinefelter syndrome affects males and is caused by two copies of the X chromosome and a single copy of the Y chromosome. Turner syndrome affects females and is caused by a missing X chromosome. So, these females only have a single copy of the X chromosome instead of having the normal two.

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