The Cell Cycle and Cell Division 7 The Cell Cycle and Cell Division

Chapter 7 The Cell Cycle and Cell Division Key Concepts 7.1 Different Life Cycles Use Different Modes of Cell Reproduction 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells 7.3 Cell Reproduction Is Under Precise Control 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity 7.5 Programmed Cell Death Is a Necessary Process in Living Organisms

Organisms have two basic strategies for reproducing themselves: Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction The lifespan of an organism is linked to cell reproduction—usually called cell division. Organisms have two basic strategies for reproducing themselves: Asexual reproduction Sexual reproduction Cell division is also important in growth and repair of tissues. See Chapter 4

Figure 7.1 The Importance of Cell Division (Part 1)

Figure 7.1 The Importance of Cell Division (Part 2)

Figure 7.1 The Importance of Cell Division (Part 3)

Figure 7.2 Asexual Reproduction on a Large Scale

Homologous chromosome Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction Gamete Chromosome Haploid(N) Diploid (2N) Fertilization Zygote Somatic Cell Homologous chromosome

Figure 7.3 All Sexual Life Cycles Involve Fertilization and Meiosis (Part 1)

Spores divide by mitosis to form the haploid generation (gametophyte). Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction Alternation of generations—most plants, some protists; meiosis gives rise to haploid spores Spores divide by mitosis to form the haploid generation (gametophyte). Gametophyte forms gametes by mitosis. Gametes then fuse to form diploid zygote (sporophyte), which in turn produces haploid spores by meiosis.

Figure 7.3 All Sexual Life Cycles Involve Fertilization and Meiosis (Part 2)

A mature organism is diploid and produces gametes by meiosis. Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction Diplontic life cycle—animals and some plants; gametes are the only haploid stage A mature organism is diploid and produces gametes by meiosis. Gametes fuse to form diploid zygote; zygote divides by mitosis to form mature organism.

Figure 7.3 All Sexual Life Cycles Involve Fertilization and Meiosis (Part 3)

Figure 7.4 Prokaryotic Cell Division – Binary Fission

Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Cytokinesis begins after chromosome segregation by a pinching in of the plasma membrane—protein fibers form a ring. As the membrane pinches in, new cell wall materials are synthesized resulting in separation of the two cells. VIDEO 7.2 Cytokinesis in the euglenoid Phacus VIDEO 7.3 Cytokinesis in a green alga, Micrasterias

Eukaryotic cells divide by mitosis followed by cytokinesis. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Eukaryotic cells divide by mitosis followed by cytokinesis. Replication of DNA occurs as long strands are threaded through replication complexes. DNA replication only occurs during a specific stage of the cell cycle. See Chapter 9

Interphase has three subphases: G1, S, and G2. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Interphase has three subphases: G1, S, and G2. G1 (Gap 1)—variable, a cell may spend a long time in this phase carrying out its functions S phase (Synthesis)—DNA is replicated G2 (Gap 2)—the cell prepares for mitosis, synthesizes microtubules for segregating chromosomes ANIMATED TUTORIAL 7.1 Mitosis

Anatomy of a Chromosome

Figure 7.5 The Phases of the Eukaryotic Cell Cycle (Part 2)

Figure 7.5 The Phases of the Eukaryotic Cell Cycle (Part 3)

Condensed chromosomes appear during prophase. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Condensed chromosomes appear during prophase. Sister chromatids—two DNA molecules on each chromosome after replication Centromere—region where chromatids are joined Kinetochores are protein structures on the centromeres, and are important for chromosome movement. See Chapter 9

Segregation is aided by other structures: Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Segregation is aided by other structures: The centrosome determines the orientation of the spindle apparatus. Each centrosome can consist of two centrioles—hollow tubes formed by microtubules. Centrosome is duplicated during S phase and each moves towards opposite sides of the nucleus.

