Chapter 12

1. Chapter 12 The Cell Cycle

2. Figure 12.1 Chromosomes in a dividing cell

3. 100 µm 200 µm 20 µm (a) Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism (LM). (b) Growth and development. This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM). (c) Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM). Figure 12.2 The functions of cell division

4. 50 µm Figure 12.3 Eukaryotic chromosomes

5. 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. Centrometers Sister chromatids Figure 12.4 Chromosome duplication and distribution during cell division

6. INTERPHASE S(DNA synthesis) G1 CytokinesisMitosis G2 MITOTIC(M) PHASE Figure 12.5 The cell cycle

7. 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 Nuclearenvelope Plasmamembrane Nucleolus Figure 12.6 Exploring The Mitotic Division of an Animal Cell

8. Prometaphase • The nuclear envelope fragments. • The microtubules of the spindle can now invade the nuclear area and interact with the chromosomes, which have become even more condensed. • Microtubules extend from each centrosome toward the middle of the cell. • Each of the two chromatids of a chromosome now has a kinetochore, a specialized protein structure located at the centromere. • Some of the microtubules attach to the kinetochores, becoming “kinetochore microtubules.” These kinetochore microtubules jerk the chromosomes back and forth. • Nonkinetochore microtubules interact with those from the opposite pole of the spindle. • G2 of Interphase • A nuclear envelope bounds the nucleus. • The nucleus contains one or more nucleoli (singular, nucleolus). • Two centrosomes have formed by replication of a single centrosome. • In animal cells, each centrosome features two centrioles. • Chromosomes, duplicated during S phase, cannot be seen individually because they have not yet condensed. • The light micrographs show dividing lung cells from a newt, which has 22 chromosomes in its somatic cells (chromosomes appear blue, microtubules green, intermediate filaments red). For simplicity, the drawings show only four chromosomes. • Prophase • The chromatin fibers become more tightly coiled, condensing into discrete chromosomes observable with a light microscope. • The nucleoli disappear. • Each duplicated chromosome appears as two identical sister chromatids joined together. • The mitotic spindle begins to form. It is composed of the centrosomes and the microtubules that extend from them. The radial arrays of shorter microtubules that extend from the centrosomes are called asters (“stars”). • The centrosomes move away from each other, apparently propelled by the lengthening microtubules between them.

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

10. Anaphase • Anaphase is the shortest stage of mitosis, lasting only a few minutes. • Anaphase begins when the two sister chromatids of each pair suddenly part. Each chromatid thus becomes a full- fledged chromosome. • The two liberated chromosomes begin moving toward opposite ends of the cell, as their kinetochore microtubules shorten. Because these microtubules are attached at the centromere region, the chromosomes move centromere first (at about 1 µm/min). • The cell elongates as the nonkinetochore microtubules lengthen. • By the end of anaphase, the two ends of the cell have equivalent—and complete—collections of chromosomes. • Metaphase • Metaphase is the longest stage of mitosis, lasting about 20 minutes. • The centrosomes are now at opposite ends of the cell. • The chromosomes convene on the metaphase plate, an imaginary plane that is equidistant between the spindle’s two poles. The chromosomes’ centromeres lie on the metaphase plate. • For each chromosome, the kinetochores of the sister chromatids are attached to kinetochore microtubules coming from opposite poles. • The entire apparatus of microtubules is called the spindle because of its shape. • Telophase • Two daughter nuclei begin to form in the cell. • Nuclear envelopes arise from the fragments of the parent cell’s nuclear envelope and other portions of the endomembrane system. • The chromosomes become less condensed. • Mitosis, the division of one nucleus into two genetically identical nuclei, is now complete. • Cytokinesis • The division of the cytoplasm is usually well underway by late telophase, so the two daughter cells appear shortly after the end of mitosis. • In animal cells, cytokinesis involves the formation of a cleavage furrow, which • pinches the cell in two.

11. Aster Centrosome MetaphasePlate Sisterchromatids Kinetochores Overlappingnonkinetochoremicrotubules Kinetochores microtubules 0.5 µm Microtubules Chromosomes Centrosome 1 µm Figure 12.7 The mitotic spindle at metaphase

12. EXPERIMENT 1 The microtubules of a cell in early anaphase were labeled with a fluorescent dye that glows in the microscope (yellow). Kinetochore Spindlepole Figure 12.8 Inquiry During anaphase, do kinetochore microtubules shorten at their spindle pole ends or their kinetochore ends?

13. 2 A Laser was used to mark the kinetochore mircotubles by eliminating the fluorescnce in a region between one spindle pole and the chromosomes. As anaphase proceeded, researches monitored the changes in the lengths of the microtubles on either side of the mark. Mark RESULTS As the chromosomes moved toward the poles, the microtubule segments on thekinetochore side of the laser mark shortened, while those on the spindle pole side stayed thesame length.

