In this explainer, 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. How many cells are there in the human body: thousands, millions, billions? There are 37.2 trillion cells in the human body. A trillion is a very large number. Onetrillion seconds, for example,
is about 30 000 years. All humans start from a single fertilized egg cell. So, how can we go from a single cell all the way to 37.2 trillion cells? Let’s look at a unicellular organism first, the amoeba, as an example of how cell division typically works. You might recall that amoeba are single-celled organisms that reproduce asexually by binary fission. In the following figure, you can see how a single amoeba can divide into two
cells. The parent amoeba duplicates its nucleus and cytoplasm and can then split into two daughter cells. However, we are not amoeba. Mitosis is the way in which our cells divide to make a copy of themselves. This is how one cell can split into two cells and then four cells, and so on. All of our cells come from preexisting cells in our body. Mitosis is a type of cell division in which one cell divides to produce two new cells that are
genetically identical. Multicellular organisms, such as humans, have two basic types of cells. The cells of the body are called somatic cells and include all cells of the body except for the egg or sperm cells. These egg or sperm cells are called reproductive or sex cells. Somatic cells are the cells that make up the body of an organism, excluding the sex cells. Somatic cells are important for growth and tissue repair, so they
need to constantly divide to make new copies. If you are cut or injured in some way, a scab forms, under which new cells need to divide to replace the damaged cells. The way these cells divide is called mitosis. Let’s look more closely at how a cell can undergo mitosis to make a copy of itself. This process takes place in the nucleus of the cell, which you can see in the following figure. The nucleus is the part of the cell that contains the genetic material of the
cell, called the DNA. The DNA is organized into chromosomes. Humans have 46 chromosomes. Normally, chromosomes are only visible in the nucleus when mitosis begins. Chromosomes are long, coiled molecules of DNA that contain proteins. Humans have 46 chromosomes, whereas other species can have a different number of chromosomes. For example, the nucleus of a wheat somatic cell contains 42 chromosomes, and the nucleus of a dog’s somatic cell
contains 78. This means that the number of chromosomes can be used to help determine the animal or plant species. Human somatic cells contain 46 chromosomes, which are arranged into two sets of 23 chromosomes each: one set from the egg cell and the other from the sperm cell. Since we have two sets of chromosomes in our somatic cells, we can call these cells diploid. The word “diploid” comes from the Greek “diploos,” which means double. You can see some of the chromosomes in the following
figure. A diploid cell is a cell that has two sets of chromosomes. Reproductive cells, such as sperm or egg cells, only have one set of chromosomes. In humans, this means that these cells have only a single set of 23 chromosomes. Since there is only one set of chromosomes in these cells, they are called haploid. The word “haploid” comes from the Greek “haploos,” meaning single. A haploid
cell is a cell that only has a single set of chromosomes. Which statement about chromosomes is true? Cells can be categorized as either somatic cells or reproductive cells. Somatic cells include all the cells in our body, excluding egg or sperm cells, and reproductive cells are egg or sperm cells. The nucleus of the cell contains DNA, which is where the genetic information is stored. The DNA is organized on structures called chromosomes. As humans, we have a total of 46 chromosomes in our somatic cells. Normally, these chromosomes are not visible in the nucleus of the cell. They only become visible as the cell prepares to divide. The number of chromosomes is usually the same across the same species. Humans usually have 46 chromosomes, whereas a different species, like dogs, usually has 78 chromosomes. Humans actually have two pairs of 23 chromosomes, making up a total of 46 chromosomes in our somatic cells. The first set of 23 chromosomes comes from the egg cell, and the other set comes from the sperm cell. Since our somatic cells contain two sets of chromosomes, they are called diploid. Reproductive cells contain only one set of chromosomes, so they are called haploid. Therefore, the statement “members of the same species usually have the same number of chromosomes” is correct regarding chromosomes. Before we can talk about mitosis, we need to first mention that mitosis is actually a small step of a larger process called the cell cycle. The cell cycle is the process that somatic cells go through when they are dividing. Before mitosis can begin, interphase must take place. The interphase is the stage at which the DNA is copied and takes much longer than mitosis. You can see this in the image below. Key Term: Cell CycleThe cell cycle is a series of steps that allow a cell to grow and divide. Key Term: InterphaseInterphase is a stage of the cell cycle at which the chromosomes are duplicated. The interphase is the stage at which the chromosomes duplicate. Generally, before duplication, each chromosome is made of one long DNA molecule. During the interphase, each chromosome is copied to make two molecules with identical DNA that are each called a chromatid. These are joined at a central point called the centromere. You can see this in Figure 5. As we will see, these chromatids are separated during mitosis to become individual chromosomes in the daughter cells. Key Term: ChromatidA chromatid is one half of a duplicated chromosome. Example 2: Recalling the Structure of a ChromosomeThe figure shows a chromosome about to go through mitosis. 
