Why Do Cells Divide Instead of Continuing to Grow

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Why do cells divide rather than continue to grow indefinitely?

Cell Growth and Division Chapter 10

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 chromosomes (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

Limits to cell size – how basic physics determines the rate of diffusion in and out of a cell 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

Section 10 -1 Ratio of Surface Area to Volume in Cells Cell Size Surface Area (length x width x 6) Volume (length x width x height) Ratio of Surface Area to Volume 3 X increase (side length) vs. 9 X increase (surface area) vs. 27 X increase (volume)

Effect of cell size on diffusion Small cell Easy access to the entire cell for diffusion in or out of the cell.

Effect of cell size on diffusion Even the middle of the small cell is still accessed by diffusion.

Effect of cell size on diffusion large cell

Effect of cell size on diffusion large cell – (with same diffusion rate as small cell)

Related phenomena - 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 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. • From Wikipedia

Related phenomena - 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

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 Asexual reproduction Sexual reproduction • Three basic types 1) Binary fission 2) Mitosis 3) Meiosis Prokaryotes Eukaryotes

Reasons for Cell Division 1. To grow • While keeping SA/V ratio high 2. Development • Cell specialization 3. Repair and replace worn out cells • homeostasis 4. Reproduction • Continuity species/heredity

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) C

Cell Cycle Outline I. Interphase A. G 1 phase B. S phase C. G 2 phase II. Cell Division A. Mitosis 1. prophase 2. metaphase 3. anaphase 4. telophase B. Cytokinesis Interphase Prophase mnemonic I_____ _ P Metaphase _____ Anaphase M Telophase _____ Cytokinesis A _____ T

• A new cell grows G 1 • It continues to grow as it duplicates its chromosomes S • The cell grows more G 2 and prepares for mitosis • The nucleus divides M • The rest of the cell divides (cytokinesis). C • G 0 is a cell not in the cell cycle-a nondividing cell

G 1 phase S phase Interphase G 2 phase Mitosis Prophase Metaphase Anaphase Telophase CELL CYCLE Cell Division Cytokinesis

How do cells make sure each daughter cell gets a full and equivalent set of hereditary material? • Chromosomes – Chromosomes made of DNA and proteins – Only visible as distinct (discrete) units during cell division (they are in the chromatin form for the rest of the cell cycle) • All organisms have a specific number of chromosomes – Not related to complexity because of large differences in size of chromosomes and other factors.

DNA Architecture • The DNA in a cell is packed into an elaborate, multilevel system of coiling and folding around proteins • Chromatin is condensed to form a chromosome • A pair of replicated, identical, and attached chromosomes are called sister chromatids DNA double helix Histones "Beads on a string" Nucleosome Tight helical fiber Supercoil Sister chromatids Centromere Figure 8. 4

ashpen [email protected] willardr 2. net Chromosomes • Chromosomes are copied in preparation of cell division (S phase). • The two halves are called sister chromatids • Held together by a centromere one chromosome A chromatid its sister chromatid centromere Sister chromatids

IDENTIFYING PHOTOGRAPHS OF STAGES OF THE CELL CYCLE

Mitosis: division of the nucleus interphase metaphase prophase anaphase metaphase telophase

Interphase chromatin nucleus

Prophase: chromosomes condense nuclear envelope disappears Spindle fibers Centriole chromosomes Figure 8. 7 x 1 c

Metaphase: chromosomes line up at the equatorial plane chromosomes centriole Spindle fibers Figure 8. 7 x 1 d

Anaphase: sister chromatids separate and are pulled to the poles Figure 8. 7 x 1 e

Telophase: cytokinesis begins and ends, nuclear envelope reappears New nucleus Cleavage furrow Figure 8. 7 x 1 f

Cytokinesis • Animals • Cleavage furrow • Plants • Cell plate forms between the new nuclei • New cell wall

• Cytokinesis in animals Cleavage furrow Contracting ring of microfilaments (a) Animal cell cytokinesis Daughter cells Figure 8. 8 a

Wall of parent cell Cell plate forming Daughter nucleus • Cytokinesis in plants Vesicles containing cell wall material Cell plate Cell wall (b) Plant cell cytokinesis New cell wall Daughter cells Figure 8. 8 b

Cytokinesis in plants

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