2. Chapter 12: The Cell Cycle http://highered.mcgraw- hill.com/olc/dl/120073/bio14.swf

3. Omnis cellula e cellula From every cell a cell – Rudolf Virchow Cell division: reproduction of cells Cell cycle: life of a cell from the time it is first formed from a dividing parent cell until it divides into 2 daughter cells Mitosis: nuclear division within a cell, followed by cytokinesis Cytokinesis: division of the cytoplasm

4. Getting the right stuff What is passed on to daughter cells? – exact copy of genetic material = DNA mitosis – organelles, cytoplasm, cell membrane, enzymes cytokinesis chromosomes (stained orange) in kangaroo rat epithelial cell  notice cytoskeleton fibers

5. Mitosis Mitosis functions in – Reproduction of single-celled organisms – Growth – Repair – Regeneration In contrast, meiosis produces gametes in sexually-reproducing organisms – Contains half the number of chromosomes of a somatic cell amoeba

6. G2G2 S G1G1 M metaphase prophase anaphase telophase interphase (G 1, S, G 2 phases) mitosis (M) cytokinesis (C) C Frequency of cell division varies by cell type – embryo cell cycle < 20 minute – skin cells divide frequently throughout life 12-24 hours cycle – liver cells retain ability to divide, but keep it in reserve divide once every year or two – mature nerve cells & muscle cells do not divide at all after maturity permanently in G 0 Frequency of cell division

7. Organization of the Genetic Material Cell division results in genetically identical daughter cells – This requires DNA replication followed by division of the nucleus Genome: genetic content of the cell – Prokaryotic cells have circular DNA Circular DNA: a single long molecule – Eukaryotic cells contain a number of DNA molecules specific to different species One eukaryotic cell has about 2 meters of DNA

8. Organizing DNA DNA is organized in chromosomes – double helix DNA molecule – wrapped around histone proteins like thread on spools – DNA-protein complex = chromatin organized into long thin fiber – condensed further during mitosis DNA histones chromatin duplicated mitotic chromosome ACTGGTCAGGCAATGTC

9. Distribution of Chromosomes During Cell Division Non-dividing cells contain chromatin – In human cells there are 46 strands of chromatin, or 23 corresponding pairs of strands Dividing cells contain chromosomes – Chromosomes becomes condensed prior to mitosis – Each duplicate chromosome has 2 sister chromatids bound by a narrow centromere Kinetochore: a structure of proteins associated with specific sections of chromosomal DNA at the centromere

10. Human male karyotype 46 chromosomes 23 pairs single-stranded

11. double-stranded mitotic human chromosomes Double- stranded

12. Chromosome Structure 0.5 µm Chromosome duplication (including DNA synthesis) Centromere Separation of sister chromatids Sister chromatids Centromeres Sister chromatids A eukaryotic cell has multiple chromosomes, one of which is represented here. Before duplication, each chromosome has a single DNA molecule. Once duplicated, a chromosome consists of two sister chromatids connected at the centromere. Each chromatid contains a copy of the DNA molecule. Mechanical processes separate the sister chromatids into two chromosomes and distribute them to two daughter cells. Figure 12.4

13. Mitotic Phase Alternates with Interphase in Cell Cycle Mitosis: M Phase includes mitosis and cytokinesis – Prophase – Prometaphase – Metaphase – Anaphase – Telophase Interphase: 90% of cell cycle; cell grows, producing organelles and proteins – G 1 : First gap – S: Synthesis, chromosome replication – G 2 : Second gap; completes preparation for cell division I’m working here! Time to Divide

14. In 24 hours… Mitosis: 1 hour G 1 : 5-6 hours (most variable) S: 10-12 hours G 2 : 4-6 hours INTERPHASE G1G1 S (DNA synthesis) G2G2 Cytokinesis Mitosis MITOTIC (M) PHASE