AnswerThe nucleus of the cell contains the DNA, which is where the genetic information is stored. The DNA is organized on structures called chromosomes. As humans, we have a total of 46 chromosomes in our somatic cells. Mitosis is a type of cell division in which one cell divides into two identical copies of itself. Before mitosis can begin, the chromosomes must first be duplicated during a process called interphase. Generally, before duplication, each chromosome is a single long molecule of DNA. During the interphase, the chromosome is copied to make two molecules with identical DNA, called the chromatids. These are joined at a point called the centromere. You can see this in the image below. Part 1 Using the description provided, we can determine that X in the image is labeling a centromere. Part 2 Using the description provided, we can determine that Y in the image is labeling a chromatid. It is actually not possible to see the individual chromosomes during the interphase. Instead, the chromosomes exist as long molecules of DNA that are all mixed up together with special proteins called histones. This is called chromatin, and you can see this in the following figure. Key Term: ChromatinChromatin is a complex formed when the DNA is associated with histone proteins. After the DNA is duplicated during the interphase, the cell can enter mitosis. The first stage of mitosis is called the prophase. Before this point, the chromosomes in the nucleus were mixed up as chromatin. During the prophase, the long molecules of DNA in the chromosomes begin to compact tightly. These condensed chromosomes are visible in the nucleus under the microscope. By the end of the prophase, the nucleolus and nuclear membrane begin to break down. Outside the nucleus, structures called centrosomes begin to form long fibers called the mitotic spindle. These spindle fibers are important because they can attach to the duplicated chromatids to separate them during mitosis. You can see these features in the following figure. Key Term: ProphaseProphase is the first step of mitosis and meiosis. It is when the chromosomes begin to condense and become visible under the microscope. Key Term: CentrosomeA centrosome is an organelle that forms the mitotic spindle, which can separate duplicated chromatids during mitosis. Key Term: Mitotic SpindleThe mitotic spindle is a group of fibers formed from the centrosome that separate duplicated chromatids during mitosis. The second stage of mitosis is called the metaphase. During the metaphase, the mitotic spindle formed from the centrosome attaches to the centromere of the duplicated chromosomes. Next, the duplicated chromosomes are pulled with the fibers of the mitotic spindle to line them up along the middle, or the equator, of the cell. You can see this in the following figure. Key Term: MetaphaseMetaphase is the second step of mitosis or meiosis at which the chromosomes are lined up along the middle, or the equator, of the cell. Example 3: Recalling Some of the Structures Involved in MitosisThe figure shows a cell going through mitosis.
AnswerPart 1 Mitosis is a type of cell division in which one cell divides into two identical copies of itself. Before mitosis can begin, the chromosomes must first be duplicated during a process called the interphase. Generally, before duplication, each chromosome is a single long molecule of DNA. During the interphase, the chromosome is copied to make two molecules with identical DNA, called chromatids. These are joined at a point called the centromere. You can see this in the image below. Mitosis is a process that involves multiple steps. In the first step, called the prophase, a structure called the centrosome begins to form long fibers called the mitotic spindle. These are important because they can attach to the centromere of the duplicated chromosome to eventually split the chromatids apart. The second step is called the metaphase. Here, the mitotic spindle attaches to the centromeres of the duplicated chromosomes and aligns them along the middle, or the equator, of the cell. You can see this in the image below. The next two steps are called the anaphase and telophase. The anaphase is when the mitotic spindles pull each chromatid to one pole of the cell, and the telophase is when the cells divide, each with a copy of the chromosomes. Therefore, structure A is called a centrosome. Part 2 Using the information provided in part 1, we can conclude that structure B is the mitotic spindle. Part 3 Using the information provided in part 1, we can conclude that the stage of mitosis shown in the figure is the metaphase. The third stage of mitosis is called the anaphase. During the anaphase, the two chromatids begin to separate as the mitotic spindle begins pulling them apart. Each chromatid migrates to one of the cell’s poles. You can see this in the following figure. Key Term: AnaphaseThe anaphase is the third step of mitosis, at which the replicated chromosome is split and the chromatids are each pulled to opposite poles of the cell. The fourth and final stage of mitosis is called the telophase. The telophase is the reverse of the prophase. During the telophase, the two chromatids arrive at the poles of the cell as the cell begins to divide into two. A new nuclear membrane begins to form around this new set of chromosomes, and the cytoplasm begins to split. The chromosomes decondense and become no longer visible. By the end of mitosis, two diploid daughter cells that are identical to one another are formed. You can see this in the following figure. Key Term: TelophaseThe telophase is the fourth stage of mitosis, at which a nuclear membrane forms around the chromosomes and the cytoplasm is split. At the end of the telophase, two daughter cells are formed. Relationship: MitosisProphase ⟶ Metaphase ⟶ Anaphase ⟶ Telophase Example 4: Recalling the Steps of MitosisWhich is the correct sequence of events that occur when a somatic cell divides?
