Cell Replication: Cell Cycle Chapter 10 * Cells must divide for the following reasons: 1. So that an organism can grow 2. So that an organism can develop 3. Repair I. Bacteria cells undergo binary fission to divide. This is a form of asexual reproduction, where the parent passes exact copies of the DNA to its offspring (RememberBacterial cells are prokaryotes!) II. Eukaryotic Cells must form chromosomes before cell division. A. Chromosome Structure Nucleus (DNA is found here) DNA is found on chromosomes inside the nucleus. Centromere- Where the 2 chromatids are attached. Chromotids – Where genes are located. One Chromosome One Gene. Most chromosomes have several hundred genes. Before cell division can take place, each chromosome must copy itself: This chromosome now has sister chromatids. They are identical in genetic material to each other. One chromosome that has copied its DNA. • G1 – Rapid Cell Growth; – Occupies most of the cell’s life. • S DNA is copied. • G2 Microtubules are assembled • Mitosis Nucleus divides • Cytokinesis Cytoplasm divides •Mitosis and Cytokinesis produce new cells that are identical to the original cell. Allows cells to grow and replace damaged tissues. 46 46 46 IV. The Cell Cycle is Controlled! A. Proteins act as signals that can permit or delay the next phase of the cell cycle. Checkpoints in the Cell Cycle: 1. G1 If the cell is healthy, then its DNA will copy. 2. G2 DNA replication is checked. If it occurred correctly, then mitosis is triggered. 3. Mitosis Signals beginning of G1. V. Cancer A. Occurs when a cell loses control of the cell cycle. The cell barrels thru the checkpoints. VI. Mitosis Division of the Nucleus Interphase • This is the time when the genetic material is actually being used to govern or regulate cell activity. Chromosomes are not visible during Interphase. Prophase • Chromosomes become visible. (Chromosome is already copies). • Nuclear envelope dissolves. Metaphase • Chromosomes line up along equator. Anaphase • Centromeres divide. • Chromatids (now called chromosomes) move toward opposite poles. Telophase • Nuclear envelope forms at each pole. • Chromosomes uncoil. • Spindle dissolves. • Cytokinesis begins. 2 genetically identical daughter cells. Review I. Mitosis DNA Replication & Division = 2 daughter cells genetically identical to the parent cell. Video of Mitosis 46 2n 46 46 2n 2n Body Cell II. Meiosis Reduction Division; Four daughter cells genetically different from the parent cell. Parent Reproductive Cell 23 n 23 n 23 n 23 n 23 n 23 n III. Phases of Meiosis A round of DNA replication occurs before stage I. Stage I Meiosis I : Prophase I • Begin with a diploid cell. • Each chromosome seeks out its corresponding homologous chromosome to form a tetrad. • Each Tetrad contains 4 chromatids. • While together, homologous chromosomes exchange genetic information during a process called crossing over. A Ex. A B C D E F B C D E F Stage II Meiosis I : Metaphase I • Homologous chromosomes (tetrads) line up across the center of the cell. Stage III Meiosis I : Anaphase I • Spindles pull the tetrads apart. Stage IV Meiosis I : Telophase I & Cytokinesis •At the end of telophase, cytokinesis occurs. •2 new daughter cells that are haploid and genetically different from the parent cell have formed. Review of Meiosis I • Meiosis Practice Stage V Meiosis II : Prophase II • A new spindle forms around the chromosomes. Stage VI Meiosis II : Metaphase II • Chromosomes line up at the equator. Stage VII Meiosis II : Anaphase II • Centromeres divide. • Sister chromatids split (now called chromosomes) and move to opposite ends of each cell. Stage VIII Meiosis II : Telophase II • A nuclear envelope forms around each set of chromosomes. • The result: 4 haploid daughter cells that are genetically different form the parent cell. Review of Meiosis II • Meiosis II Practice Meiosis In Males vs. Females Spermatogenesis Oogenesis Genetic Variation • 3 events give rise to genetic variation in sexually reproducing individuals. 1) Crossing Over Homologous chromosomes exchange genes. 2) Independent Assortment which cell each homologous chromosome will end up in is random. 3) Random Fertilization The random mating of organisms in nature. Genetic Variation Speeds Up Evolution!! It offers new combinations of traits that the environment can choose for or against.