Cell Growth and Division

Cell Growth Limits to growth Stress on DNA Difficulty moving nutrients/wastes across membrane Ratio of surface area to volume Volume increases at a faster rate than surface area

Cell Division Before a cell becomes too large, it divides, producing 2 daughter cells Each daughter cell is an exact replica of the parent cell Before the cell divides, the DNA is replicated, so each new cell will have the same genetic information as the parent cell

Cell Division 2 stages (eukaryotes) Mitosis- division of the cell nucleus Cytokinesis- division of the cytoplasm Asexual reproduction (esp for unicellular organisms) Also used for new cells as an organism grows and develops

Chromosomes Threadlike structure within the nucleus containing the genetic information (DNA) that is passed from one generation of cells to the next Cells of every organism have a specific number of chromosomes Fruit flies = 8; Humans = 46; carrots = 18

Chromosomes Chromatin- granular material visible within the nucleus; consists of DNA tightly coiled around proteins Chromatid- one of two identical “sister” parts of a duplicated chromosome Centromere- an area where the chromatids are attached to one another

The Cell Cycle 2 major phases Interphase Mitosis

Interphase 3 phases G1 phase= cells do most of their growing Increase in size and synthesize new proteins and organelles S phase= chromosomes are replicated and the synthesis and DNA molecules takes place Usually if a cell enters S phase and begins replication, it completes the rest of the cycle G2 phase= many of the organelles and molecules required for cell division are produced Shortest of the 3 phases of interphase

Mitosis Divided into 4 phases Followed with Cytokinesis Prophase Metaphase Anaphase Telophase Followed with Cytokinesis Depending on cell- may last a few minutes to several days

Prophase 1st and longest phase of mitosis Events Chromosomes become visible Centrioles separate and move to opposite sides of the cell Chromosomes become attached to fibers in the spindle at the centromere Chromosomes coil more tightly Nucleolus disappears Nuclear envelope breaks down

Metaphase Often lasts only a few minutes Events Chromosomes line up across the center of the cell Microtubules connect the centromere of each chromosome to the two poles of the spindle

Anaphase Centromeres split Sister chromatids separate and move to opposite poles Anaphase ends when chromosomes stop moving

Telophase Chromosomes begin to disperse into a chromatin Nuclear envelope re-forms around each cluster of chromosomes Spindle begins to break apart Nucleolus becomes visible

Cytokinesis Occurs at the same time as telophase Animal cells: Cell membrane is drawn inward until the cytoplasm is pinched into 2 nearly equal parts Plant cells: Cell plate forms midway between the divided nuclei Cell wall begins to appear in the cell plate Result? 2 new identical cells

Controls on Cell Division Effects of controlled cell growth can be seen by placing some cells in a petri dish containing nutrient broth Cells grow until they form a thin layer covering the bottom of the dish Cells stop growing when they come into contact with other cells If cells are removed, the remaining cells will begin dividing again Something can turn cell division on or off

Regulating Cell Growth Cyclins- proteins that regulate the timing of the cell cycle in eukaryotic cells Internal regulators: proteins that respond to events inside the cell i.e. make sure all chromosomes have been replicated; make sure all chromosomes are attached to the spindle before entering anaphase External regulators: proteins that respond to events outside the cell i.e. embryonic development; wound healing

Uncontrolled Cell Growth Cancer- disorder in which some of the body’s own cells lose the ability to control growth Cancer cells do not respond to the signals that regulate the growth of most cells P53 gene halts the cell cycle until all chromosomes have been properly replicated A damaged or defective p53 gene causes the cells to lose the information needed to respond to signals that would normally control their growth