1. The Life and Death of Cells (in eukaryotic cells) Every hour about one billion (10 9 ) cells die in your body, and the same number of cells are made. As multicellular organisms grow, the number of cells increases. Why does the number of cells increase instead of the size of the cells? When you look in the mirror, what we see is the dead skin. We lose 30,000 to 40,000 skin cells every minute! Why doesn’t our skin wear away?

2. Here is a summary of what you will learn in this section: Cell division allows organisms to reproduce, to grow and to repair. The cell cycle, is the series of events that take place in a cell, leading to its division and replication. As cells grow and divide, they go through 3 stages in cell cycle.

3. Cell Cycle http://www.youtube.com/watch?v=Q6ucKWIIF mg The 3 distinct stages of cell cycle are:  Interphase  Mitosis  cytokinesis

4. Interphase The longest stage of cell cycle, Cells carry out all their regular activities, including growth, plus copying DNA DNA is duplicated More organelles are formed  DNA (deoxyribonucleic acid) is very long, thin, invisible strand.

5. Cell Division Cell division occurs in two stages: 1)Mitosis – the division of the contents of the nucleus (every thing is still in the same cell membrane). 2)Cytokinesis – the complete division of the cell, producing two genetically identical cells called daughter cells.

6. Mitosis Is composed of 4 phases: Prophase Metaphase Anaphase Telophase PMAT

7. Prophase The long strands of DNA become condense and compact to form chromosomes. Each chromosome is a long piece of coiled DNA and proteins. Chromosomes are visible under light microscopes. Each chromosome consists of two identical strands called sister chromatids. The sister chromatids are held together by centromere. The nuclear membrane dissolves during prophase.

8. Metaphase Chromosomes line up in the middle of the cell by the mitotic spindles (spindle fibers). Anaphase The centromere splits by the pull of the mitotic spindles, and the sister chromatids separate to form daughter chromosomes. The daughter chromosomes move to the opposite parts of the cell Telophase The daughter chromosomes stretch out to become thinner and no longer visible under microscope. A new nuclear membrane forms around each group of daughter chromosomes. At this stage cell has two nuclei.

9. Cytokinesis Final stage of cell cycle Cytoplasm divides producing two genetically identical daughter cells. In an animal cell the cell membrane is pinched off in the centre. In a plant cell, a plate between the daughter cells develops into a new cell wall.

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11. Do the cells grow at the same rate at different ages? Do the cells of different organisms grow at the same rate? Do the cells of different organs in an organism grow at the same rate? What factors affect cell growth?

13. Checkpoints in the cell cycle To keep the organisms healthy, during the cell cycle, the cell activities are controlled by the nucleus at specific points. A cell should remain in interphase and not divide if: Signals from surrounding cells tell the cell not to divide, There are not enough nutrients for cell growth, The DNA has not been replicated, The DNA is damaged. If one or more of the checkpoints fail, the cell and all of the subsequent daughter cells will continue to divide uncontrollably.