1. Cell Growth and Division Cell Cycle Mitosis Cytokinesis

2. Cell Growth  Rather than growing indefinitely, cells divide Maintains large SA/V ratio!!! Does not put too many demands on DNA Cell is more efficient in transport of materials in and out

3. Cell Division  Process by which a cell divides into two new identical cells Called daughter cells Reduces cell volume Each daughter cell receives a full set of genetic instruction

4. Reasons for cell division  Growth of organism  Reproduction  Renewal  Repair

5. Genetic Material  DNA  Most of the time, the DNA along with proteins, is found in the nucleus in the form of chromatin (spread out and unfolded)  Before division, the chromatin condenses to form chromosomes Can be seen with light microscope

6. Genetic Material

7. Chromosomes  After replication, consists of two sister chromatids connected by a centromere “x” = doubled Each chromatid is a chromosome

8. Number of chromosomes is specific to an organism…  Human – 46  Cat – 38  Dog – 78  Mouse – 40  Frog – 26  Fruit Fly – 8  Potato – 48  Corn – 20  Carrot – 18  Yeast - 32

9. The Cell Cycle  Series of events as cells grow and divide  Consists of: Interphase (resting stage) Mitosis (nuclear division) Cytokinesis (cytoplasm division)

10. Cell Cycle

11. Interphase  Preparation time between cell divisions  Cell is in resting phase and performing regular cell functions  Could be for a long period of time, depending on cell Usually majority of cell cycle (about 90%)  Consist of three phases: G1 – growth, protein and organelle synthesis S – DNA replication; “synthesis” G2 – growth and prep for division

12. M phase of Cell Cycle  Consists of mitosis and cytokinesis  Mitosis – actual division begins Nucleus is divided PMAT – prophase, metaphase, anaphase, and telophase Time varies for different cell types  Cytokinesis – actual splitting of cells Cytoplasm pinches in half

13. Mitosis  Prophase Longest phase Chromatin condenses into chromosomes (now visible) Centrosomes move apart to different sides of the cell and spindle is formed Nuclear envelope breaks down and nucleolus disappears Spindle attaches to chromosomes at kinetichores near the centromere

14. Mitotic Spindle  Begins to form during prophase  Consists of microtubule fibers and proteins  Assembly starts in the centrosome (organelle that organizes microtubules) Animal cells have centrioles in the center Microtubules extend from the centrosomes in radial arrays called asters Spindle fibers elongate by adding subunits of the protein tubulin

15. Mitosis  Metaphase Chromosomes line up along the center of the cell (metaphase plate) Centromeres align on this spot Hint: metaphase  middle

16. Mitosis  Anaphase Centromeres split Sister chromatids are separated Spindle pulls the chromatids to opposite sides of the cell as microtubules shorten Each end has a complete set of chromosomes

17. Mitosis  Telophase Chromosomes lose condensed shape and return to chromatin form Nuclear envelope begins to reform on each side Spindle breaks apart Nucleolus become visible Now have two identical nuclei

19. Cytokinesis  Cytoplasm is divided  Cell membrane draws inward (cleavage furrow in animals) until the cytoplasm is pinched in two  Usually occurs at the same time as telophase  In plants, a cell plate forms between the two nuclei which will later develop into the cell wall

20. Cytokinesis

21. Prokaryotes vs. Eukaryotes  Prokaryotes don’t have nuclei, so cannot do mitosis  Prokaryotes reproduce by binary fission – “division in half” (asexual) Chromosomes are replicated Chromosomes are separated to opposite ends (not yet discovered exactly how) Plasma membrane grows inward and new cell wall is deposited

22. Regulating the Cell Cycle  Not all cells move through the cycle at the same rate Nerve and muscle cells don’t usually divide after initial development (G 0 – non-dividing state) Skin, digestive, and bone marrow cells grow and divide rapidly Depends on need or cell type

23. Regulation  Events of cell cycle are directed by a control system  Checkpoints are critical points in the cycle where stop and go signals regulate the next step G1 checkpoint – “restriction point”  Check for damaged DNA  usually if cell receives “go ahead” at this point, the cycle will be completed  If signal not received, it will exit the cycle and switch to non-dividing state called G 0 phase G2 checkpoint– check for proper replication M checkpoint – check for spindle assembly and alignment of chromosomes

24. Regulators  Protein kinases – enzymes that activate or inactivate other proteins by phosphorylation Must attach to a cyclin protein to be active Called cyclin-dependent kinases, Cdks Activity of Cdks change with the concentration of cyclins Example is MPF, a cyclin-Cdk complex  “maturation promoting factor” – triggers the cells passage to the M phase Stages of the cell cycle seem to be controlled by cyclin-Cdk complexes

25.  Cyclins – proteins that regulate the cell cycle determine when cells will divide Family of similar proteins Fluctuate in concentration in the cell  Other types of regulatory proteins Internal and external Provide the stop-and-go signals at the checkpoints

26. Internal Regulators  Proteins that respond to a stimulus from inside the cell that trigger and regulate division Prevents different phases from occurring until ready Helps to ensure consistency and equal chromosome distribution

27. External Regulators  Proteins that respond to events or conditions outside the cell Direct cells to speed up or slow down cycle Include growth factors  Important in embryonic development and healing  Many cells won’t divide unless growth factors are present Molecules on the surface of other cells can affect the cycle of neighboring cells  Prevent excessive growth

28. External Regulators  Density-dependent inhibition – crowded cells stop dividing  Anchorage dependence – must be attached to something in order to divide Could be a culture jar or the extracellular matrix of a tissue

29. Uncontrolled Cell Growth  Cancer – when cells lose the ability to control their own growth Single cell in a tissue undergoes transformation Results from mutation of genes that regulate the cell cycle “defective” cells divide uncontrollably (proliferate) and form tumors May break from tumors and spread to other parts of the body (metastasis) Can disrupt normal activity and cause tissue damage

30. Cancer  Benign- when abnormal cells remain at original site usually removable Not yet compromising to tissues  Malignant – invasive enough to impair organs or tissues

31. Causes of Cancer  Numerous and many unknown  Some include: Smoking Radiation exposure Viral infections Toxic materials Genetics