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Key Concept Review 1. Cell cycle events: Interphase (growing, making, repairing): • first growth phase (G 1 ): cell produces new proteins and organelles • synthesis phase (S): cell makes an entire copy of DNA and produces key proteins associated with chromosomes • second growth phase (G 2 ): cell produces organelles and structures needed for division Mitosis (cell division): • prophase: chromatin condenses and forms chromosomes; centrioles move toward poles and spindle fibres form • metaphase: chromosomes line up at centre of cell • anaphase: sister chromatids separate and move to opposite poles • telophase: cell splits into two (cytokinesis) 2. Mitosis in plant cells involves a cell plate forming to separate the cytoplasm during cytokinesis. In animal cells, the centrioles move to the poles during prophase. Mitotic spindles attach to centromeres and centrioles. During telophase, the cell membrane pinches inward to make two cells. 3. Apoptosis is the normal death of a cell that is no longer useful or is not functioning normally. Death of cells allows space for new, healthy cells to grow. 4. Students’ answers will vary. The similarities between mitosis in plant and animal cells include the condensing of the chromosomes during prophase, the lining up of chromosomes in the centre of the cell during metaphase, the formation of the mitotic spindle, and the separation of the chromatids during anaphase. The differences between mitosis in plant and animal cells include the formation of a cell plate during cytokinesis in plant cells, the pinching inward of the cell membrane during telophase in animal cells, and the centrioles that attach to mitotic spindles and move to opposite poles in the animal cell during prophase. 5. A cancer cell is a cell that divides uncontrollably. 6. Mitosis ensures genetic continuity because cell division produces two daughter cells that each contains the same number of chromosomes and the same genetic information as the parent cell. 7. Mitosis makes growth and repair of cells possible because during cell division, two daughter cells are formed. These new cells replace cells that are damaged or that are no longer functioning effectively. Because there are limits to how large a cell can grow, an organism grows by forming new cells. Page 37 1.2 48 Unit A: Tissues, Organs, and Systems of Living Things 8. Cells spend most of their cycle in interphase because it is during that stage that cells grow, duplicate DNA, and prepare for cell division. 9. As a cell prepares to divide, the chromosomes are duplicated. 10. A cancer cell continuously divides. Cancer cells do not undergo apoptosis, do not stick to other cells and do not stop reproducing. Cancer cells behave independently and may move to other locations in the body. Connect Your Understanding 11. Differences between mitosis in plant and animal cells include the formation of a cell plate during cytokinesis in plant cells, the pinching inward of the cell membrane during telophase in animal cells, and the centrioles that attach to mitotic spindles and move to opposite poles in the animal cell during prophase. 12. Cell division must be controlled for cells to be healthy because the uncontrolled production of cells can lead to tumours and excessive numbers of cells. Cells need to reproduce to replace injured cells or cells that have died. Some cells are produced for specific purposes, such as fighting disease, and need to be removed once the job is complete so that energy and resources in the organism are used efficiently. 13. If spindle fibres do not form, the sister chromatids cannot be pulled apart and cell division cannot be completed successfully. This would mean that cells would not divide and growth would not occur. 14. When a daughter cell is formed that does not contain the correct amount of chromosomes, the cell is missing some of its genetic information. This cell would not be able to survive. 15. The photo shows anaphase. At this stage, the sister chromatids are separating into individual chromosomes, which are moving to the opposite ends (poles) of the cell. 16. Since nerve cells rarely undergo mitosis, nerve cells that are damaged may not be replaced. If a specific region containing nerve cells was damaged, there may not be a nerve cell to undergo mitosis, which results in loss of that nerve centre. 17. Ultraviolet light can cause cells to mutate, which can potentially cause cancer cells to develop. Sunscreen reduces the amount of ultraviolet light that reaches the skin cells and reduces the chances of cellular mutation. 18. Cancer researchers can use their knowledge about mitosis to decrease a cancer cell’s rate of mitosis without affecting normal cells. They can also look at stage-specific treatments to stop cancer cells from reproducing.