1. Why do Cells Divide? Why do Cells Divide? Cell Size Limits

2. 1. Replacement of Cells  Humans need to replace old, worn out cells from normal wear and tear

3. 2. Rate of Diffusion  Remember, the cell membrane allows nutrients (ex. Glucose, oxygen, etc.) in and wastes out (diffusion)  Diffusion is fast over short distances but slow over long distances  So, b/c of slow rate of diffusion, cells can’t be giants b/c they would starve to death or be poisoned from the build up of wastes!!

4. Diffusion Animation  Remember diffusion works best over short distances!!  diffusion animation diffusion animation diffusion animation

5. 3. Need enough DNA to support protein needs of cell  Since DNA codes for making protein there needs to be enough to support the protein needs of the cell ex. Think how much protein is in the ex. Think how much protein is in the cell membrane and if the cell cell membrane and if the cell size increases than you would size increases than you would need more protein to build it need more protein to build it

6. 4. Surface Area to Volume Ratio  Need a proper SA:Vol ratio to support the needs of the cell  You need the cell to remain small!!  For example, if the cell size doubles it would need 8x more nutrients to survive and it would create 8x the waste to excrete!!  The vol. increases faster than the surface area of the cell membrane!!

7. Reminder from cell theory  All cells come from pre-existing cells!! -Cell Division follows this b/c it’s the process where new cells are made from 1 cell -Cell Division follows this b/c it’s the process where new cells are made from 1 cell  Cell Division makes 2 new cells that are identical to the original

8. Cell Division Requirements  I) Cell Division  A. Mitosis- nuclear division *  B. Cytokinesis- Division of the cytoplasm cytoplasm

9. Cell Cycle- Interphase Interphase (Nucleus enclosed in nuclear membrane, nucleolus present, chromosomes appear as chromatin) Interphase (Nucleus enclosed in nuclear membrane, nucleolus present, chromosomes appear as chromatin)  a) G1 phase- intense cellular activity and growth activity and growth  b) S phase- DNA synthesis- chromosomes replicate chromosomes replicate

10. Interphase continued  c) G2 phase- spindle fibers are assembled/ centrioles assembled/ centrioles replicate replicate - cell prepares for cell division - cell prepares for cell division

11. Interphase Diagram

12. MITOSIS PHASES

13. Prophase (1 st stage) Prophase- 1st stage of actual cell division Prophase- 1st stage of actual cell division  a.) Chromosomes condense and shorten, become distinguishable become distinguishable b.) Chromatids attached near center by centromere centromere  c.) Centrioles separate, migrate to the poles  d.) Spindle fibers form between centrioles, asters form asters form  e.) Nuclear membrane disintegrates, nucleolus disappears (in order to nucleolus disappears (in order to separate contents of nucleus, it must separate contents of nucleus, it must disappear ) disappear )

14. Prophase Early prophase Late prophase

15. Metaphase- 2 nd stage (shortest)  a.) chromosomes line up in pairs at equator (center of cell) at equator (center of cell)  b.) centromere of each chromosome is attached to a chromosome is attached to a separate spindle fiber separate spindle fiber

16. Metaphase diagram Metaphase

17. Anaphase- 3 rd stage  a.) Centromere of each chromosome separates separates  b.) Separation of the chromatids in each pair pair  c.) Spindle fibers appear to shorten, pulling the chromatids apart at the pulling the chromatids apart at the centromere (now called centromere (now called chromosomes) chromosomes)  d.) migration of the chromosomes ends with the arrival at the poles and the with the arrival at the poles and the formation of clusters formation of clusters

18. Anaphase diagram Anaphase

19. Telophase – 4 th stage  a.) Cleavage furrow forms b/w 2 cells and spindle fibers disappear and spindle fibers disappear  b.) Nuclear membrane forms around each set of chromosomes (forms 2 each set of chromosomes (forms 2 new nuclei) new nuclei)  c.) Chromosomes uncoil to form chromatin net (getting ready for chromatin net (getting ready for interphase) interphase)  d.) Nucleoli reorganized

20. Telophase Diagram Telophase

21. Cytokinesis  Cytokinesis- division of the cytoplasm  In animal cells- cell membrane pinches together, furrow forms along the equator.  -In plant cells- cell plate forms in the middle of the dividing cell middle of the dividing cell