Chapter 11 - The Cell Cycle- Review Learning Objectives: • Explain the four phases of the eukaryotic cell cycle. • Explain the difference between mitosis and cytokinesis. • Understand the various ways the cell cycle is controlled. • Explain the consequences of uncontrolled cell division. All cells come from preexisting cells via cell division. Mitosis is cell division that occurs in somatic cells. • Mitosis generates daughter cells that are genetically identical to the parent cell. • Mitosis is usually accompanied by cytokinesis, the division of cytoplasm. I. Mitosis and the Cell Cycle A. Mitosis and cytokinesis are responsible for three key events:Growth, Wound repair, Reproduction B. What is a chromosome? (see section II below)C. Cells alternate between M phase and interphase 1. The cell cycle is the order of events that occur between the formation of a eukaryotic cell and mitosis. 2. Interphase is the time of the cell cycle during which the cell is not dividing but is replicating itsDNA (S phase) and ? 3. M phase, or mitosis, is the phase of the cell cycle in which cell division occurs, when the cell separates the replicated DNA copies. 4. Mitosis generates two genetically identical cells. D. The discovery of the gap phases1. Because there was a 4- to 5-hour lag time between when scientists exposed the cells to the radioactive thymidine and when they observed mitosis, they concluded that there is an intervening phase, which they called the second gap, or G2. 2. Because they observed M-phase cells for 4 to 5 hours after the radioactive thymidine exposure, they concluded that was the duration of mitosis. 3. Because the entire cell cycle lasted about 18 to 24 hours, they concluded that the length of time between the end of mitosis and the next synthesis phase was 7 to 9 hours. They called this the first gap, or G1. F. The cell cycle1. Figure 11.3 is a comprehensive view of the cell cycle. 2. G1 allows the parent cell to grow large enough and generate enough organelles that the daughter cells will function normally. 3. G1 gives the cell time to do its job in your body. 4. S phase  duplication of DNA (replication)5. G2 allows the cell to build the protein machinery necessary for mitosis. II. How Does Mitosis Take Place? A. Chromosome structure (Fig. 11.4) 1. DNA is complexed with proteins (histones) into chromatin. 2. At the beginning of mitosis, chromatin condenses to form a compact, mobile structure. a. Each replicated chromosome is called a chromatid. b. Each set of chromatids is joined at a centromere. c. Chromatids from the same chromosome are called sister chromatids and are exact copies of one another. B. Events in Mitosis1. Prophase (Fig. 11.5, step 2) 1a. Prophase is preceded by DNA replication - interphase. b. DNA condenses. c. The mitotic spindle is built. (1) Mitotic spindles are built from microtubules. (2) In animals, mitotic spindles arise from centrosomes. (3) Mitotic spindles extend into the center of the cell toward the nucleus. 2. Prometaphase (Fig. 11.5, step 3) a. The nuclear envelope breaks down. b. Spindle fibers from each centrosome attach to one of a pair of sister chromatids at the kinetochore. c. The centrosomes begin to move to opposite poles of the cells. d. Spindle fibers attached to kinetochores begin to move the chromosomes to the middle of the cell. 3. Metaphase (Fig. 11.5, step 4) a. The centrosomes complete their migration and reach the opposite poles. b. This results in all of the chromosomes becoming lined up with their kinetochores at the metaphase plate. 4. Anaphase (Fig. 11.5, step 5) a. Spindles attached to kinetochores begin to shorten, exerting a force on the sister chromatids thatpulls them apart. b. Spindle fibers that are not attached to kinetochores begin to push against each other via motor proteins that connect them, causing the poles of the cell to be separated even more. c. Spindle fibers continue to shorten, pulling chromatids to opposite poles. d. This process ensures that each daughter cell gets identical sets of chromosomes. 5. Telophase (Fig. 11.5, step 6) a. The nuclear envelope re-forms around the two new nuclei. b. The spindle fibers break down. C. Cytokinesis results in two daughter cells. (Fig. 11.5, steps 7 and 8) 1. In plants, a cell plate forms. (Fig. 11.6a) a. Vesicles from the Golgi apparatus carrying cell-wall material migrate to the middle of the cell between the two new nuclei. b. The vesicles build up and fuse, forming a new cell membrane and a cell wall dividing the two new nuclei. 2. In animals, a cleavage furrow forms between the two nucleia. The furrow is made from a ring of microfilaments that surround the inside circumference of the cell. b. The microfilaments contract, causing the cell membrane to constrict and pinch the cell in two. 3. The mechanism of cell division in bacteria is similar to cytokinesis in animals. Ftsz protein ring  Fission (Fig. 11.7) D. How do chromosomes move during mitosis? 1. Mitotic spindle forces: Spindle fibers are made of microtubules. a. Microtubules are lengthened and shortened by the addition and loss of tubulin subunits. b. Mitotic spindle shortening during anaphase is a result of the loss of tubulin subunits. III. Control of the Cell Cycle A. The discovery of cell-cycle regulatory molecules 1. Researchers conducted cell fusion experiments to identify the existence of a molecular cell-cycle switch in cells. (Fig. 11.10) 2. Experiments using frog eggs (oocytes) confirmed this hypothesis. 2a. The researchers extracted cytoplasm from frog eggs going through various stages of the cellcycle and injected it into cells at various stages of the cell cycle. (1) When they injected cytoplasm from an egg in M phase into a cell in G2, the G2-phase eggs entered M phase. (2) The cytoplasm from an interphase cell could not remove the G2-phase cells from their current phase. b. These experiments demonstrated that there was some molecular signal in the cytoplasm thatinitiated mitosis. This factor was eventually isolated and named mitosis-promoting factor (MPF). MPF induces mitosis in all eukaryotic cells. 3. MPF contains a protein and a cyclin. a. MPF is composed of two polypeptide subunits. (1) One polypeptide functions as a protein kinase that can activate and inactivate other proteins by phosphorylation. This subunit is expressed at constant levels throughout the cell cycle. (2) The other subunit is a cyclin whose concentration fluctuates throughout the cell cycle.