15. Mitotic Spindle is Necessary for Nuclear Division Mitotic spindle: used to segregate sister chromatids in anaphase Consists of microtubules and proteins Microtubules are able to change in length – Elongate by adding Tubulin subunits – Shorten by loss of Tubulin subunits CentrosomeAster Sister chromatids Metaphase Plate Kinetochores Overlapping nonkinetochore microtubules Kinetochores microtubules Centrosome Chromosomes Microtubules 0.5 µm 1 µm

16. Mitotic Spindle Interphase: the centrosome is replicated and the two centrosomes remain paired near the nucleus – Centrosome or Microtubule Organizing Center (MTOC): contains two centrioles Prophase: spindle formation Early Prometaphase: migration of centrosomes to poles, spindle microtubules grow Late Prometaphase: centrosomes are at the poles, asters form – Aster: radial array of short microtubules – The mitotic spindle includes the centrosomes, spindle microtubules and asters

17. Cellular Changes Indicate M- Phase Initiation G 2 of Interphase: nuclear envelope intact – Nucleus and nucleolus present – Centrosome Replication Animal cells contain centrioles – Chromosomes are not condensed and therefore not individually visible

18. Prophase Nucleoli disappear Chromosomes condense Sister chromatids form – Two genetically identical arms joined by the centromere and cohesin proteins – Mitotic spindle forms using microtubules; asters form – Centrosome migration as microtubules lengthen

19. Prometaphase Nuclear envelope fragments and microtubules invade nuclear area Nonkinetochore microtubules elongate Chromosomes condense further and gain kinetochore proteins Spindle fibers (microtubules) interact with kinetochores

20. Mitotic Spindle Prometaphase: Kinetochore: protein structure associated with specific sections of the chromosomal DNA at the centromere Kinetochore microtubules: attach kinetochore to spindle Metaphase Plate: created as microtubules attach to kinetochore, creation indicates Metaphase start

21. Metaphase Longest stage of mitosis – Can last up to 20 minutes Metaphase plate: site of chormosome alignment Centrosomes are at poles Sister chromatid kinetochores attach to kinetochore microtubules

22. Anaphase Shortest stage of mitosis Cohesins between sister chromatids are cleaved by enzymes – Sister chromatids are now separate chromosomes Spindle fibers shorten causing chromosomes to move to opposite poles – Spindles shorten (tubulin breakdown) at kinetochore ends Nonkinetochore microtubules elongate cell

23. Kinetochores use motor proteins that “walk” chromosome along attached microtubule – microtubule shortens by dismantling at kinetochore (chromosome) end, NOT from the spindle pole side Chromosome movement

24. Telophase Daughter nuclei form in cell Chromosomes loosen and become less dense Nucleoli reappear

25. Cytokinesis Occurs in animal cells only Relies on cleavage marked by a cleavage furrow Cytoplasmic side of cleavage furrow contains actin microfilaments and myosin – As the actin and myosin interact the ring contracts and the cleavage furrow deepens

26. Mitosis in animal cells

27. Mitosis in whitefish blastula

28. Cellular Division in Plant Cells Plant cells form a cell plate following mitosis Golgi vesicles contain cell wall materials and migrate toward the center of the cell – forming the cell plate, a new cell wall

29. Binary Fission Asexual eukaryotes can utilize mitosis for reproductive purposes – this is called binary fission Asexual prokaryotes perform binary fission that does not involve mitosis

30. Evolution of Mitosis Some proteins were highly conserved In prokaryotes, protein resembling eukaryotic actin may help with cell division – Tubulin-Like proteins may help separate daughter cells Attach to nuclear envelope Reinforce spatial arrangement of nucleus Spindle inside nucleus

31. Cell Cycle Regulation Cytoplasmic regulators as shown in Mammalian cell Fusion experiment by Johnson and Rao (1970) Conclusion: molecules in the cytoplasms of cells in the S and M phase control the progression of phases