14. CONCLUSION This experiment demonstrated that during anaphase, kinetochore microtubules shorten at their kinetochore ends, not at their spindle pole ends. This is just one of the experiments supporting the hypothesis that during anaphase, a chromosome tracks along a microtubule as the microtubule depolymerizes at its kinetochore end, releasing tubulin subunits Chromosomemovement Kinetochore Tubulinsubunits Microtubule Motorprotein Chromosome

15. 100 µm Cleavage furrow Vesiclesforming cell plate 1 µm Wall of patent cell Cell plate New cell wall Contractile ring of microfilaments Daughter cells (a) Cleavage of an animal cell (SEM) (b) Cell plate formation in a plant cell (SEM) Figure 12.9 Cytokinesis in animal and plant cells Daughter cells

16. 1 2 3 4 5 Chromatinecondensing Nucleus Chromosome Nucleolus Metaphase. The spindle is complete,and the chromosomes,attached to microtubulesat their kinetochores, are all at the metaphase plate. Prometaphase.We now see discretechromosomes; each consists of two identical sister chromatids. Laterin prometaphase, the nuclear envelop will fragment. Telophase. Daughternuclei are forming. Meanwhile, cytokinesishas started: The cellplate, which will divided the cytoplasm in two, is growing toward the perimeterof the parent cell. Prophase. The chromatinis condensing. The nucleolus is beginning to disappear.Although not yet visible in the micrograph, the mitotic spindle is staring to from. Anaphase. Thechromatids of each chromosome have separated, and the daughter chromosomesare moving to the ends of cell as their kinetochoremicrotubles shorten. Figure 12.10 Mitosis in a plant cell

17. Origin of replication 4 3 1 2 Cell wall Plasma Membrane E. coli cell Bacterial Chromosome Two copies of origin Chromosome replication begins. Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell. Replication continues. One copy ofthe origin is now at each end of the cell. Origin Origin Replication finishes. The plasma membrane grows inward, and new cell wall is deposited. Two daughter cells result. Figure 12.11 Bacterial cell division (binary fission)

18. (a) Prokaryotes. During binary fission, the origins of the daughter chromosomes move to opposite ends of the cell. The mechanism is not fully understood, but proteins may anchor the daughter chromosomes to specific sites on the plasma membrane. Bacterial chromosome Chromosomes Dinoflagellates. In unicellular protists called dinoflagellates, the nuclear envelope remains intact during cell division, and the chromosomes attach to the nuclear envelope. Microtubules pass through the nucleus inside cytoplasmic tunnels, reinforcing the spatial orientation of the nucleus, which then divides in a fission process reminiscent of bacterial division. (b) Microtubules Intact nuclear envelope (c) Diatoms. In another group of unicellular protists, the diatoms, the nuclear envelope also remains intact during cell division. But in these organisms, the microtubules form a spindle within the nucleus. Microtubules separate the chromosomes, and the nucleus splits into two daughter nuclei. Kinetochore microtubules Intact nuclear envelope Kinetochore microtubules (d) Most eukaryotes. In most other eukaryotes, including plants and animals, the spindle forms outside the nucleus, and the nuclear envelope breaks down during mitosis. Microtubules separate the chromosomes, and the nuclear envelope then re-forms. Centrosome Fragments of nuclear envelope Figure 12.12 A hypothetical sequence for the evolution of mitosis

19. In each experiment, cultured mammalian cells at two different phases of the cell cycle were induced to fuse. EXPERIMENTS CONCLUSION RESULTS Experiment 2 Experiment 1 S G1 M G1 S S M M When a cell in the S phase was fused with a cell in G1, the G1 cellimmediately entered the S phase—DNA was synthesized. When a cell in the M phase was fused with a cell in G1, the G1 cell immediately began mitosis— a spindle formed and chromatin condensed, even though the chromosome had not been duplicated. The results of fusing cells at two different phases of the cell cycle suggest that molecules present in the cytoplasm of cells in the S or M phase control the progression of phases. Figure 12.13 Inquiry Are there molecular signals in the cytoplasm that regulate the cell cycle?

20. G1 checkpoint Control system S G1 G2 M M checkpoint G2 checkpoint Figure 12.14 Mechanical analogy for the cell cycle control system

21. G0 G1 checkpoint G1 G1 (b) If a cell does not receive a go-ahead signal at the G1checkpoint, the cell exits the cell cycle and goes into G0, a nondividing state. (a) If a cell receives a go-ahead signal at the G1 checkpoint, the cell continues      on in the cell cycle. Figure 12.15 The G1 checkpoint

22. 3 G1 G1 M G2 G2 (a) Fluctuation of MPF activity and cyclin concentration during the cell cycle S S M MPF activity Cyclin Relative Concentration Time (b) Molecular mechanisms that help regulate the cell cycle 1 Synthesis of cyclin begins in late S phase and continues through G2. Because cyclin is protected from degradation during this stage, it accumulates. 5 During G1, conditions in the cell favor degradation of cyclin, and the Cdk component of MPF is recycled. G1 S Cdk M G2 DegradedCyclin G2checkpoint 2 Cdk Cyclin is degraded Cyclin MPF MPF promotes mitosis by phosphorylating various proteins. MPF‘s activity peaks during metaphase. 4 During anaphase, the cyclin component of MPF is degraded, terminating the M phase. The cell enters the G1 phase. Figure 12.16 Molecular control of the cell cycle at the G2 checkpoint Accumulated cyclin molecules combine with recycled Cdk mol- ecules, producing enough molecules of MPF to pass the G2 checkpoint and initiate the events of mitosis.

