1. Mitosis and the cell cycle AS Biology

2. Mitosis and the cell cycle The cell cycle is the series of events in the life of a cell. The cell is either actively dividing – mitosis, or it is between divisions- interphase

3. Interphase This used to be called the resting phase but in fact some important events occur in interphase This stage divides into three sections G1 S G2

4. Still with Interphase G1- the cell enlarges after division and organelles like mitochondria replicate. S- the cell commits to division in this stage because DNA replicates. One chromosome becomes two chromatids G2-Reserves of ATP build up and the centrioles replicate

5. Mitosis The cell division part of the cell cycle is a continuous process but it is described in four named stages Prophase Metaphase Anaphase Telophase

6. Prophase During interphase the chromosomes have appeared as diffuse granular material called chromatin but now they begin to shorten by coiling up. They appear as thin threads

7. Prophase The pairs of centrioles move to opposite poles of the cell and begin to form a spindle of microtubules The chromosomes condense and can be seen to be composed of two identical chromatids held together by centromeres

8. Metaphase The chromosomes align at the equator of the cell and the centromeres attach to the microtubules of the spindle

9. Metaphase On this slide you can clearly see chromosomes aligned at the cell equator

10. Anaphase As the microtubules shorten the two chromatids of each chromosome separate and are pulled to opposite poles of the cell

11. Anaphase The central cell is in anaphase Each cluster of chromosomes contains one copy of each chromosome

12. Telophase Each cluster of chromosomes becomes surrounded by a new nuclear membrane The spindle disappears and the chromosomes uncoil The cytoplasm of the cell separates into two, a process known as cytokinesis

13. Summary

14. Importance of Mitosis Two new cells are formed from one original cell Each cell is a genetically identical copy of the original cell –a clone Each new cell contains the same number and type of chromosomes as the original cell

15. Where does mitosis happen In animals mitosis occurs in all tissues in the embryo, foetus and juvenile for growth. In adults mitosis is used for replacement of tissues like skin, hair, blood and mucous membranes, and for wound healing. Plants don’t grow all over their bodies like animals but have regions of cell division called meristems. These are found at the tips of the root and main and side shoots, and in woody plants, in a ring around the stem.

16. Homework Look at the following web page www.biology.arizona.edu/default.html Click on Cell biology and onion root tips Read the pages and do the activity ( the kinetochore is part of the centromere) Email me the answer (percentage of cells in each stage of the cell cycle) at hwellington@stvincent.ac.uk