Lesson Video: Mitosis | Nagwa Lesson Video: Mitosis | Nagwa

Lesson Video: Mitosis Science

In this video, we will learn how to describe the events of interphase and mitosis, outline the significance of this process for unicellular and multicellular organisms, and outline how cancer can develop.

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Video Transcript

In this video, we will learn how to describe the different stages of mitosis as well as interphase. We will explain the importance of mitosis in living organisms, and we will discuss the relationship between mitosis and cancer.

So what exactly is mitosis? You may be surprised to learn that you’re experiencing lots of mitosis right now. Some scientists have estimated that around 300 billion cells die and are replaced in the human body every day. And all of these cells are being replaced by mitosis. Mitosis is a process that’s mainly used for the growth, maintenance, and repair of multicellular organisms like you and me.

First, let’s talk a little bit about growth. Years ago, you started off as a single cell called a zygote. That zygote divided into two cells, and then those two cells divided into four. Four became eight, eight became 16, and so on. This process continued until you had all of the cells that you needed to be born. Over the years, that same process has allowed you to grow into who you are today. The trillions of cells that you’re made of right now all came from that one cell that you started out as. And all of these cells were made by mitosis.

We typically call the type of cell division used for growth and repair of organisms mitosis. However, it’s useful to know that mitosis is specifically the name of the set of steps that allows the nucleus of a eukaryotic cell to make an exact copy of itself. You will recall that eukaryotic cells are cells that have a nucleus. All multicellular organisms are made of eukaryotic cells. Multicellular organisms include animals, plants, some fungi, and even some protists.

While the cells of these different types of organisms have different features, one thing that they all have in common is that their genetic material called DNA is stored and protected inside of an organelle called a nucleus. The genetic material, or DNA, works the same way in all living things. DNA holds the code that tells the cell what to do. DNA is responsible for determining all of the traits of different organisms.

The DNA inside of the nucleus is arranged in chromosomes. In sexually reproducing organisms, the chromosomes usually occur in pairs. This means that the cell actually has two sets of similar chromosomes, one set that came from one parent and a second set that came from the other. We call these kinds of cells that have two sets of chromosomes diploid. Di- is a prefix that means two. Human diploid cells have 46 chromosomes, or 23 pairs. And those chromosomes are illustrated here.

Your typical house cat is made of cells with 38 chromosomes, or 19 pairs. The cells of a chicken have 78 chromosomes, or 39 pairs. A common species of tomato plant has 24 chromosomes within each diploid cell, or 12 pairs. The cells of a common fungus that we typically call bread mold have 14 chromosomes, or just seven pairs. Organisms that belong to the same species usually have the same number of chromosomes within the nucleus of each of their cells. In order to make new cells for growth or repair, these cells use mitosis to copy their nucleus and the chromosomes inside so that the new daughter cells are exactly the same or identical to the cells that they came from.

Mitosis occurs as a series of four steps. These steps are also known as the phases of mitosis. We call these four phases prophase, metaphase, anaphase, and telophase. However, cells are not always dividing. Most cells spend the majority of their life cycle in what we call interphase. Inter- is a prefix that means in between. Interphase is how the cell lives in between mitosis cycles. During interphase, the cell carries out its normal functions. Then at a certain point in interphase, the cell begins to prepare for mitosis. This preparation mainly occurs inside of the nucleus.

Instead of a human cell that has 46 chromosomes, let’s illustrate this process with the cell of a fruit fly that has eight chromosomes. In order to prepare for mitosis, during interphase the chromosomes of the cell make exact copies of themselves. This is called DNA replication. The cell will continue to exist in interphase with these replicated chromosomes until mitosis begins.

Mitosis begins with prophase. During prophase, several changes occur within the cell. The chromosomes become more visible. During interphase, the chromosomes are stretched out in long, thin strands and are not typically visible under a light microscope. These long strands are called chromatin. During prophase, the chromosomes coil into tightly condensed structures. This makes them easier to see. Each chromosome will be roughly X shaped.

Each half of the replicated chromosome is called a sister chromatid. The sister chromatids are exact copies of each other. In fact, when they separate from each other, each sister chromatid will be considered an individual chromosome. The sister chromatids were made when the chromosome was replicated during interphase. The place where the sister chromatids connect, which makes them look like an X, is called a centromere.

