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Chapter 8

Chapter 8. The Cell Cycle. The Life of a Eukaryotic Cell. The ability of organisms to reproduce best distinguishes living things from nonliving matter The continuity of life is based upon the reproduction of cells, or cell division.

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Chapter 8

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  1. Chapter 8 The Cell Cycle

  2. The Life of a Eukaryotic Cell • The ability of organisms to reproduce best distinguishes living things from nonliving matter • The continuity of life is based upon the reproduction of cells, or cell division

  3. In unicellular organisms, division of one cell reproduces the entireorganism • Ex. Yeast cells, amoeba • Multicellular organisms begin as a single fertilized egg cell that will go through many cycles of division • Multicellular organisms depend on cell division for: • Development from a fertilized cell • Growth • Repair • Cell division is an integral part of the cell cycle, the life of a cell from formation to its own division

  4. LE 12-2 100 µm 200 µm 20 µm Reproduction Growth and development Tissue renewal

  5. Cell division results in genetically identical daughter cells • Cells duplicate their genetic material before they divide, ensuring that each daughter cell receives an exact copy of the genetic material, DNA • A dividing cell duplicates its DNA, allocates the two copies to opposite ends of the cell, and only then splits into daughter cells

  6. Chromosomes: Cellular Organization of the Genetic Material • A cell’s endowment of DNA (its genetic information) is called its genome • DNA molecules in a cell are packaged into chromosomes

  7. Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus • Ex. Human……. 46 • Alligator…… 32 • Amoeba ……. 50 • Corn………… 20

  8. Somatic (nonreproductive, also called autosomal) cells have two complete sets of chromosomes • Gametes (reproductive cells: sperm and eggs) have half as many chromosomes as somatic cells (one complete set)

  9. Chromosomes • Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein (histones) that condenses during cell division • Bacteria typically contain one singlecircular chromosome • When cell prepare to divide: • Chromatinchromosomes • Does this by: • chromatin duplicates ( DNA replication) creating a complete second copy of the DNA strand • strands begin to coil tightly until a rod shaped chromatid is formed

  10. Structure of Chromosomes

  11. LE 12-3 25 µm

  12. Distribution of Chromosomes During Cell Division • In preparation for cell division, DNA is replicated and the chromosomes condense • Each duplicated chromosome has two sister chromatids, which separate during cell division • The centromere is the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached

  13. LE 12-4 0.5 µm Chromosome duplication (including DNA synthesis) Centromere Sister chromatids Separation of sister chromatids Centromeres Sister chromatids

  14. Human and animal chromosomes are categorized as either • Sex chromosomes: in humans, X and Y. (Human females=XX; males=XY) • Autosomes: all of the other chromosomes • Humans have two sex chromosomes and 44autosomes for a total of 46

  15. In sexually reproducing organisms, chromosomes occur in pairs • Homologous chromosomes or homologs: • Each member of a pair; they are the same size, shape, and carry genes for the same trait, but may have different forms of the gene.

  16. A cell containing homologous pairs is called • Diploid = 2N, where N = the number of different kinds of chromosomes. • Ex. The 2N, or diploid number of chromosomes for humans = 46, where N=23 in regular body cells (not sperm or egg) • Haploid = 1N, where only one member of a pair is present. • Ex. 1N or haploid number of chromosomes for humans = 23 (sex cells, sperm and egg)

  17. Karyotype • A diagram of all 46 human chromosomes is shown on a karyotype • Karyotypes are can be used to diagnose a chromosomal abnormality in a child • Mistakes in chromosome number may result in a genetic disorder

  18. Normal Human Karyotype

  19. Chromosomal Abnormalities • Deletion: a piece of a chromosome is lost • Inversion: a segment flips around and reattaches • Translocation: a piece from one chromosomes attaches to another • Duplication: a segment doubles itself • Nondisjunction: failure of the two chromatids to separate during division resulting in too few or too many chromosomes

  20. Eukaryotic cell division consists of: Mitosis, the division of the nucleus Cytokinesis, the division of the cytoplasm Gametes are produced by a variation of cell division called meiosis Meiosis yields nonidentical daughter cells that have only one set of chromosomes, half as many as the parent cell Phases of Cell Cycle

  21. Phases of the Cell Cycle • The cell cycle consists of • Mitotic (M) phase (mitosis and cytokinesis) • Interphase (cell growth and copying of chromosomes in preparation for cell division) • Interphase (about 90% of the cell cycle) can be divided into subphases: • G1 phase (“first gap”) • S phase (“synthesis”) • G2 phase (“second gap”)

  22. LE 12-5 INTERPHASE S (DNA synthesis) G1 Mitosis Cytokinesis G2 MITOTIC (M) PHASE

  23. Mitosis is conventionally divided into five phases: • Prophase • Prometaphase • Metaphase • Anaphase • Telophase • Cytokinesis is well underway by latetelophase