Polar microtubules form a spindle and overlap in the center. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Centrosomes serve as mitotic centers or poles; the spindle forms between the poles from two types of microtubules: Polar microtubules form a spindle and overlap in the center. Kinetochore microtubules—attach to kinetochores on the chromatids. Sister chromatids attach to opposite halves of the spindle. VIDEO 7.5 Formation of mitotic spindles

Figure 7.6 The Phases of Mitosis (1)

Figure 7.6 The Phases of Mitosis (2)

Kinetochore

Chromosome Walking

Experiments: Spindle fibers shorten during anaphase from the end attached to the chromosome, not the centrosome. Fig. 12.7b Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Figure 7.7 Cytokinesis Differs in Animal and Plant Cells (Part 1)

Figure 7.7 Cytokinesis Differs in Animal and Plant Cells (Part 2)

Each daughter cell contains all of the components of a complete cell. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells After cytokinesis: Each daughter cell contains all of the components of a complete cell. Chromosomes are precisely distributed. The orientation of cell division is important to development, but organelles are not always evenly distributed.

Concept 7.3 Cell Reproduction Is Under Precise Control The eukaryotic cell cycle has four stages: G1, S, G2, and M. Progression is tightly regulated—the G1-S transition is called R, the restriction point. Passing this point usually means the cell will proceed with the cell cycle and divide.

Figure 7.8 The Eukaryotic Cell Cycle

Concept 7.3 Cell Reproduction Is Under Precise Control Specific signals trigger the transition from one phase to another. Evidence for substances as triggers came from cell fusion experiments. Nuclei in cells at different stages, fused by polyethylene glycol, both entered the phase of DNA replication (S).

Figure 7.9 Regulation of the Cell Cycle (Part 1)

Concept 7.3 Cell Reproduction Is Under Precise Control Transitions also depend on activation of cyclin- dependent kinases (Cdk’s). A protein kinase is an enzyme that catalyzes phosphorylation from ATP to a protein. Phosphorylation changes the shape and function of a protein by changing its charges. See Chapter 5

Concept 7.3 Cell Reproduction Is Under Precise Control Cdk is activated by binding to cyclin (by allosteric regulation); this alters its shape and exposes its active site. The G1-S cyclin-Cdk complex acts as a protein kinase and triggers transition from G1 to S. Other cyclin-Cdk’s act at different stages of the cell cycle, called cell cycle checkpoints. See Chapter 3

Cyclins Protein kinases provide the “go” signal at G1 and G2 checkpoints. Cyclins work with and activate Cyclin-dependent kinases (Cdk’s) to form complexes, like Maturation promoting factor (MPF). MPF permits cell to move past G2 checkpoint into M phase. MPF phosphorylates other proteins to create events of mitosis, like destruction of the nuclear membrane. Late in metaphase, MPF switches itself off by starting destruction of its own cyclin. Cdk remains, until activated again. This protein degradation is important for getting past M checkpoint and starting anaphase. G1 checkpoint has its own set of Cdk proteins and cyclins.

Cdk constant; cyclin varies. When complexed, have active MPF.

Concept 7.3 Cell Reproduction Is Under Precise Control Example of G1-S cyclin-Cdk regulation: Progress past the restriction point in G1 depends on retinoblastoma protein (RB). RB normally inhibits the cell cycle, but when phosphorylated by G1-S cyclin-Cdk, RB becomes inactive and no longer blocks the cell cycle. See Chapters 3 and 5

Growth Factors Cell signals that influence. Internal: Starting anaphase – unattached kinetochores produce a signal molecule; stop when spindle fibers attach to kinetochores. When no more signal, anaphase-promoting complex is activated. Prevents cell division in presence of extra chromosomes. When APC active, triggers breakdown of cyclin AND inactivation of proteins at centromere that attach sister chromatids. External In cell culture, must have nutrients or cells won’t divide. If provide every thing necessary, will often still need presence of growth factors in order to divide. Platelet derived growth factor (PDGF) required for fibroblasts (see slide). Works through tyrosine-kinase receptors. Also find PDGF released in animals around site of a wound.

Density normally inhibits growth Anchorage dependence Senescence and Immortals (HeLa Cell Line) Density dependent inhibition of growth; also anchorage-dependence. Help to ensure growth at a particular site and to a particular density. Senescence – some signal usually tells cells to stop dividing after a certain number of division cycles (usually between 20 and 50 in mammals). This is lost or ignored in some cancer cells.

