The Cell Cycle

1. The Cell Cycle Chapter 11

2. The key roles of cell division • Reproduction • Asexual and sexual reproduction is based on cell division • Growth and development • Allows development from a single cell into multicellular organisms • Repair • Replacing cells that die normally • Repairing damaged cells

3. Reproduction • Unicellular organisms and some multicellular organisms reproduce by cell division • Amoebas • Plants • Hydras

4. Growth and Repair • Multicellular organisms depend on cell division for • Development from a fertilized cell • Growth • Repair

5. Cell Division • Two types of cell division: • Meiosis and Mitosis

6. Meiosis • Produces daughter cells with 1/2 the amount of hereditary material found in the parent cell. • Production of gametes (eggs and sperm) • Basis of sexual reproduction and genetic inheritance.

7. Mitosis • Produces daughter cells genetically identical to the parent cell • Basis of asexual reproduction • Involved only in the production of somatic (body) cells

8. Cell Division • The compliment of DNA given to each cell is called its genome • Prokaryotes have one long strand • Eukaryotes have their genome in several long strands • Cells duplicate their genetic material before they divide, ensuring that each daughter cell receives an exact copy of the genetic material, DNA

9. Eukaryotic Cell Division

10. Eukaryotic DNA • Long double helix of DNA wrapped around proteins • Each eukaryotic species has a characteristic number of chromosomes per nucleus • Human somatic cells have 23 sets of chromosomes • Reproductive cells (gametes) have ½ that number

11. Chromosomes • Before cell division and during replication strands are long chromatin fibers • After duplication the strands condense • Chromosomes condense around histones • Wrap around and around them

12. The Cell Cycle • Mitotic phase (mitosis and cytokinesis) is the shortest phase of the cell cycle • Interphase accounts for 90% of the cell cycle • Preparation for cell division • Cell grows by producing proteins and cytoplasmic organelles • Divided into subphases: • G1-Gap one • S phase- synthesis • G2-gap two

13. The Cell Cycle • G1phase lasts 7 to 9 hours, S phase 6 to 8 hours, and G2 phase 4 to 5 hours • Organelles replicate and additional cytoplasm is made • Normal cell functions during G1 phase

14. Mitosis

15. Mitosis • During interphase the cell replicates its chromosomes • Sister chromatids joined at centromere

16. 0.5 µm A eukaryotic cell has multiplechromosomes, one of which is represented here. Before duplication, each chromosomehas a single DNA molecule. Chromosomeduplication(including DNA synthesis) Once duplicated, a chromosomeconsists of two sister chromatidsconnected at the centromere. Eachchromatid contains a copy of the DNA molecule. Centromere Sisterchromatids Separation of sister chromatids Mechanical processes separate the sister chromatids into two chromosomes and distribute them to two daughter cells. Centromeres Sister chromatids Sister Chromatids

17. Mitosis • During mitosis chromatids separate during mitosis to form independent chromosomes • One copy for each daughter cell • Each daughter cell has the same genetic information as the parent cell.

18. PROMETAPHASE G2 OF INTERPHASE PROPHASE Centrosomes(with centriole pairs) Aster Fragmentsof nuclearenvelope Early mitoticspindle Kinetochore Chromatin(duplicated) Centromere Nonkinetochoremicrotubules Kinetochore microtubule Chromosome, consistingof two sister chromatids Nucleolus Nuclearenvelope Plasmamembrane Mitosis

19. METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS Metaphaseplate Cleavagefurrow Nucleolusforming Nuclear envelopeforming Daughter chromosomes Centrosome at one spindle pole Spindle Mitosis

20. Prophase • Chromosomes condense • Spindle apparatus forms

21. Prometaphase • Nucelar envelope disappears • Nucleolus disappears • Spindle fibers attach to each sister chromatid at kinetochores • Microtubules move chromosomes toward the middle of the cell

22. Metaphase • Chromosomes line up along the metaphaseplate

23. Anaphase • Centromeres split and sister chromatids are pulled toward opposite poles of the cell

24. Telophase • New nuclear envelope and nucleolus • Spindle apparatus disappears • Two nuclei

25. Cytokinesis • Division of the cytoplasm to form two daughter cells • Occurs immediately after mitosis • Occurs differently between animals and plants

27. Vesiclesforming cell plate 1 µm Wall of patent cell Cell plate New cell wall Daughter cells (b) Cell plate formation in a plant cell (SEM)

29. Cell Cycle Control

30. Cell Cycle Length • Can vary greatly among cell types • Simpler organisms less DNA • G1 phase is practically eliminated in rapidly dividing cells. • Nondividing cells get permanently stuck in G1 phase • Called the G0 state. • Suggests cell cycle regulation

31. Cell-Cycle Checkpoints • Many protein complexes are involved in regulating the cell cycle • Holding cells in a particular stage • Stimulating their passage to the next stage. • A cell-cycle checkpoint is a critical point in the cell cycle that is regulated • Cells without effective cell-cycle checkpoints keep growing and form a tumor

33. G1 Checkpoint Factors that affect whether cells pass the G1 checkpoint are • (1) cell size—cells must be large enough to split into two functional daughter cells • (2) nutrient condition—there must be enough food for cells to grow • (3) social signals—signaling molecules from other cells in a multicellular organism • (4) tumor suppressors—regulatory proteins that can stop the cell cycle.

34. (a) Normal mammalian cells. The availability of nutrients, growth factors, and a substratum for attachment limits cell density to a single layer. Cells anchor to dish surface and divide (anchorage dependence). When cells have formed a complete single layer, they stop dividing (density-dependent inhibition). If some cells are scraped away, the remaining cells divide to fill the gap and then stop (density-dependent inhibition). 25 µm G1 Checkpoints

35. Other Checkpoints • G2 checkpoint- Cells stop growing here if chromosome replication has not proceeded properly or if DNA is damaged • Prevents improper chromosome number • M checkpoint- stops mitosis if chromosomes are not properly attached to the mitotic spindle • Prevents incorrect chromosome separation

36. Cancer • All cancers derive from cells in which cell-cycle checkpoints have failed • A tumor forms when one or more cells in a multicellular organism begins to divide uncontrollably • Malignant tumors are invasive and can spread throughout the body via the blood or lymph and initiate new tumors • Called metastasis

37. 4 3 1 2 Lymphvessel Tumor Bloodvessel Glandular tissue Cancer cell MetastaticTumor A tumor grows from a single cancer cell. Cancer cells invade neighboring tissue. A small percentage of cancer cells may survive and establish a new tumor in another part of the body. Cancer cells spread through lymph and blood vessels to other parts of the body. Cancer

38. Cancer • Normally, mammalian cell cultures will not grow unless growth factors are present • Cancer cells divide without growth factors • No control at the G1 checkpoint • Most cancers result from multiple defects in cell-cycle regulation • Each type of cancer is caused by a unique combination of errors.