  24. LE 12-6ca G2 OF INTERPHASE PROPHASE PROMETAPHASE

  25. LE 12-6da 10 µm TELOPHASE AND CYTOKINESIS METAPHASE ANAPHASE

  26. Video: Animal Mitosis Video: Sea Urchin (time lapse) Animation: Mitosis (All Phases) Animation: Mitosis Overview Animation: Late Interphase Animation: Prophase Animation: Prometaphase Animation: Metaphase Animation: Anaphase Animation: Telophase

  27. LE 12-10 Chromatin condensing Nucleus 10 µm Chromosomes Cell plate Nucleolus Prometaphase. We now see discrete chromosomes; each consists of two identical sister chromatids. Later in prometaphase, the nuclear envelope will fragment. Prophase. The chromatin is condensing. The nucleolus is beginning to disappear. Although not yet visible in the micrograph, the mitotic spindle is starting to form. Metaphase. The spindle is complete, and the chromosomes, attached to microtubules at their kinetochores, are all at the metaphase plate. Telophase. Daughter nuclei are forming. Meanwhile, cytokinesis has started: The cell plate, which will divide the cytoplasm in two, is growing toward the perimeter of the parent cell. Anaphase. The chromatids of each chromosome have separated, and the daughter chromosomes are moving to the ends of the cell as their kinetochore micro- tubules shorten.

  28. Cytokinesis: A Closer Look • In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow • In plant cells, a cell plate forms during cytokinesis Animation: Cytokinesis

  29. LE 12-9a 100 µm Cleavage furrow Daughter cells Contractile ring of microfilaments Cleavage of an animal cell (SEM)

  30. LE 12-9b Vesicles forming cell plate Wall of parent cell 1 µm New cell wall Cell plate Daughter cells Cell plate formation in a plant cell (TEM)

  31. Meiosis • Meiosis: a process of nuclear division that reduces the number of chromosomes in new cells to half the number in the original cell. • In humans, produces gametes, or reproductive cells (sperm and egg)

  32. Meiosis I • Stages of meiosis: • Meiosis I • Prophase I • Spindle fibers appear • Nuclear membrane and nucleolus disappear • Chromosomes line up next to homologue to form tetrad (4 chromatids twisted around each other) • Tetrads may engage in crossing over (exchange of pieces of DNA between chromatids)

  33. Meiosis I • Metaphase I • Homologous pairs line up at equator with pairs remaining together

  34. Anaphase I • Homologous pairs separate as they are pulled by spindle fibers to opposite poles of the cell • Chromatids are still attached

  35. Telaphase I • Cytoplasm divides, forming two daughter cells (not identical)

  36. Meiosis II: occurs to each cell formed in meiosis I • Prophase II • New spindle fibers form

  37. Metaphase II • Chromosomes (still made up of two chromatids) move to equator of cell

  38. Anaphase II • Centromere connecting chromatids divide, and identical chromatids are pulled apart to opposite poles of the cell

  39. Telophase II • Spindle dissolves • Nuclear membrane reforms around chromatids • Four new daughter cells are formed

  40. Gamete formation • Spermatogenesis : Sperm formation • Oogenesis: egg formation

  41. The Cell Cycle Control System • The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clock • The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received

  42. LE 12-14 G1 checkpoint Control system S G1 G2 M M checkpoint G2 checkpoint

  43. For many cells, the G1 checkpoint seems to be the most important one • If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide • If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G0 phase

  44. LE 12-15 G0 G1 checkpoint G1 G1 If a cell does not receive a go-ahead signal at the G1 checkpoint, the cell exits the cell cycle and goes into G0, a nondividing state. If a cell receives a go-ahead signal at the G1 checkpoint, the cell continues on in the cell cycle.

  45. The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases • Two types of regulatory proteins are involved in cell cycle control: cyclins and cyclin-dependent kinases (Cdks) • The activity of cyclins and Cdksfluctuates during the cell cycle

  46. LE12-16a G1 S G2 G1 M M M S G2 MPF activity Cyclin Relative concentration Time Fluctuation of MPF activity and cyclin concentration during the cell cycle

  47. LE12-16b G1 Cyclin S Cdk M Degraded cyclin G2 accumulation G2 checkpoint Cdk Cyclin is degraded Cyclin MPF Molecular mechanisms that help regulate the cell cycle

  48. Stop and Go Signs: Internal and External Signals at the Checkpoints • An example of an internal signal is that kinetochoresnot attached to spindle microtubules send a molecular signal that delays anaphase • Some external signals are growth factors, proteins released by certain cells that stimulate other cells to divide • For example, platelet-derived growth factor (PDGF) stimulates the division of human fibroblast cells in culture

  49. LE 12-17 Scalpels Petri plate Without PDGF With PDGF Without PDGF With PDGF 10 mm

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