Also during prophase, the nucleus begins to break down. The nuclear envelope, which is the membrane surrounding the nucleus, begins to break apart and dissolve. In animal cells like our fruit fly cell, a special structure called a centrosome begins to form what we call a mitotic spindle. The mitotic spindle helps to keep everything organized during mitosis. It’s very important in the phases to come. Note that not all cells have a centrosome. In fact, plant cells do not have a centrosome. Those cells use other types of structures to move their chromosomes around during mitosis.

The second phase of mitosis is metaphase. During metaphase, the mitotic spindle attaches to the centromere of each of the chromosomes. The mitotic spindle helps the chromosomes to line up along the middle of the cell. We call this place where the chromosomes line up the metaphasic plate. We also sometimes refer to this location as the equator of the cell. The word equator means middle line, just like the equator that’s the imaginary middle line around our planet.

The third phase of mitosis is called anaphase. During anaphase, the sister chromatids separate from each other. The mitotic spindle pulls the chromatids towards opposite ends of the cell. Similar to how we describe the cell as having an equator, we sometimes refer to the opposite ends of the cell as poles, sort of like the North and South Pole on our planet. Remember that each of the sister chromatids is an exact copy of the other. So at the end of this step, each side of the cell has a full identical set of chromosomes.

The fourth and last phase of mitosis is called telophase. During telophase, two new nuclei begin to form. The chromosomes unwind and become less visible. The nucleus of the cell has successfully made an exact copy of itself. Now the cytoplasm is ready to divide in two. After mitosis, the cell and its contents split into two new cells. The phases of mitosis describe how one cell can split into two identical daughter cells.

Mitosis is generally used for the growth, maintenance, and repair of multicellular organisms. As organisms become larger and develop over their lifespan, mitosis creates the new cells that they need. When old cells wear out and die, mitosis replaces them with new ones. And when organisms get hurt, mitosis creates new cells to repair the damage.

Another function of mitosis is a certain kind of reproduction. Mitosis is also used for asexual reproduction in some simple multicellular organisms. For example, a hydra is a type of very small, simple animal that lives in fresh water. During reproduction by budding, a smaller hydra grows from the side of its parent organism. It starts off as a small clump of cells. And over time, it develops into a new individual. Once the offspring is large enough to survive on its own, it separates from its parent. And the offspring begins its life elsewhere. Because this offspring is the result of mitosis, it will be genetically identical to the parent that it came from.

Mitosis can be used in asexual reproduction, but be careful not to confuse it with binary fission. Mitosis primarily occurs in multicellular organisms. Although some unicellular organisms, like yeast, also reproduce by mitosis. Binary fission is the term we generally use to describe asexual reproduction in unicellular organisms.

Before we wrap up our lesson, let’s examine how mitosis plays a role in a common type of disease.

We mentioned earlier that most cells typically spend the majority of their life cycle in interphase. Interphase occurs between mitosis cycles, and it’s when the cells carry out their normal function. Sometimes something will happen to a cell that will make it skip over many of the important parts of interphase. In these cells, the cell cycle has been broken. This causes these cells to repeat mitosis without stopping. And as a result, they grow and multiply rapidly. When this occurs, the illness that it causes is called cancer.

Cancer cells grow and divide much more rapidly than normal cells. They usually first form a mass that we call a tumor. Sometimes, these rapidly dividing cells break away from the tumor and move around the body. When this happens, we say that the cancer has metastasized. The moving cancer cells can grow into new tumors in other places in the body and cause serious health issues. Since cancer is caused by the body’s own cells multiplying out of control, it can be very difficult to treat.

Let’s wrap up our lesson by taking a moment to review what we’ve learned about mitosis. In this video, we learned about mitosis. We learned that mitosis produces genetically identical daughter cells. We learned that mitosis functions in the growth, maintenance, and repair of multicellular eukaryotic organisms. And we learned that mitosis also plays a role in some types of asexual reproduction. We learned about the four phases of mitosis. Those phases are prophase, metaphase, anaphase, and telophase.

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