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Chapter 8 – Cellular Reproduction In order for organisms to grow and reproduce, cells must divide. Cell Division (8.1) Cell division is part of asexual reproduction • Single-celled organisms reproduce by simple cell division • There is no fertilization of an egg by a sperm Sexual reproduction • Requires fertilization of an egg by a sperm using a special type of cell division called meiosis. • Thus, sexually reproducing organisms use: – Meiosis for reproduction – Mitosis for growth and maintenance Chromosomes Contain Our Genes • Long stretches of DNA and proteins • Humans have 23 different chromosomes • Diploid – 2 of each chromosome (our body cells) • Haploid – 1 of each chromosome (our sex cells) DNA double helix Histones TEM “Beads on a string” Nucleosome Tight helical fiber Looped domains TEM Duplicated chromosomes (sister chromatids) Centromere Figure 8.4 Chromosomes 8.2 Cell Cycle 1. 2. 3. 4. Cells grow and do “cell” things Cells copy DNA Each copy moves to opposite ends of cell. Cells divide; each new cell has a copy of DNA Chromosomes are duplicated before cell division Cell Cycle (8.6) Eukaryote cells divide by mitosis or meiosis • Mitosis – One cell divides producing two identical nuclei followed by cell division – Almost all cell reproduction • Meiosis (only in testes and ovaries) – One cell divides twice to produce four genetically different cells – Produces egg and sperm cells (gametes) A Cell’s life cycle is divided into phases. • Most of the time a cell is in Interphase • During interphase, a cell: – Performs its normal functions – Doubles everything in its cytoplasm – Grows in size Interphase and Prophase (8.7) Prophase • Chromosomes condense and are visible under light microscope • Nuclear membrane breaks down • Spindle microtubules grow from two centrosomes, clouds of cytoplasmic material that in animal cells contain centrioles. Metaphase and Anaphase • Metaphase • Mitotic spindle guides the separation of two sets of daughter chromosomes. • Chromosomes line up along middle axis of cell (metaphase plate) • Anaphase • Spindle fibers pull the chromosomes away from center • Centromeres break and each sister chromosome goes to opposite poles of the cell. Telophase and Cytokinesis • Chromosomes unwind at the poles • New nuclear membrane forms • Cytokinesis typically: –Occurs during telophase –Divides the cytoplasm –Is different in plant and animal cells Cytokinesis (8.8) Cytokinesis (8.8) Meiosis • Two cell divisions (Meiosis I and II) • Four daughter cells result each containing half of the chromosomes as the parent cell • In males meiosis results in the production of 4 sperm cells • In females meiosis results in the production of 1 large egg cell and 3 small polar bodies Haploid gametes (n 23) Egg cell n n Sperm cell FERTILIZATION MEIOSIS Multicellular diploid adults (2n 46) 2n MITOSIS and development Diploid zygote (2n 46) Key Haploid (n) Diploid (2n) Figure 8.12 Meiosis has two stages • Meiosis I • Meiosis II Meiosis I • Similar to mitosis except during first metaphase chromosome pair up side by side • During first anaphase one set of chromosomes move to either pole of the cell. • Sister chromotids stay joined in the new daughter cells Meiosis I (8.14) Meiosis II • Anaphase II sister chromotids separate • Daughter cells have only one of each chromosome (Haploid) Meiosis II (8.14) Crossing Over Increases Genetic Variation The number of times a cell can divide is limited. Cloning (NIB) Dolly was euthanized • When chromosomes replicate they lose part of their ends (telomeres) • Linked to aging • Dolly’s chromosomes came from an adult sheep with already shortened telomeres • Dolly developed age-related illnesses Dolly taught us a lot about cellular aging. Mitosis and Meiosis