Chapter 10 Notes 10-1 The larger a cell becomes, the more demands the cell places on its DNA. In addition, the cell has trouble moving enough nutrients and wastes across the cell membrane. 1. Living things grow by producing more cells. When a cell is small, the info stored in DNA is able to meet all of cell’s needs. In time, cell’s DNA wouldn’t be able to serve increasing needs of growing cells. 2. The rate at which waste products leave the cell depends on surface area of cell. Rate at which food and O are used up and waste products are produced depends on cell’s volume. 3. The surface are of a cell would be equal to length x width x number of sides. The volume of the cell equal to length x width x height. To get ratio of surface area to volume, divide surface area by volume. 4. If length of cell doubled, cell’s surface would be 2 x 2 x 6=24 cm2. Volume increases more rapidly than surface area, causing ratio to decrease. If a cell got too large, it would be hard to get sufficient amounts of O and nutrients in and waste products out. 1 x 1 x 1 cm 2 x 2 x 2 cm 3 x 3 x 3 cm 1 cm x 1 cm x 6= 6 cm 2 cm x 2 cm x 6= 24 cm 3 cm x 3 cm x 6= 54 cm 1 cm x 1 cm x 1 cm= 1 cm 6/1 = 6:1 2 cm x 2 cm x 2 cm = 8 cm 24 / 8 =3 : 1 3 cm x 3 cm x 3 cm= 27 cm 54 / 27 = 2 : 1 Cell Size Surface Area (length x width x 6) Volume (length x width x height) Ratio of surface area to Volume 5. Before cell becomes too large, it divides forming 2 “daughter” cells through cell division. Before cell division occurs, cells replicates all of its DNA. This replication of DNA solves problem of info storage b/c each daughter cell gets one complete set of genetic info. 6. Cell division also solves problem of increasing size by reducing cell volume. Each daughter cell has increased ratio of surface area to volume, allowing efficient exchange of materials with environment. 10-2 During the cell cycle, a cell grows, prepares for division, and divides to form two daughter cells, each of which then begins the cycle again. Biologists divide the events of mitosis into four phases: prophase, metaphase, anaphase, and telophase. Mitosis insures that each daughter cell has the same genetic info as the parent cell. During prophase in animal cells, the centrioles separate and take up positions on opposite sides of the nucleus. In addition, chromosomes condense and the spindle appears. During metaphase, the chromosomes line up across the center of the cell. Microtubules connect the chromosome to each pole of the spindle. During anaphase, the centromeres that join the sister chromatids split, and the sister chromatids separate and become individual chromosomes. In telophase, the chromosomes, which were distinct and condensed, uncoil and disperse as the nuclear envelope re-forms. Cytokinesis is the division of the cytoplasm. 1. Every cell must first copy its genetic info before cell division begins. Each daughter cell then gets a complete copy of that info. In most prokaryotes, rest of cell division is separating the contents of cell into 2 parts. In eukaryotes, cell division is more complex and occurs in 2 main stages: mitosis and cytokinesis. 2. Reproduction of mitosis is classified as asexual, since cells produced by mitosis are genetically identical to parent cell. Mitosis is source of new cells when multicellular organism grows and develops. 3. In eukaryotic cells, genetic info that’s passed from one generation of cells to next is carried by chromosomes, which are made of DNA and proteins. Cells of every organism has specific number of chromosomes (humans- 46 chromosomes). 4. Chromosomes are visible in cells b/c DNA and protein molecules are spread throughout nucleus, but at start of cell division, chromosomes condense into compact, visible structures. 5. Before cell division, each chromosome is copied into “sister” chromatids. When cell divides, “sister” chromatids separate from each other. One chromatid goes to each of 2 new cells. Each pair of chromatids is attached to the centromeres. 6. Cell cycle has 4 phases. Mitosis and cytokinesis takes place during M phase. Chromosome replication take place during S phase. When cell copies chromosomes, it makes duplicate set of DNA. Between M and S phases are G1 and G2. “G” in the names of these phases stands for “gap”, but these are periods of intense growth and activity. 7. During normal cell cycle, interphase can be long. Interphase can be divided into 3 phases: G1, S, and G2. G1 phase is period of activity where cells do most of their growing. Cells increase in size and synthesize new proteins and organelles. 8. G1 followed by S phase, where chromosomes are replicated and synthesis of DNA molecules takes place. Key proteins associated with chromosome are synthesized during S phase. Once cell enters S phase and begins replication of its chromosomes, it completes rest of cell cycle. 9. When DNA replication is completed, cell enters G2 phase, which is shortest of the 3 phases of interphase. During this phase, many of organelles and molecules required for cell division are produced. When events of G2 phase are completed, cell enters M phase and begins process. 