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Chapter 10: Cell Growth and Division Section 10-1 Cell Growth How Do Living Things Grow? Cell division allows living things to grow, and replace old cells. Some organisms use it for asexual reproduction. Limits to Cell Growth Cells can only grow so big for two reasons: 1. DNA Overload: DNA (1 copy) holds information for an entire cell so as a cell gets larger it puts more demands on its DNA. Example: A library has only a certain number of books. If a town doubles or triples in population there wouldn’t be enough books for everyone. 2. Exchanging materials: Nutrients enter and waste products leave cells through the cell membrane. Lower surface area to volume ratio = worse at exchanging nutrients and waste. Ratio of Surface Area to Volume As a cell grows, surface area to volume ratio decreases. Area = L x W x 6 sides Volume = L x W x H 1 cm 2 cm 4 cm A = 6 cm2 A = 24 cm2 A = 96 cm2 V = 1 cm3 V = 8 cm3 V = 64 cm3 Surface Area / Volume = 6/1 3/1 1.5/1 Section 10-2 Cell Division Cell Division Cell division – process by which a cell divides into two new daughter cells This solves problem of material exchange by reducing cell volume. Before cell division, cells replicate their DNA. This solves problem of DNA overload because each new cell gets a copy of DNA. Chromosomes Chromosome = a very long strand of DNA tightly wound around proteins. Every species’s cells contain a certain number of chromosomes. (ex. Human cells – 46, jack jumper ant cells - 2, adders-tongue fern cells 1260) Chromosomes Cells (with one exception, see Ch. 11) have two non-identical copies of every kind of chromosome. These copies are called homologous chromosomes. One copy came from Mom, one copy came from Dad. Human cells have 23 kinds of homologous chromosomes = 46 total chromosomes in each cell. Chromosomes When a cell is preparing to divide, it replicates its DNA, making a perfect identical copy of each chromosome. The identical copies remain attached to each other. They’re called sister chromatids. Chromosomes Practice Are these pairs of homologous chromosomes or sister chromatids? Chromosomes Practice This image shows homologous chromosomes after DNA replication into sister chromatids. Point to a pair of each… The Cell Cycle Cell Cycle – “life cycle of a cell,” the series of events that cells go through as they grow and divide After the cell divides, the two new daughter cells begin the cell cycle again. Cell Cycle Cell Cycle The Cell Cycle Phases of the cell cycle: • G1 phase – cell growth • S phase – DNA replication interphase • G2 phase – rapid metabolism • M phase – Mitosis - division of the nucleus Cytokinesis - division of the cytoplasm http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animation__how_the _cell_cycle_works.html Interphase – G1, S, and G2 Interphase = “normal life.” DNA is unwound and chromosomes are not visible. Nucleus bound by nuclear membrane Purpose and Results of Mitosis and Cell Division Purpose – to create more of the same type of cell so an organism can grow and replace old cells Results – 2 new daughter cells that are genetically identical to each other and to the original cell M phase (includes Mitosis) Mitosis is divided into four phases: 1. 2. 3. 4. Prophase Metaphase Anaphase Telophase Cytokinesis (part of M phase but not part of mitosis) Prophase In Preparation for division, DNA condenses into visible chromosomes. Centrioles move to opposite sides of the cell and spindle fibers form. Nuclear membrane breaks down. Metaphase Chromosomes line up across the Middle of the cell and are connected to the spindle. Anaphase Spindle fibers shorten, pulling the duplicated chromosomes Apart to opposite sides of the cell. Telophase Two new nuclear membranes form around the chromosomes at opposite ends of the cell. Chromosomes unravel and lose their distinct shape. Cytokinesis Cytoplasm pinches in half, the new daughter cells move apart. Each daughter cell immediately goes back into G1 of interphase. Models Long string = Cell membrane Short string = Nuclear membrane Jujubes = Centrioles Twizzlers = Spindle fibers Each cell has 6 chromosomes Gummi worms = Chromosome #1 Sour worms = Chromosome #2 Different colors = homologous chromosomes, same color = sister chromatids Different colors = homologous chromosomes, same color = sister chromatids Good n’ Plenties = Chromosome #3 Different colors = homologous chromosomes, same color = sister chromatids Section 10-3 Regulating the Cell Cycle Controls on Cell Division Normally, cells stop dividing when they come in contact with other cells. Example: Cells placed in a petri-dish grow and divide until they form a thin layer covering the bottom. If cells are removed, cells bordering the open space divide until they have filled the space and then stop. Uncontrolled Cell Growth Cancer – mutations to DNA in genes that control the cell cycle result in rapid, uncontrolled cell growth cancer normal http://scienceeducation.nih.gov/supplements/nih1/cancer/activities/activity2_animations .htm Section 11-4 Meiosis Reminder: homologous chromosomes Example: A body cell in an adult fruit fly has 8 chromosomes, as shown in the drawing below. 4 chromosomes came from dad, and 4 came from mom. The two sets of chromosomes are homologous. Diploid and Haploid Cells Diploid (2n) – cells containing homologous chromosomes Example: all body cells except sex cells Haploid (n) – cells containing only one of each kind of chromosome Example: sex cells or gametes (sperm and egg cells) Diploid and Haploid Cells Diploid vs Haploid before DNA replication (no sister chromatids) Diploid and Haploid Cells Diploid vs Haploid after DNA replication (with sister chromatids) Why are reproductive cells haploid? • A sperm cell (haploid) and an egg cell (haploid) combine to create the first cell of a new organism (diploid). Sperm 1 set 1 set Egg Humans Zygote (Baby) 2 sets Meiosis Meiosis - a process of nuclear division that produces gametes, new cells with half of the original cell’s genome Meiosis Meiosis goes through two divisions instead of one. Meiosis Crossing Over Crossing over – portions of homologous chromosomes are exchanged Happens in Prophase I, at the very start of meiosis Helps ensure that every sperm and egg cell is unique Meiosis Result: Meiosis produces four haploid daughter cells (gametes) that are all genetically different from each other, and from the original cell. Animations http://highered.mcgrawhill.com/sites/0072495855/student_view0/cha pter28/animation__how_meiosis_works.html http://highered.mcgrawhill.com/sites/0072495855/student_view0/cha pter28/animation__comparison_of_meiosis_a nd_mitosis__quiz_1_.html Mitosis Meiosis to grow and replace old cells make gametes or reproductive cells 2 4 Chromosome Number (diploid or haploid) diploid (2 sets) haploid (1 set) Genetic Relationship to Original Cell identical different body cells gametes Purpose for Organism Number of Cells Produced Type of Cells Produced Mitosis Meiosis to grow and replace old cells make gametes or reproductive cells 2 4 Chromosome Number (diploid or haploid) Diploid->Diploid Diploid->haploid Genetic Relationship to Original Cell identical different body cells gametes Purpose for Organism Number of Cells Produced Type of Cells Produced