1. Why do Cells Divide? Cell Size Limits

2. Why do cells divide? A. If they get too big – Surface area to volume ratio, etc. 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!! When you increase the size of the cell, the vol. increases faster than the surface area of the cell membrane!!

4. Why do cells divide? B. Growth of organism – In order for an organism to grow, it needs more cells C. Repair – New cells replace old/damaged ones D. Reproduction – ASEXUAL- offspring is identical to parent – SEXUAL- offspring is a combo of both parents

5. Cell Division Mitosis – Occurs in somatic cells (body cells) – Daughter cells are exactly like parent – Diploid 2n In humans n=23Meiosis –Produces gametes (germ cells/sex cells) –Daughter cells have half the chromo- somes as parent “Reduction division” –Haploid 1nWhy?

6. Mitosis- divides somatic cells to get tissues and organs built; makes individual grow into an adult

7. DNA VOCAB 1.Chromatin- – when a cell is not dividing it’s DNA is a tangled mass of thin threads. 2.Chromosome- – When the DNA is ready to divide it condenses into individual chromosomes In humans- there are 23 HOMOLOGOUS PAIRS of chromosomes The two chromosomes in a pair are SIMILAR but not exactly alike One is from mom and one from dad 3.Sister Chromatid- – When the DNA replicates (to make one copy for each cell) the two copies join together by a centromere and each copy is called a SISTER CHROMATID – These sisters are EXACTLY ALIKE!

10. TYPICAL CHROMOSOME PAIR

11. The Cell Cycle (Somatic Cells) A.INTERPHASE: growth and preparation 1.G1 2.S 3.G2 B.MITOSIS: Nucleus divides 1.Prophase 2.Metaphase 3.Anaphase 4.Telophase C.CYTOKINESIS: Division of cytoplasm

12. 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 makes 2 new daughter cells that are identical to the parent cell

13. Cell Cycle- The life of a cell

14. Cell Cycle- Interphase Interphase (Nucleus enclosed in nuclear membrane, nucleolus present, chromosomes appear as chromatin (in other words, chromosomes are not yet visible)) a) G1 phase- intense cellular activity and growth; organelles double b) S phase- DNA synthesis- chromosomes replicate (copy)

15. Chromatin to Chromosomes

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

17. Interphase Diagram

18. MITOSIS PHASES

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

20. Prophase Early prophase Late prophase

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

22. Metaphase diagram Metaphase

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

24. Anaphase diagram Anaphase

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

26. Telophase Diagram Telophase

27. 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