Why do cells divide rather than continue to grow indefinitely? When an animal gets larger does each cell get larger or does the animal make more cells? Cell Growth and Division Chapter 10 Figure 8.1x Unicellular organisms grow and then divide into 2 organisms. Multicellular organisms grow by division of cells (increasing in number) rather than just continually getting larger--why? 1. DNA Overload - the number of (the amount of DNA) is constant (there would not be enough DNA to provide a really large cell with enough new protein). 2. Laws of physics prevent cells from growing too large Diffusion rate 1
Limits to cell size how basic physics determines the rate of diffusion in and out of a cell Section 10-1 Ratio of Surface Area to Volume in Cells 1. Cell must be able to move materials in and out of the cell 2. As cells increase in size Volume increases faster than surface area All exchanges occurs over the cell membrane 3. Decrease in surface area/volume ratio Slows transport of materials Cell Size Surface Area (length x width x 6) Volume (length x width x height) Ratio of Surface Area to Volume 3X increase (side length) vs. 9X increase (surface area) vs. 27X increase (volume) As things get bigger (whether it s a single cell or a polar bear), volume increases faster than surface area! In zoology, Bergmann's Rule is a principle that correlates environmental temperature with body mass in warm-blooded animals. It asserts that within a species, the body mass increases with latitude and colder climate. Among mammals and birds, individuals id of a particular species in colder areas tend to have greater body mass than individuals in warmer areas. For instance, White-tailed Deer are larger in Canada than in the Florida Keys. The rule is named after a nineteenth-century German biologist, Christian Bergmann. Volume increases faster than surface area! Gigantothermy is a phenomenon with significance in biology and paleontology, whereby large, bulky ectothermic (cold-blooded) animals are more easily able to maintain a constant, relatively high body temperature than smaller animals by virtue of their greater volume to surface area ratio. A bigger animal has proportionately less of its body close to the outside environment than a smaller animal of otherwise similar shape, and so it gains heat from, or loses heat to, the environment much more slowly. The largest leatherback turtle ever found however was a little over three meters from head to tail and weighed over 900 kilograms. From Wikipedia From Wikipedia Small cell Easy access to the entire cell for diffusion in or out of the cell. Even the middle of the small cell is still accessed by diffusion. 2
large cell large cell 10-2 Cell Division Cell Division The result of cell division is a pair of identical daughter cells. Each daughter cell receives a full set of genetic instructions (DNA). Cell Division Three basic types Asexual reproduction 1) Binary fission: prokaryotic organisms 2) Mitosis: eukaryotic organisms Both result in two identical daughter cells Sexual reproduction 3) Meiosis: eukaryotic cells Results in four genetically different daughter cells Reasons for Cell Division 1. To grow While keeping SA/V ratio high 2. Development Cell specialization i 3. Repair and replace worn out cells homeostasis 4. Reproduction Continuity species/heredity 3
Cell Cycle Cell s life Regulated by the organism Varies from: Species to species Cell types within an organism Divided into phases Interphase: period between divisions Mitosis: division of nucleus Cytokinesis: division of cell (cytoplasm) Cell Cycle Outline I. Interphase A. G1 phase B. S phase C. G2 phase II. Cell Division A. Mitosis 1. prophase 2. metaphase 3. anaphase 4. telophase B. Cytokinesis Interphase Prophase Metaphase Anaphase Telophase Cytokinesis mnemonic I P M A T C Section 10-2 Figure 10 4 The Cell Cycle Thus, a cell grows (G1), continues to grow as it duplicates its (S), grows more and prepares for mitosis (G2), the nucleus divides (M), and the rest of the cell divides (cytokinesis). M phase G 1 phase S phase G 2 phase How do cells make sure each daughter cell gets a full and equivalent set of hereditary material? Chromosomes DNA and protein All organisms have a specific number of Is the number of related to the complexity of the organism? Lettuce and carrots 18 Garden pea 20 Fruit fly 8 Cat 32 Earthworm 36 Human 46 Chimpanzee 48 Horse 64 Dog 78 Adder s tongue ferns 1,262 4
DNA Architecture Chromosomes The DNA in a cell is packed into an elaborate, multilevel system of coiling and folding Chromosome chromatid DNA double helix Histones Beads on a string Tight helical fiber Sister chromatids Nucleosome Supercoil Centromere Figure 8.4 Chromosomes are copied in preparation of cell division (S phase). Sister chromatids Held together by a centromere one chromatid its sister chromatid centromere Sister chromatids Interphase G 1 Growth and maturation S Synthesis of DNA Replication of M phase G 2 Growth, Preparation for division G 0 Non-dividing cell Not in the cell cycle G 2 phase G 1 phase S phase I. Interphase A. G1 phase B. S phase C. G2 phase II. Cell Division A. Mitosis division of the nucleus 1. prophase 2. metaphase 3. anaphase 4. telophase B. Cytokinesis division of the cytoplasm Cell Cycle Outline Mitosis: division of the nucleus Interphase nucleus interphase prophase metaphase metaphase anaphase telophase 5
Prophase: condense nuclear envelope disappears Metaphase: line up at the equatorial plane Spindle fibers Centriole centriole Spindle fibers Figure 8.7x1c Figure 8.7x1d Anaphase: sister chromatids separate and are pulled to the poles Telophase: cytokinesis begins and ends, nuclear envelope reappears New nucleus Cleavage furrow Figure 8.7x1e Figure 8.7x1f Cytokinesis Animals Cleavage furrow Plants Cell plate forms between the new nuclei New cell wall animals Cleavage furrow Cleavage furrow Contracting ring of microfilaments (a) Animal cell cytokinesis Daughter cells Figure 8.8a 6
Wall of parent cell Cell plate forming Daughter nucleus plants Vesicles containing cell wall material Cell wall Cell plate New cell wall (b) Plant cell cytokinesis Daughter cells Figure 8.8b 7