10. Depending on type of cell, 4 phases of mitosis can last from a few minutes to several days. 11. First and longest phase of mitosis, prophase, takes 50-60% of mitosis time. During prophase, chromosomes become visible. The centrioles separate and take positions on opposite sides of nucleus. The centrioles lie in region called centrosome that helps to organize spindle. During prophase, condensed chromosomes become attached to fibers in spindle at point near centromeres of each chromatid. Plant cells don’t have centrioles. Near end of prophase, chromosomes coil more tightly and nucleolus disappears and nuclear envelope breaks down. 12. Second phase of mitosis, metaphase, lasts only a few minutes. During this phase, chromosomes line up across center of cell. Microtubules connect centromeres of each chromosome to 2 poles of spindle. 13. Anaphase is 3rd phase of mitosis where centromeres that join the sister chromatids split, allowing sister chromatids to separate and become individual chromosomes. Chromosomes continue to move until they’ve separated into 2 groups near poles of spindle. Anaphase ends when chromosomes stop moving. 14. Following anaphase is telophase, the final phase of mitosis. In this phase, chromosomes begin to disperse into tangle of dense material. A nuclear envelope reforms around each cluster of chromosomes. Spindle begins to break apart and nucleolus becomes visible in each daughter nucleus. 15. As result of mitosis, 2 nuclei, each w/ duplicate set of chromosomes, are formed, usually within cytoplasm of single cell. All that remains to complete M phase of cycle is cytokinesis, which usually occurs at same time as telophase. 16. Cytokinesis can take place in number of ways. In most animal cells, cell membrane drawn inward until cytoplasm is pinched into 2 equal parts. Each part contains its own nucleus and cytoplasmic organelles. 17. In plants, a cell plate forms midway between divided nuclei. Cell plate gradually develops into separating membrane. A cell wall then begins to appear in cell plate. 10-3 Cyclins regulate the timing of the cell cycle in eukaryotic cells. Cancer cells do not respond to the signals that regulate the growth of most cells. 1. Scientists observe effects of controlled cell growth in lab by placing some cells in Petri dish with nutrient broth, which provides food for cells. Most cells will grow until they form thin layer covering bottom of dish. Then, cells stop growing. When cells come into contact with other ells, they respond by not growing. 2. If cells are removed from center of dish, cells bordering open space will begin dividing until they have filled empty space. These experiments show that controls on cell growth and cell division can be turned on and off. 3. When an injury like a cut in skin occurs, cells at edges of injury are stimulated to divide rapidly. This produces new cells, starting process of healing. When healing process nears completion, rate of cell division slows. 4. Cells in mitosis contained a protein that when injected into a nondividing cell, would cause a mitotic spindle to form. The amount of this protein in the cell rose and fell in time with cell cycle. There are 2 types of regulatory proteins: those that occur inside cell and those that occur outside cell. 5. Proteins that respond to events inside cell are internal regulators, which allow cell cycle to proceed only when certain processes have happened inside cell. Several regulatory proteins make sure that a cell doesn’t enter mitosis until all its chromosomes have been regulated. 6. Proteins that respond to events outside cell are external regulators, which direct cells to speed up or slow down cell cycle. Growth factors are among most important external regulators. They are important during embryonic development and wound healing. Molecules found on surfaces of neighboring cells often cause cells to slow down or stop their cell cycles. 7. Principal reason why cell growth is regulated carefully may be that consequences of uncontrolled cell growth in multicellular organism are very severe. Cancer cells divide uncontrollably and from masses of cells called tumors that can damage surrounding tissues; theses cells may break loose from tumors and spread throughout the body, disrupting normal activities and causing serious medical problems or death. 8. The various forms of cancer have many causes like smoking tobacco, radiation exposure, or viral infections. The control over the cell cycle has broken down in all cancers. Some cancer cells will no longer respond to external growth regulators and others fail to produce internal regulators that ensure orderly growth. 9. A huge number of cancer cells have defect in gene called p53, which normally halts cell cycle until all chromosomes have been properly replicated. Damaged p53 genes cause cells to lose info needed to respond to signals that would normally control their growth. 10. Cancer is serious disease. Cancer is disease of cell cycle, and conquering cancer will require much deeper understanding of processes that control cell division.