2. Why do cells divide? 4. For the reproduction of unicellular organisms (like bacteria) 1. To heal/repair tissue 2. For multicellular organisms to grow in size 3. To keep cell sizes small for increased efficiency

3. The Cell Cycle The Cell Cycle is the preparation and division of the nucleus of a cell to form two new cells Interphase and Mitosis make up the Cell Cycle

4. Stages of the cell cycle INTERPHASEINTERPHASE – G1 stage = "GAP 1” – S stage = "Synthesis” This is the stage when DNA replication occursThis is the stage when DNA replication occurs – G2 stage = "GAP 2” MITOSISMITOSIS – M stage = mitosis & cytokinesis

6. Cell Cycle

7. What is Mitosis? Nuclear division plus division of the cell itself (cytokinesis)Nuclear division plus division of the cell itself (cytokinesis) Produces two identical daughter cells.Produces two identical daughter cells.

8. Regulation of the Cell Cycle Very complexVery complex Errors can lead to cancer.Errors can lead to cancer. –In cancer, regulation of the cell cycle goes awry –Normal cell growth & behavior is lost.

9. Regulatory Enzymes Cdk (cyclin dependent kinase)Cdk (cyclin dependent kinase) Major control switch, along with other cyclinsMajor control switch, along with other cyclins –causes cell to move from G1 to S or G2 to M. MPF (Maturation Promoting Factor)MPF (Maturation Promoting Factor) –triggers progression through cell cycle.

10. Cancer Promotors p53 = protein that blocks the cell cycle if DNA is damaged. Cell can die.p53 = protein that blocks the cell cycle if DNA is damaged. Cell can die. p53 mutation is the most frequent mutation leading to cancer.p53 mutation is the most frequent mutation leading to cancer. p27 = protein that blocks entry into S phasep27 = protein that blocks entry into S phase –Low levels may mean eventual breast cancer

11. Chromosome Characteristics - Ploidy Number of sets of chromosomes in a cellNumber of sets of chromosomes in a cell Haploid (n)-- one set chromosomesHaploid (n)-- one set chromosomes Diploid (2n)-- two sets chromosomesDiploid (2n)-- two sets chromosomes Most plant and animal adults are diploid (2n)Most plant and animal adults are diploid (2n) Eggs and sperm are haploid (n)Eggs and sperm are haploid (n)

12. Chromosome Characteristics Diploid set for humans; 2n = 46Diploid set for humans; 2n = 46 = (2n=46)zygote (n=23)sperm + (n=23)egg = (2n=46)zygote Autosomes = body chromosomesAutosomes = body chromosomes Female-sex chromosomes = homologous (XX)Female-sex chromosomes = homologous (XX) Male-sex chromosomes = non-homologous (XY)Male-sex chromosomes = non-homologous (XY)

13. Interphase - G 1 S G 2 The cell is metabolizing, growing, replicating DNA and preparing for mitosisThe cell is metabolizing, growing, replicating DNA and preparing for mitosis

14. Interphase – G 1 S G 2 Chromosomes are not clearly seenChromosomes are not clearly seen Nucleolus may be visibleNucleolus may be visible In animal cells ONLY, a pair of centrioles may start moving into position.In animal cells ONLY, a pair of centrioles may start moving into position.

15. Interphase summary Three Phases: The cell grows & develops (Gap 1)The cell grows & develops (Gap 1) DNA is replicated (S)DNA is replicated (S) Necessary materials are made (Gap 2)Necessary materials are made (Gap 2) ANIMAL PLANT

16. Most of these cells are in Interphase How do you know these cells are in interphase? Intact Nucleus

17. Mitosis (M Phase) The M Phase consists of: Mitosis and CytokinesisMitosis and Cytokinesis Mitosis: the division of the nucleus Cytokinesis: the division of the cytoplasm Four Phases of Mitosis: P rophaseP rophase M etaphaseM etaphase A naphaseA naphase T elophaseT elophase

18. Late Interphase: G 2 Preparing for Division Chromosome consisting of two sister chromatids becomes visible Centromere Early Spindle Apparatus Prophase Nuclear membrane disappears

19. Prophase Chromatin begins to condense and becomes visible.Chromatin begins to condense and becomes visible. Nucleolus disappears.Nucleolus disappears. Centrioles begin moving to opposite ends of the cellCentrioles begin moving to opposite ends of the cell

20. How is DNA packaged into chromosomes? DNA coils around set of 4 histone proteins, creating a “bead on a string” called a nucleosome The nucleosome coils into structures called chromatin Chromatin supercoils = chromosome!

