Video Transcript
In this video, we’ll learn about karyotypes, what they are, and what they represent. We’ll learn about chromosomes and the different number of chromosomes present in different types of cells. And we’ll learn about the procedure that scientists use to make the images that we call karyotypes.
In eukaryotic cells, the chromosomes are found within the nucleus. Chromosomes are the condensed form of our DNA, or genetic material. DNA is a long strands-like molecule with a double helical shape. When DNA condenses into chromosomes, it coils around special proteins which spiral around each other, eventually packing tightly together into what we call a chromatid. You may be used to seeing chromosomes represented in diagrams like this, or maybe you’re more familiar with this.
We get our best view of chromosomes when cells divide because all of the DNA that was stored in the nucleus is condensed into chromosomes. During cell division, each strand of DNA is first copied so that each daughter cell gets its own complete set of chromosomes. The copied strands of DNA are identical, and they’re called sister chromatids. They join together, forming the familiar shape at a point called the centromere. So even though these two representations look different, they’re both considered to be a single chromosome. The difference is that the X-shaped diagram shows a pair of identical replicated chromatid.
A karyotype is a picture of all of the chromosomes found within a cell. The word part karyo- means kernel or nucleus just like in the word eukaryote. So how do scientists take a picture of chromosomes? In order to karyotype a cell, a scientist will start by growing it in a lab. They will wait until the cell is about to divide. At this point, called metaphase, the chromosomes are all condensed and arranged in preparation for cell division. The scientist will use a chemical to freeze the cell at this exact moment. Then the cell is broken open, and the chromosomes are spread apart. Next, they’ll use a stain that colors the chromosomes with dark and light bands. Finally, they’ll use a microscope to take a picture of the chromosomes. The finished karyotype will look something like this.
There are two important things to notice about the finished karyotype image. First, the chromosomes are arranged in what we call homologous pairs. Sexually reproducing organisms get half of their chromosomes from one parent and half from another. Even though the chromosomes from each parent are not exactly the same, they match in some important ways. The homologous pairs have the same genes in the same places. Their centromeres are located in the same place, and the chromosomes are the same length. The second thing you’ll notice about the karyotype image is that the homologous pairs are arranged by size from largest to smallest.
Karyotypes are useful for a few things. They were originally developed as a way to compare and contrast the chromosomes of different species by botanists and taxonomists. In humans, the chromosomes laid out and arranged like this allow scientists and healthcare professionals to check for disorders and abnormalities. Here, we have a diagram of a karyotype of a healthy human cell. Every cell in the human body has 23 pairs of chromosomes, or 46 individual chromosomes.
The chromosomes are arranged and numbered in pairs by descending size, except for the last pair of chromosomes. These chromosomes are called sex chromosomes because they influence the biological sex of an organism. In humans, if this pair of chromosomes match, we call it XX, and the individual is likely female. If the chromosomes do not match, we call it XY, and this organism is likely male. The sex chromosomes are an exception to the size order rule. If we put them in size order, the X chromosome would actually be eighth in line. But by convention, the sex chromosomes are always pictured last.
We’ve already mentioned that one of the chromosomes in each homologous pair comes from one parent and that the second chromosome in each pair comes from the other parent. The genetic information on these chromosomes combines and works together to give an individual their unique traits. In order for this individual to reproduce, they need to pass on half their genetic material, one chromosome from each pair, to their offspring. They do this by producing sex cells, also called gametes. Since this individual had an X and a Y for their 23rd pair of chromosomes, they’re male, and the gamete that they produce will be a sperm. The female gamete is called an egg cell or an ova. These cells each have half of the genetic material of a typical body cell.
During reproduction, two gametes combine into a zygote, which will eventually develop into an offspring. Since the mother and the father each contributed half of their genetic material, the zygote has 46 chromosomes, or 23 pairs, which we’ve already learned is the correct number of chromosomes for normal human body cells. A typical human body cell is called a somatic cell. Soma- is a word part that means body. Somatic cells possess 46 chromosomes, or 23 pairs. The 23rd pair of chromosomes are called sex chromosomes. All of the rest of the chromosomes in a somatic cell are called autosomes. Somatic cells are referred to as diploid because their chromosomes are all paired. Di- is a prefix that means two.
On the other hand, gametes have half the number of chromosomes that a somatic cell has. So a gamete has 23 chromosomes, and these chromosomes are not in homologous pairs. Gametes are considered to be haploid since they possess half of the normal number of chromosomes. This pattern of diploid somatic cells and haploid gametes is the same in many reproducing organisms, including most animals and many plants. However, these organisms will likely have a different number of chromosomes than humans do. Let’s look at an example.
For example, your typical cattle possesses 60 chromosomes, or 30 pairs of chromosomes, in each somatic cell. That means that 60 is the diploid number of chromosomes for cattle. If 60 is the diploid number of chromosomes, what do you think the haploid number will be? Absolutely, the haploid number of chromosomes will be 30 since it’s half the diploid number. The cattle gametes will also have the haploid number of chromosomes. So we can expect for the sperm and the eggs of cattle to have 30 chromosomes each. You’ll recall that gametes have half of the normal number of chromosomes so that when they combine, the offspring will have the normal, or diploid, number of chromosomes.
Let’s try one more example. Picture we have a karyotype of a skin cell of a cat. With just this information, can you fill in all the blanks? Pause your video here and give it a try yourself. Well, the karyotype is for a skin cell, which we know is a type of somatic cell. Remember that soma- means body, and we can count 19 pairs of chromosomes, or 38 individual chromosomes. These 38 chromosomes are in pairs, which reminds us that 38 is the diploid number since di- means two. Half of that is the haploid number, or 19. And since gametes are haploid cells, they’ll each have 19 chromosomes.
The sex chromosomes always come last in a karyotype, and they’re recognizable because they’re not in high order. And we know that all of the chromosomes that are not sex chromosomes are autosomes. So the cat has 36 autosomes. Finally, since the sex chromosomes appear to be a homologous pair, we know that we would call those XX. And since they match, this cat is most likely female. We can learn a lot by looking at the karyotype of an organism.
Now that we’ve learned about karyotypes, what they are, how they’re made, and how to read them, let’s take a moment to review what we’ve learned. In this video, we learned about karyotypes, which are pictures of all of the chromosomes in a cell arranged from largest to smallest in homologous pairs. We learned that somatic cells, or typical body cells, have the diploid number of chromosomes, while gametes, or sex cells, have the haploid number of chromosomes. And we learned about the difference between autosomes and sex chromosomes.