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Mitosis - Advanced Douglas Wilkin, Ph.D. Say Thanks to the Authors Click http://www.ck12.org/saythanks (No sign in required) To access a customizable version of this book, as well as other interactive content, visit www.ck12.org CK-12 Foundation is a non-profit organization with a mission to reduce the cost of textbook materials for the K-12 market both in the U.S. and worldwide. Using an open-content, web-based collaborative model termed the FlexBook®, CK-12 intends to pioneer the generation and distribution of high-quality educational content that will serve both as core text as well as provide an adaptive environment for learning, powered through the FlexBook Platform®. Copyright © 2015 CK-12 Foundation, www.ck12.org The names “CK-12” and “CK12” and associated logos and the terms “FlexBook®” and “FlexBook Platform®” (collectively “CK-12 Marks”) are trademarks and service marks of CK-12 Foundation and are protected by federal, state, and international laws. Any form of reproduction of this book in any format or medium, in whole or in sections must include the referral attribution link http://www.ck12.org/saythanks (placed in a visible location) in addition to the following terms. Except as otherwise noted, all CK-12 Content (including CK-12 Curriculum Material) is made available to Users in accordance with the Creative Commons Attribution-Non-Commercial 3.0 Unported (CC BY-NC 3.0) License (http://creativecommons.org/ licenses/by-nc/3.0/), as amended and updated by Creative Commons from time to time (the “CC License”), which is incorporated herein by this reference. Complete terms can be found at http://www.ck12.org/terms. Printed: January 15, 2015 AUTHOR Douglas Wilkin, Ph.D. www.ck12.org C HAPTER Chapter 1. Mitosis - Advanced 1 Mitosis - Advanced • Identify and describe the main processes in mitosis. How do all your cells stay genetically identical? Mitosis, of course. As you can see, mitosis is a multistage process that ensures separation of sister chromatids, and ensures that daughter cells are just like the parent cell. Mitosis Mitosis is the division of the cell’s nucleus, the final step before two daughter cells are produced. Mitosis begins immediately at the conclusion of interphase, specifically at the end of the G2 phase. The cell enters mitosis as it approaches its size limitations. Four distinct phases of mitosis have been recognized: prophase, metaphase, anaphase, and telophase, with each phase merging into the next one ( Figure 1.1). See All cells arise from pre-existing cells at http://www.dnaftb.org/7/animation.html for an animation of Walther Flemming discussing his description of mitosis. http://www.biology.arizona.edu/Cell_bio/tutorials/cell_cycle/MitosisFlash.html has an excellent animation of mitosis. The Phases The phases of mitosis are discussed in the video: http://www.youtube.com/watch?v=LLKX_4DHE3I (20:42). 1 www.ck12.org FIGURE 1.1 Mitosis is the phase of the eukaryotic cell cycle that occurs between DNA replication and the formation of two daughter cells. What happens during mitosis? During mitosis, the nucleus divides, paving the way for two cells to be produced after cell division, each with a complete makeup of genetic material. MEDIA Click image to the left or use the URL below. URL: http://www.ck12.org/flx/render/embeddedobject/271 Prophase Prophase is the first and longest phase of mitosis, see Figure 1.2. During prophase, the chromatin (DNA) coils up into visible chromosomes, each made up of two sister chromatids held together by the centromere. Also during this phase, the nucleolus disappears, and the spindle begins to form from the centrioles. Most eukaryotic cells contain structures known as centrosomes, consisting of a pair of centrioles. During prophase, the centrioles begin to move to opposite ends, or poles, of the cell. As the centrioles migrate, the fiber-like spindle begins to elongate between the centrioles. The spindle is a thin, cage-like structure made out of microtubules. In plant cells, the spindle forms without centrioles. The spindle plays an essential role moving chromosomes and in the separation of sister chromatids. Preprophase As plant cells have some structural differences compared to an animal cell, an additional stage prior to prophase is necessary. In plant cells only, prophase is preceded by a preprophase stage. Plant cells have a large central vacuole encompassing the center of the cell. Prior to the division of the nucleus, the nucleus must migrate to the center of the plant cell. To accomplish this, the cell forms a phragmosome, a sheet of cytoplasm that bisects the middle of the cell. The phragmosome suspends the cell nucleus in the center of the cell in preparation for prophase. Additionally, during this phase the plane of cell division is established. The accurate control of division planes, which establishes the placement of the future cell wall, is crucial for the correct structure of plant tissues and organs. The mitotic spindle also starts to form during preprophase in plant cells. Unlike animal cells, plant cells do not have 2 www.ck12.org Chapter 1. Mitosis - Advanced FIGURE 1.2 The spindle starts prophase of mitosis. to form during Kinetochores on the spindle attach to the centromeres of sister chromatids. centrosomes to organize their mitotic spindles. Instead, in these cells, the nuclear envelope acts as a microtubule organizing center (MTOC) for spindle formation. The preprophase spindle forms by self-assembly of microtubules in the cytoplasm surrounding the nuclear envelope. Prometaphase During early prometaphase, the nuclear membrane disintegrates and microtubule spindles invade the center of the cell. Also during this phase, the spindle attaches to the centromere of each chromatid. Specifically, the spindle attaches to the kinetochore, a protein structure on the centromere where the spindle fibers attach. Metaphase During metaphase, the centromeres of the chromosomes line up along the metaphase plate or equatorial plane, in essence the approximate middle of the cell. This orientation of the chromosomes at the equator of the cell helps to ensure proper chromosome separation. This alignment allows the spindle fibers to correctly pull the chromatids to either pole of the cell, resulting in separation of sister chromatids from a chromosome, see Figure 1.3. FIGURE 1.3 Chromosomes, consisting of sister chromatids, line up at the equator (metaphase plate) of the cell during metaphase. 