21. sister chromatids centromere The chromosome is duplicated (during S)The chromosome is duplicated (during S) Each duplicated chromosome consists of two.Each duplicated chromosome consists of two. These are connected by a.These are connected by a.

22. Late Prophase Nuclear membrane dissolvesNuclear membrane dissolves Proteins attach to centromeres creating the kinetochores.Proteins attach to centromeres creating the kinetochores. Microtubules attach at centromeres/kinetochores and chromosomes begin moving.Microtubules attach at centromeres/kinetochores and chromosomes begin moving.

23. Metaphase Chromosomes line up at an imaginary line between the centrosomes at opposite ends of the cell. The spindle attaches to the centromeres.

24. Metaphase Spindle fibers align the chromosomes along metaphase plate (middle).Spindle fibers align the chromosomes along metaphase plate (middle). This helps to ensure each new nucleus will receive one copy of each chromosome.This helps to ensure each new nucleus will receive one copy of each chromosome.

25. Anaphase Spindles pull the “daughter” chromosomes apart

26. Anaphase Paired chromosomes separate at kinetochores and move to opposite sidesPaired chromosomes separate at kinetochores and move to opposite sides

27. Anaphase Remember: the DNA in these chromosomes is identical on each end! Remember: the DNA in these chromosomes is identical on each end! IDENTICAL DNA

28. Telophase Chromatids arrive at opposite polesChromatids arrive at opposite poles Membranes re-form around daughter nuclei.Membranes re-form around daughter nuclei. Chromosomes & spindles disperse and are no longer visible.Chromosomes & spindles disperse and are no longer visible. Cytokinesis may beginCytokinesis may begin

29. Telophase & Cytokinesis Telophase Two nuclei become visible In animal cells, a Cleavage Furrow is visible In plant cells, a cell plate is now visible Cytokinesis The cytoplasm and organelles divide.

30. Cytokinesis: Animals Fiber ring of protein called actin around center of cell contracts and pinches cell into two identical daughter cellsFiber ring of protein called actin around center of cell contracts and pinches cell into two identical daughter cells

31. Plants In plant cells, the rigid wall requires that a cell plate be synthesized between the two daughter cells.In plant cells, the rigid wall requires that a cell plate be synthesized between the two daughter cells.

32. The END RESULT of the CELL CYCLE Two identical cells as the original, ready to go through interphase and mitosis, repeating the process Two identical cells as the original, ready to go through interphase and mitosis, repeating the process

33. Mitosis & Cytokinesis animal cell division

34. What is the purpose of the Cell Cycle? To replace somatic cells (skin, bone, lining, etc.) Somatic cells are body cells Some cells like your nerve, muscle and red blood cells do not undergo division. (One reason stem cell research is so important.)

35. Asexual Reproduction Uses only mitosisUses only mitosis Cells duplicate to form 2 exact copies (clones) of each otherCells duplicate to form 2 exact copies (clones) of each other Most primitive form of reproductionMost primitive form of reproduction –NO GENETIC VARIATION Used by higher organisms when conditions are not favorable for sexual reproductionUsed by higher organisms when conditions are not favorable for sexual reproduction

36. Chromosome # during Mitosis Diploid cell Diploid cell =2n (46 in =2n (46 in humans) humans) 2(2n) = 92 due to DNA replication 2(2n) = 92 due to DNA replication chromosomes chromosomes in humans in humans one cell division occurs one cell division occurs Diploid cell Diploid cell Diploid cell Diploid cell =2n (46 in =2n (46 in =2n (46 in =2n (46 in humans) TWO humans) humans) TWO humans) IDENTICAL IDENTICAL CELLS CELLS