Reduce the chromosome number from diploid to haploid Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity Meiosis consists of two nuclear divisions but DNA is replicated only once. The function of meiosis is to: Reduce the chromosome number from diploid to haploid Ensure that each haploid has a complete set of chromosomes Generate diversity among the products ANIMATED TUTORIAL 7.2 Meiosis

Sexual Reproduction

Figure 7.11 Mitosis and Meiosis: A Comparison

Figure 7.12 Meiosis: Generating Haploid Cells (Part 1)

Figure 7.12 Meiosis: Generating Haploid Cells (Part 2)

Figure 7.12 Meiosis: Generating Haploid Cells (Part 3)

Figure 7.12 Meiosis: Generating Haploid Cells (Part 4)

Figure 7.12 Meiosis: Generating Haploid Cells (Part 5)

In prophase of meiosis I homologous chromosomes pair by synapsis. Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity In prophase of meiosis I homologous chromosomes pair by synapsis. The four chromatids of each pair of chromosomes form a tetrad,or bivalent. The homologs seem to repel each other but are held together at chiasmata. Crossing over is an exchange of genetic material that occurs at the chiasma. Crossing over results in recombinant chromatids and increases genetic variability of the products

In-Text Art, Ch. 7, p. 138

Figure 7.13 Crossing Over Forms Genetically Diverse Chromosomes

Prophase I may last a long time. Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity Prophase I may last a long time. Human males: Prophase I lasts about 1 week, and 1 month for entire meiotic cycle Human females: Prophase I begins before birth, and ends up to decades later during the monthly ovarian cycle

Either results in aneuploidy—chromosomes lacking or present in excess Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity Meiotic errors: Nondisjunction—homologous pairs fail to separate at anaphase I—sister chromatids fail to separate, or homologous chromosomes may not remain together Either results in aneuploidy—chromosomes lacking or present in excess

Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity Organisms with triploid (3n), tetraploid (4n), and even higher levels are called polyploid. This can occur through an extra round of DNA duplication before meiosis, or the lack of spindle formation in meiosis II. Polyploidy occurs naturally in some species, and can be desirable in plants.

Concept 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity If crossing over happens between non- homologous chromosomes, the result is a translocation. A piece of chromosome may rejoin another chromosome, and its location can have profound effects on the expression of other genes. Example: Leukemia See Chapters 10 and 11

In-Text Art, Ch. 7, p. 140

Cell death occurs in two ways: Concept 7.5 Programmed Cell Death Is a Necessary Process in Living Organisms Cell death occurs in two ways: In necrosis, the cell is damaged or starved for oxygen or nutrients. The cell swells and bursts. Cell contents are released to the extracellular environment and can cause inflammation. See Concept 31.1 APPLY THE CONCEPT Programmed cell death is a necessary process in living organisms

Apoptosis is genetically programmed cell death. Two possible reasons: 7.5 ProConcept 7.ammed Cell Death Is a Necessary Process in Living Organisms Apoptosis is genetically programmed cell death. Two possible reasons: The cell is no longer needed, e.g., the connective tissue between the fingers of a fetus. Old cells may be prone to genetic damage that can lead to cancer—blood cells and epithelial cells die after days or weeks.

Cell detaches from its neighbors Concept 7.5 Programmed Cell Death Is a Necessary Process in Living Organisms Events of apoptosis: Cell detaches from its neighbors Cuts up its chromatin into nucleosome-sized pieces Forms membranous lobes called “blebs” that break into fragments Surrounding living cells ingest the remains of the dead cell VIDEO 7.10 Apoptosis

Figure 7.14 Apoptosis: Programmed Cell Death (Part 1)

Cell death cycle is controlled by signals: Concept 7.5 Programmed Cell Death Is a Necessary Process in Living Organisms Cell death cycle is controlled by signals: Lack of a mitotic signal (growth factor) Recognition of damaged DNA External signals cause membrane proteins to change shape and activate enzymes called caspases—hydrolyze proteins of membranes.

Figure 7.14 Apoptosis: Programmed Cell Death (Part 2)

Answer to Opening Question Human papilloma virus (HPV) stimulates the cell cycle when it infects the cervix. Two proteins regulate the cell cycle: Oncogene proteins are positive regulators of the cell cycle—in cancer cells they are overactive or present in excess Tumor suppressors are negative regulators of the cell cycle, but in cancer cells they are inactive—can be blocked by a virus such as HPV VIDEO 7.11 Human melanoma cells dividing in culture

Figure 7.15 Molecular Changes Regulate the Cell Cycle in Cancer Cells