3 www.ck12.org Anaphase Anaphase is the phase in which the sister chromatids separate. The sister chromatids are pulled apart by the shortening of the microtubules of the spindles, similar to the reeling in of a fish by the shortening of the fishing line. One sister chromatid moves to one pole of the cell, and the other sister chromatid moves to the opposite pole. This process occurs when the proteins that bind sister chromatids together are cleaved, resulting in unattached identical chromosomes, essentially separate daughter chromosomes. These separate chromosomes are pulled apart by shortening spindle fibers, and pulled toward the centrosomes to which they are attached. At the end of anaphase the spindle fibers degrade. At this time, each pole of the cell has a complete set of chromosomes, identical to the amount of DNA at the beginning of G1 of the cell cycle. Telophase Telophase is essentially the opposite of prophase and prometaphase. The chromosomes begin to unwind back into chromatin in preparation to direct the cell’s metabolic activities. A new nucleus forms around each set of chromosomes. This is followed by cytokinesis, the division of the cytoplasm, resulting in two genetically identical cells, ready to enter G1 of the next cell cycle. The phases of mitosis are summarized in Figure 1.4. FIGURE 1.4 Mitosis in the Eukaryotic Cell Cycle. Mitosis is the multi-phase process in which the nucleus of a eukaryotic cell divides. In this diagram, prometaphase is not included as a separate phase, but incorporated into prophase. Cytokinesis Cytokinesis is the final step in cell division. It often occurs concurrently with telophase, though it is a separate process. Cytokinesis ( Figure 1.6) differs between plant and animal cells. In animal cells, the plasma membrane pinches inward along the cell’s equator until two cells are formed. Specifically, a cleavage furrow containing a contractile ring develops in approximately the middle of the cell (similar to the position of the metaphase plate), essentially pinching off the two nuclei and forming separate cells. In plant cells, a cell plate forms along the cells equator. A new membrane grows along each side of the cell plate, with a new cell wall forming on the outside of each new membrane. At the end of cytokinesis, each daughter cell has a complete copy of the genome of its parent cell. The end of cytokinesis marks the end of the M-phase, the end of one cell cycle, and the beginning of G1 and interphase of the next cell cycle. 4 www.ck12.org Chapter 1. Mitosis - Advanced FIGURE 1.5 This is a representation of dividing plant cells. Cell division in plant cells differs slightly from animal cells as a cell wall must form. Note that most of the cells are in interphase. Can you find examples of the different stages of mitosis? FIGURE 1.6 Cytokinesis is the final stage of eukaryotic cell division. It occurs differently in animal (left) and plant (right) cells. FIGURE 1.7 In this electron micrograph of a cell, the formation of two new cells is almost complete, as new membrane grows and divides the parent cell. Inside the Cell: The Stages of Mitosis can be viewed at http://www.youtube.com/watch?v=2WwIKdyBN_s (1:22). 5 www.ck12.org MEDIA Click image to the left or use the URL below. URL: http://www.ck12.org/flx/render/embeddedobject/272 Vocabulary • anaphase: Phase of mitosis during which sister chromatids separate and move to opposite poles of the cell. • cell plate: Forms during cytokinesis in plant cells; a new membrane grows along each side of the cell plate, with a new cell wall forming on the outside of each new membrane. • centriole: A cylindrical shaped cell structure composed of nine triplets of microtubules; structure from which spindle fibers originate. • cytokinesis: Division of the cytoplasm, forming two daughter cells. • kinetochore: The protein structure on chromatids where the spindle fibers attach during cell division. • metaphase: Phase of mitosis during which chromosomes line up at the equator of the cell. • metaphase plate: The center (equator) of a cell during mitosis; chromosomes line up at the metaphase plate to ensure the proper separation of the sister chromatids. • mitosis: The division of the nucleus into two genetically identical nuclei. • phragmosome: A sheet of cytoplasm that forms in highly vacuolated plant cells to prepare for mitosis; forms in the approximate center of the cell. • preprophase: Phase of mitosis prior to prophase; only in plant cells; ensures the nucleus is positioned in the center of the cell. • prometaphase: Phase of mitosis during which the nuclear envelope breaks down, centrioles separate, and a spindle moves through the center of the cell. • prophase: First phase of mitosis during which chromatin condense into chromosomes, the nuclear envelope breaks down, centrioles separate, and a spindle begins to form. • telophase: Last stage of mitosis during which chromosomes uncoil to form chromatin, the spindle breaks down, and new nuclear membranes form. Summary • Mitosis is the division of the nucleus. • Mitosis is the division of the nucleus; five distinct phases of mitosis have been recognized: prophase, prometaphase, metaphase, anaphase, and telophase. • Cytokinesis is the division of the cytoplasm. Cytokinesis occurs after mitosis. • At the end of cytokinesis there are two genetically identical daughter cells. 6 www.ck12.org Chapter 1. Mitosis - Advanced Explore More • Animal Cell Mitosis at http://www.cellsalive.com/mitosis.htm . Review 1. 2. 3. 4. Order the phases of mitosis. Describe the main processes involved in mitosis. Differentiate cytokinesis in animal and plant cells. Four phases of mitosis: can you describe what happens in each phase? References 1. 2. 3. 4. 5. 6. 7. Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation. . CC BY-NC 3.0 Courtesy of Nogales group and Lawrence Berkeley National Laboratory. . Public Domain Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation. . CC BY-NC 3.0 Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation. . CC BY-NC 3.0 Edmund Beecher Wilson. . Public Domain Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation. . CC BY-NC 3.0 Wadsworth Center. . Public Domain 7