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7.3: Mitotic Phase - Mitosis and Cytokinesis

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    22486
  • Divide and Split

    Can you guess what this colorful image represents? It shows a eukaryotic cell during the process of cell division. In particular, the image shows the nucleus of the cell dividing. In eukaryotic cells, the nucleus divides before the cell itself splits in two; and before the nucleus divides, the cell’s DNA is replicated, or copied. There must be two copies of the DNA so that each daughter cell will have a complete copy of the genetic material from the parent cell. How is the replicated DNA sorted and separated so that each daughter cell gets a complete set of the genetic material? To answer that question, you first need to know more about DNA and the forms it takes.

    Mitosis-fluorescent
    Figure \(\PageIndex{1}\): (Mortadelo2005, public domain; via Wikimedia.org; Mitosis fluorescent.jpg#file)

    The Forms of DNA

    Replicated Chromosome
    Figure \(\PageIndex{2}\): Chromosome. After DNA replicates, it forms chromosomes like the one shown here. 1. Chromatid, 2. Centromere, 3. short arm, 4. long arm (Original version: Magnus Manske, this version by Dietzel65 [CC BY-SA 3.0]; via Wikimedia.org)

    Except when a eukaryotic cell divides, its nuclear DNA exists as a grainy material called chromatin. Only when a cell is about to divide and its DNA has replicated does DNA condense and coil into the familiar X-shaped form of a chromosome, like the one shown below. Because DNA has already replicated (during S phase of interphase) when it coils into a chromosome, each chromosome actually consists of two identical copies. The two copies of a chromosome are called sister chromatids. Sister chromatids are joined together at a region called a centromere.

    The process in which the nucleus of a eukaryotic cell divides is called mitosis. During mitosis, the two sister chromatids that make up each chromosome separate from each other and move to opposite poles of the cell. This is shown in the figure below. Mitosis actually occurs in four phases. The phases are called prophase, metaphase, anaphase, and telophase. They are shown in Figure \(\PageIndex{3}\) and described in detail below.

    Mitosis schematic diagram
    Figure \(\PageIndex{3}\): Mitosis is the phase of the eukaryotic cell cycle that occurs between DNA replication and the formation of two daughter cells. Mitosis has four substages, prophase, metaphase, anaphase, and telophase. (Original: Jpablo cad and juliana osorio translation: MattDerivative work: M3.dahl [CC BY-SA 3.0 ]; via Wikipedia.org; Mitosis schematic diagram-en.svg)

    Prophase

    Spindle and chromatids
    Figure \(\PageIndex{4}\): Spindle. The spindle starts to form during prophase of mitosis. Kinetochores on the spindle attach to the centromeres of sister chromatids during metaphase. (lbl.gov/Science-Articles; Courtesy of Nogales group and Lawrence Berkeley National Laboratory; CC BY-NC 3.0)

    The first and longest phase of mitosis is prophase. During prophase, chromatin condenses into chromosomes, and the nuclear envelope (the membrane surrounding the nucleus) breaks down. In animal cells, the centrioles near the nucleus begin to separate and move to opposite poles of the cell. Centrioles are small organelles found only in eukaryotic cells that help ensure the new cells that form after cell division each contain a complete set of chromosomes. As the centrioles move apart, a spindle starts to form between them. The spindle, shown in the diagram below, consists of fibers made of microtubules.

    Metaphase

    During metaphase, spindle fibers attach to the centromere of each pair of sister chromatids. As you can see in the figure below, the sister chromatids line up at the equator, or center, of the cell. The spindle fibers ensure that sister chromatids will separate and go to different daughter cells when the cell divides. Some spindles do not attach with the centromeres of chromosomes, rather, they attach with each other and grow longer. The elongation of spindles not attached to the centromeres. They elongate the whole cell. This is visible in the figure below:

    Mitotic Metaphase
    Figure \(\PageIndex{5}\): Chromosomes, consisting of sister chromatids, line up at the equator or middle of the cell during metaphase. The blue lines are spindles, and the orange rectangles at the cell poles are centrioles. (LadyofHats [Public domain]; via Wikipedia.org; Mitotic Metaphase.svg)

    Anaphase

    During anaphase, sister chromatids separate and the centromeres divide. The sister chromatids are pulled apart by the shortening of the spindle fibers. This is a little like reeling in a fish by shortening the fishing line. One sister chromatid moves to one pole of the cell, and the other sister chromatid moves to the opposite pole (see figure below). At the end of anaphase, each pole of the cell has a complete set of chromosomes

    Mitotic Anaphase

    Figure \(\PageIndex{5}\): Anaphase: Sister chromatids break apart and move to the opposite pole with the help of spindles. The newly separated sister chromatids are called chromosomes now. (LadyofHats [Public domain]; via Wikipedia.org; Mitotic Anaphase.svg)

    Telophase

    the chromosomes reach the opposite poles and begin to decondense (unravel), relaxing once again into a stretched-out chromatin configuration. The mitotic spindles are depolymerized into tubulin monomers that will be used to assemble cytoskeletal components for each daughter cell. Nuclear envelopes form around the chromosomes, and nucleosomes appear within the nuclear area.

