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15.3: Mitotic Phase - Mitosis and Cytokinesis

  • Page ID
    30733
  • 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 genetic material? To answer that question, you first need to know more about DNA and the forms it takes.

    Fluorescent labeled cell during mitosis
    Figure \(\PageIndex{1}\): Dividing cell stained with fluorescent dyes. You can see chromosomes in blue and spindles in green.

    The Forms of DNA

    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 in Figure \(\PageIndex{2}\). Because DNA has already replicated, 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.

    Replicated Chromosome  with sister chromatids and centromere
    Figure \(\PageIndex{2}\): Chromosome. After DNA replicates, it forms X-shaped chromosomes like the one shown here. 1. Chromatid, 2. Centromere, 3. short arm, 4. long arm. Centromere contains proteins called kinetochores (not shown) where spindles attach during mitosis.

    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. Mitosis 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.

    Cell going through Mitotic steps
    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.

    Prophase

    Prometaphase

    Figure \(\PageIndex{4}\): Prophase in later stage is called prometaphase. The spindle starts to form during the prophase of mitosis. The spindles start to attach to the Kinetochores of centromeres of sister chromatids during Prometaphase.

    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 blue spindle, shown in Figure \(\PageIndex{4}\), consists of fibers made of microtubules.

    Metaphase

    During metaphase, spindle fibers fully attach to the centromere of each pair of sister chromatids. As you can see in Figure \(\PageIndex{5}\), 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:

    Metaphase of Mitosis
    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. Some spindles from the opposing centrioles attach with each other, and some spindles attach to the kinetochores of the sister chromosomes from their respective sides. Each chromosome is attached to two spindles.

    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 \(\PageIndex{6}\)). At the end of anaphase, each pole of the cell has a complete set of chromosomes

    A cell with 2n=4 in its anaphase of mitosis
    Figure \(\PageIndex{6}\): 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.

    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 (see Figure \(\PageIndex{7}\).

    A cell in Mitotic Telophase
    Figure \(\PageIndex{7}\): Telophase: The chromosomes decondense, spindles start to disappear, two nuclei form in a cell.

    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 in Figure \(\PageIndex{8}\). In animal cells, the plasma membrane of the parent cell pinches inward along the cell’s equator until two daughter cells form. In the 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 in Eukaryotic animal and plant Cells
    Figure \(\PageIndex{8}\): Cytokinesis is the final stage of eukaryotic cell division. It occurs differently in animal (left) and plant (right) cells. You can see a microfilament ring forming at the center of the elongated animal cell. This creates a depression called cleavage furrow. This invagination ultimately separates the cell cytoplasm into two cells. A cell plate forms at the center of the elongated plant cell. Then a new plasma membrane and cell wall form along each side of the cell plate.

    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:
      1. How DNA always looks in eukaryotic cells
      2. How DNA in eukaryotic cells looks once it is replicated and the cell is about to divide
      3. Female sex chromosomes only
      4. How DNA appears immediately after cytokinesis
    6. Which of the following is not part of a chromosome in eukaryotic cells?
      1. Centriole
      2. Centromere
      3. Chromatid
      4. 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:
      1. separation of sister chromatids
      2. DNA replication
      3. cytokinesis
      4. lining up of chromosomes in the center of the cell
      5. condensation and coiling of DNA into a chromosome
    9. Why do you think the nuclear envelope breaks down at the start of mitosis?
    10. What are 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.

    Attributions

    1. Mitosis fluorescent by US government, public domain via Wikimedia Commons
    2. Chromosome by Dietzel65, CC BY-SA 3.0 via Wikimedia Commons
    3. Mitosis schematic by M3.dahl, CC BY-SA 3.0 via Wikimedia Commons
    4. Prometaphase by LadyofHats, Public domain, via Wikimedia Commons
    5. Metaphase by Matt, released into the public domain via Wikimedia Commons
    6. Anaphase by Matt, released into the public domain via Wikimedia Commons
    7. Telophase by Matt, released into the public domain via Wikimedia Commons
    8. Cytokinesis by LadyofHats for CK-12 licensed CC BY-NC 3.0
    9. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0