23. 3 2 1 EXPERIMENT Scalpels A sample of connective tissue was cut up into small pieces. Petriplate Enzymes were used to digest the extracellular matrix,resulting in a suspension of free fibroblast cells. Cells were transferred to sterile culture vessels containing a basic growth medium consisting of glucose, amino acids, salts, and antibiotics (as a precaution against bacterial growth). PDGF was added to half the vessels. The culture vessels were incubated at 37°C. Without PDGF With PDGF Figure 12.17 Inquiry Does platelet-derived growth factor (PDGF) stimulate the division of human fibroblast cells in culture?

24. a b In a basic growth medium without PDGF (the control), cells failed to divide. CONCLUSION RESULTS Without PDGF In a basic growth medium plus PDGF, cells proliferated. The SEM shows cultured fibroblasts. With PDGF 10 µm This experiment confirmed that PDGF stimulates the division of human fibroblast cells in culture.

25. Cells anchor to dish surface and divide (anchorage dependence). When cells have formed a complete single layer, they stop dividing (density-dependent inhibition). (a) Normal mammalian cells. The availability of nutrients, growth factors, and a substratum for attachment limits cell density to a single layer. If some cells are scraped away, the remaining cells divide to fill the gap and then stop (density-dependent inhibition). 25 µm Cancer cells do not exhibitanchorage dependence or density-dependent inhibition. (b) Cancer cells. Cancer cells usually continue to divide well beyond a single layer, forming a clump of overlapping cells. 25 µm Figure 12.18 Density-dependent inhibition and anchorage dependence of cell division

26. 2 4 3 1 Lymphvessel Tumor Bloodvessel Glandular tissue Cancer cell MetastaticTumor A small percentage of cancer cells may survive and establish a new tumor in another part of the body. A tumor grows from a single cancer cell. Cancer cells invade neighboring tissue. Cancer cells spread through lymph and blood vessels to other parts of the body. Figure 12.19 The growth and metastasis of a malignant breast tumor

27. Unnumbered Figure p. 235

28. Using the data in the table below, the best conclusion concerning the difference between the S phases for betaand gamma is that gamma contains more DNA than beta. beta and gamma contain the same amount of DNA. beta contains more RNA than gamma. gamma contains 48 times more DNA and RNA than beta. beta is a plant cell and gamma is an animal cell. 0 Minutes Spent in Cell Cycle Phases

29. 0 • Cytokinesis usually, but not always, follows mitosis. If a cell completed mitosis but not cytokinesis, what would be the result? • a cell with a single large nucleus • a cell with high concentrations of actinand myosin • a cell with two abnormally small nuclei • a cell with two nuclei • a cell with two nuclei but with half theamount of DNA

30. 0 • Taxol is an anticancer drug extracted from the Pacific yew tree. In animal cells, taxol disrupts microtubule formation by binding to microtubules and accelerating their assembly from the protein precursor, tubulin. Surprisingly, this stops mitosis. Specifically, taxol must affect • the fibers of the mitotic spindle. • anaphase. • formation of the centrioles. • chromatid assembly. • the S phase of the cell cycle.

31. 0 • Movement of the chromosomes during anaphase would be most affected by a drug that • reduced cyclin concentrations. • increased cyclin concentrations. • prevented elongation of microtubules. • prevented shortening of microtubules. • prevented attachment of the microtubules to the kinetochore.

32. 0 • Measurements of the amount of DNA per nucleus were taken on a large number of cells from a growing fungus. The measured DNA levels ranged from 3 to 6 picograms per nucleus. In which stage of the cell cycle was the nucleus with 6 picograms of DNA? • G0 • G1 • S • G2 • M

33. 0 • A group of cells is assayed for DNA content immediately following mitosis and is found to have an average of 8 picograms of DNA per nucleus. Those cells would have __________ picograms at the end of the S phase and __________picograms at the end of G2. • 8 ... 8 • 8 ... 16 • 16 ... 8 • 16 ... 16 • 12 ... 16

34. A particular cell has half as much DNA as some of the other cells in a mitotically active tissue. The cell in question is most likely in • G1. • G2. • prophase. • metaphase. • anaphase.

35. In some organisms, mitosis occurs without cytokinesis occurring. This will result in • cells with more than one nucleus. • cells that are unusually small. • cells lacking nuclei. • destruction of chromosomes. • cell cycles lacking an S phase.

36. The rhythmic changes in cyclin concentration in a cell cycle are due to • its increased production once the restriction point is passed. • the cascade of increased production once its enzyme is phosphorylated by MPF. • its degradation, which is initiated byactive MPF. • the correlation of its production with the production of Cdk. • the binding of the growth factor PDGF.

37. A cell in which of the following phases would have the least amount of DNA? • G0 • G2 • prophase • metaphase • anaphase