2. Science 9 Cells and Reproduction The Cell Cycle Mitosis

3. Understanding: What is mitosis?  Mitosis is the process by which cells divide.  Mitosis is the division of the nucleus  It results in cells with identical genetic information.  Mitosis happens as part of the cell cycle.

4. The Cell Cycle  The cell cycle is an ordered set of events.  Mitosis happens the same way every time for every cell.  It finishes with cell growth and the division of the cell into two daughter cells.

5. The Cell Cycle  The Stages of Mitosis  Mitosis is nuclear division plus cytokinesis, and produces two identical daughter cells during  Interphase  prophase,  metaphase,  anaphase  telophase.  cytokinesis

6. The Cell Cycle  Interphase is technically not part of mitosis, but rather is the growth and production phases of the cell cycle.

7. The Cell Cycle Interphase  The cell is engaged in metabolic activity (making what it makes) and preparing for mitosis.  Chromosomes are not clearly discerned in the nucleus, although a dark spot called the nucleolus may be visible.  The Cell is just “doing its thing”

8. The Cell Cycle Prophase  During prophase the chromosomes coil up. They become shorter and fatter. The nuclear envelope seems to disappear.  If you squash the cell and look at it under a microscope it is possible to see chromosomes and count them. You would see 46 chromosomes in a normal human cell.  This stage may take over one hour.

9. Prophase

10. The Cell Cycle Prophase to Metaphase… Prometaphase  The nuclear membrane dissolves, marking the beginning of prometaphase.  The chromosomes begin moving.

11. The Cell Cycle Metaphase  This is a very short stage in mitosis. The chromosomes move and line themselves up at the equator of the cell.  It should be possible with a good microscope to see that each chromosome has divided into two daughter chromatids. These are genetically identical to each other.  This stage only takes about 15 minutes

12. Metaphase

13. The Cell Cycle Anaphase  During anaphase the chromatids separate (are pulled apart) and move to the poles (opposite ends of the cell).  You may be able to see spindle fibres which help to pull the chromatids apart.  Although this stage only takes about ten minutes, it is the most interesting stage because it shows that the cell division is genetically exact.

14. Early Anaphase

15. Anaphase

16. Late Anaphase

17. Telophase  In the human cell, there are now 46 daughter chromatids at each end of the cell.  Each one uncoils: it gets longer and thinner.  It is no longer possible to see and count individual chromosomes.  This stage takes much longer but it is not so interesting.

18. Telophase

19. The Cell Cycle Cytokenesis  After mitosis has taken place (the division of the nucleus) the cell divides into two cells.  This process is called cytokinesis. New cell membrane is formed dividing the cell into two.  In animal cells, cytokinesis results when a fiber ring composed of a protein around the center of the cell contracts pinching the cell into two daughter cells, each with one nucleus.

20. The Cell Cycle Cytokenesis  In plant cells, the rigid wall requires that a cell plate be synthesized between the two daughter cells.

21. The Cell Cycle  Meiosis is another kind of cell division. This is the one which makes sperms and eggs. We’ll learn about that later.

22. The Cell Cycle Cells Alive Video

23. The Cell Cycle Think about….  You are made of billions of cells, they are all genetically identical to each other and identical to your original self when you were a fertilized egg or zygote.  If a cell divides once every day, how many cells will there be in a week?  How many in a month?  How many in a year?