    Mitotic Telophase

    Figure \(\PageIndex{6}\): Telophase: The chromosomes decondense, spindles start to disappear, two nuclei form in a cell. (LadyofHats [Public domain]; via Wikipedia.org; Mitotic Anaphase.svg)

    Cytokinesis

    Cytokinesis is the final stage of cell division in eukaryotes as well as prokaryotes. During cytokinesis, the cytoplasm splits in two and the cell divides. The process is different in plant and animal cells, as you can see from the diagrams below. In animal cells, the plasma membrane of the parent cell pinches inward along the cell’s equator until two daughter cells form. In plant cells, a cell plate forms along the equator of the parent cell. Then, a new plasma membrane and cell wall form along each side of the cell plate.

    Cytokinesis Eukaryotic animal and plant Cells
    Figure \(\PageIndex{7}\): Cytokinesis is the final stage of eukaryotic cell division. It occurs differently in animal (left) and plant (right) cells. (LadyofHats; CK-12 Foundation; CC BY-NC 3.0)

    Summary

    • Except when a eukaryotic cell divides, its nuclear DNA exists as a grainy material called chromatin. After DNA replicates and the cell is about to divide, the DNA condenses and coils into the X-shaped form of a chromosome. Each chromosome actually consists of two sister chromatids, which are joined together at a centromere.
    • Mitosis is the process in which the nucleus of a eukaryotic cell divides. During this process, sister chromatids separate from each other and move to opposite poles of the cell. This happens in four phases, called prophase, metaphase, anaphase, and telophase.
    • Cytokinesis is the final stage of cell division, during which the cytoplasm splits into two and two daughter cells form.

    summary of mitosis and cytokinesis

    Figure \(\PageIndex{8}\). Karyokinesis (or mitosis) is divided into five stages—prophase, prometaphase, metaphase, anaphase, and telophase. We should note that this is a continuous process and that the divisions between the stages are not discrete. The pictures at the bottom were taken by fluorescence microscopy (hence, the black background) of cells artificially stained by fluorescent dyes: blue fluorescence indicates DNA (chromosomes) and green fluorescence indicates microtubules (spindle apparatus). (credit “mitosis drawings”: modification of work by Mariana Ruiz Villareal; credit “micrographs”: modification of work by Roy van Heesbeen; credit “cytokinesis micrograph”: Wadsworth Center/New York State Department of Health; scale-bar data from Matt Russell)

    Review

    1. Describe the different forms that DNA takes before and during cell division in a eukaryotic cell.
    2. Identify the four phases of mitosis in an animal cell, and summarize what happens during each phase.
    3. Explain what happens during cytokinesis in an animal cell.
    4. What are the main differences between mitosis and cytokinesis?
    5. The familiar X-shaped chromosome represents:

      A. How DNA always looks in eukaryotic cells

      B. How DNA in eukaryotic cells looks once it is replicated and the cell is about to divide

      C. Female sex chromosomes only

      D. How DNA appears immediately after cytokinesis

    6. Which of the following is not part of a chromosome in eukaryotic cells?

      A. Centriole

      B. Centromere

      C. Chromatid

      D. DNA

    7. What do you think would happen if the sister chromatids of one of the chromosomes did not separate during mitosis?

    8. Put the following processes in order of when they occur during cell division, from first to last:

      A) separation of sister chromatids; B) DNA replication; C) cytokinesis; D) lining up of chromosomes in the center of the cell; E) condensation and coiling of DNA into a chromosome

    9. Why do you think the nuclear envelope breaks down at the start of mitosis?

    10. The fibers made of microtubules that attach to the centromeres during mitosis are called ____________.

    11. True or False. Chromosomes begin to uncoil during anaphase.

    12. True or False. During cytokinesis in animal cells, sister chromatids line up along the equator of the cell.

    13. True or False. After mitosis, the result is typically two daughter cells with identical DNA to each other.

    Explore More

    Watch the video below to visualize mitosis.

    The animation below demonstrates how nondisjunction can occur.