AnswerMitosis is a type of cell division in which one cell divides into two identical copies of itself. Before mitosis can begin, the chromosomes must first be duplicated during a process called the interphase. Generally, before duplication, each chromosome is a single long molecule of DNA. During the interphase, the chromosome is copied to make two molecules with identical DNA, called chromatids. These are joined at a point called the centromere. You can see this in the image below. After the chromosomes are duplicated, mitosis can begin. Mitosis involves several steps. The first step, called the prophase, is when the duplicated chromosomes become visible under the microscope. Here, a structure called the centrosome begins forming the mitotic spindle, which is a group of fibers that are used to separate the two chromatids. During the metaphase and anaphase, the duplicated chromosomes line up along the middle of the cell, where each chromatid is separated to opposite poles of the cell. The final step is called the telophase and is when the cytoplasm of the cell and the cell itself divide to form two genetically identical cells. Therefore, the correct sequence is the following: chromosomes duplicate ⟶ chromosomes become visible ⟶ duplicated chromosomes separate ⟶ cytoplasm divides in two. Mitosis is a necessary part of an organism’s life. This process allows cells to divide to repair damaged tissue or, for an organism, to grow. In some organisms, mitosis can also be used for asexual reproduction. Mitosis is a highly regulated process and, if left unchecked, can lead to uncontrolled cell division or cancer. This can result in a mass of tissue called a tumor. Key Term: CancerCancer is a group of diseases that are characterized by uncontrolled cell division. Key Term: TumorA tumor is a mass of cells produced by uncontrolled cell division. Some tumors develop in a localized manner and do not spread to other areas of the body. These are called benign tumors and usually do not cause any harm. Malignant tumors are tumors that can spread throughout the body and grow elsewhere and may lead to death. Cancer is the second leading cause of death worldwide and in 2018 was responsible for 9.6 million deaths. There are numerous therapies available for treating cancer. For example, one therapy involving nanotechnology is now being studied. Nanotechnology is a field of study that makes use of incredibly tiny particles to benefit humankind in some way. These particles are on the scale of a nanometre, typically 1–100 nm. To give you an idea of how small that is, the thickness of a human hair is 80 000 nanometres. Key Term: NanotechnologyNanotechnology is a field of study that makes use of very tiny particles on the scale of a nanometre. For cancer treatment, nanotechnology is being used to target tumors. For example, a company has developed gold-plated nanoparticles that can be injected into the bloodstream, where they can circulate around the body and accumulate in tumors. Then, when a laser is used, these gold particles are heated up, destroying the tumor cells around them. Another interesting application of mitosis involves liver transplants. In general, the liver plays an important role in the detoxification of certain molecules and produces chemicals needed for digestion and growth. Liver transplants do not always require a full liver, and in some cases a partial liver transplant is sufficient. This involves taking only a portion of the liver from the donor to implant in the recipient. This can be done because the cells of the liver exhibit a high rate of mitosis and can replace themselves in both the donor and the recipient. Let’s recap some of the key points we have covered in this explainer. Key Points
What phase divides the cytoplasm into 2?During the mitotic (M) phase, the cell separates its DNA into two sets and divides its cytoplasm, forming two new cells.
During which phase of the cell cycle does cytoplasm divide?Cytokinesis. Cytokinesis is the second part of the mitotic phase during which cell division is completed by the physical separation of the cytoplasmic components into two daughter cells.
What is it called when the cytoplasm divides into 2 identical cells?Mitosis is a process of nuclear division in eukaryotic cells that occurs when a parent cell divides to produce two identical daughter cells.
In which stage the cell begins a stage by which the cytoplasm divides resulting in two cells with identical nuclei?Telophase and Cytokinesis
The nuclear membrane then reforms, and the chromosomes begin to decondense into their interphase conformations. Telophase is followed by cytokinesis, or the division of the cytoplasm into two daughter cells. The daughter cells that result from this process have identical genetic compositions.
|