32. Cell Cycle Control System Control systems vary cell to cell – Skin cells divide frequently, liver cells only divide when needed (after injury), and nerve and muscle cells never divide Cytoplasmic molecules signal cell cycle as shown by Johnson and Rao Cell cycle events are regulated cyclicly – These are referred to as Cell Cycle Checkpoints

33. Cell Cycle Control Systems Cell Cycle Checkpoint: control point in cell cycle where stop and go-ahead signals can regulate the cycle; – relies on signal transduction pathways controlled by internal and external molecular signals – G1 checkpoint: acts as a mammalian restriction point “Go Signal” permits G 1, S, G 2 and M “Stop signal” causes G 0 phase: non-dividing state (many cells of human body) – G 2 checkpoint – M checkpoint

34. G 0 phase – non-dividing, differentiated state – most human cells in G 0 phase  liver cells  in G 0, but can be “called back” to cell cycle by external cues  nerve & muscle cells  highly specialized  arrested in G 0 & can never divide

35. Checkpoint control system 3 major checkpoints: – G 1 /S can DNA synthesis begin? – G 2 /M has DNA synthesis been completed correctly? commitment to mitosis – spindle checkpoint are all chromosomes attached to spindle? can sister chromatids separate correctly?

36. Cell Cycle Clock The rate of cell cycle is controlled by two proteins: – Cyclins – Cyclin-dependent kinases (Cdks)

37. Cell Cycle Clock Kinases: activate or inactivate other proteins via phosphorylation – Inactive most of the time – Kinases are activated by cyclins Cyclins: regulatory proteins that have a cyclic (fluctuating) concentration in the cell Cyclin-dependent kinases (Cdks) – Activity fluctuates with cyclin concentration

38. Cell Cycle Clock MPF: maturation- promoting factor or M- phase promoting factor – MPF is a Cyclin-Cdk complex Triggers cell passage from G 2 to M G 2 checkpoint: As cyclin builds during G 2 it binds with Cdk – Resulting MPF phosphorylates proteins, initiating Mitosis

39. During anaphase, MPF inhibits itself by destroying its own cyclin – Cdk persists in the cell

40. Stop and Go Signals Internal signals – M Phase checkpoint relies on kinetochore signaling – Allows for enzymatic cleavage of cohesins External factors 1. Nutrients 2. Growth Factor Dependency 3. Density-dependent inhibition 4. Anchorage dependence

41. External signals Growth factors – communication between cells – protein signals released by body cells that stimulate other cells to divide Platelet Derived Growth Factor (PDGF) – made by platelets in blood clots – binding of PDGF to cell receptors stimulates cell division in connective tissue: to heal wounds Connective tissue Fibroblast cells PDGFNo PDGF present

42. E2F nucleus cytoplasm cell division nuclear membrane Growth factor protein kinase cascade nuclear pore chromosome Cdk cell surface receptor P P P P P E2F Rb Growth factor signals

43. External signals Density-dependent inhibition – crowded cells stop dividing – each cell requires growth factor and nutrients » not enough activator left to trigger division in any one cell Anchorage dependence – to divide cells must be attached to a substrate » “touch sensor” receptors

44. Loss of Cell Cycle Controls Cancer cells – Exhibit neither density-dependent inhibition nor anchorage dependence 25 µm Cancer cells do not exhibit anchorage dependence or density-dependent inhibition. Cancer cells. Cancer cells usually continue to divide well beyond a single layer, forming a clump of overlapping cells. (b) Figure 12.18 B – Do not respond normally to the body’s control mechanisms – Form tumors

45. Malignant tumors invade surrounding tissues and can metastasize – Exporting cancer cells to other parts of the body where they may form secondary tumors Cancer cells invade neighboring tissue. 2 A small percentage of cancer cells may survive and establish a new tumor in another part of the body. 4 Cancer cells spread through lymph and blood vessels to other parts of the body. 3 A tumor grows from a single cancer cell. 1 Tumor Glandular tissue Cancer cell Blood vessel Lymph vessel Metastatic Tumor Figure 12.19