1. CELL REPLICATION Why do cells divide? To produce new cells To help the organism grow and increase in size To replace ones that have worn out

2. To make an organism grow. All vertebrates begin at a single cell stage. Egg and sperm fuse to make a zygote. This then divides, and then the next two cells divide and then the next two until you end up with millions of cells. Along the way the cells will differentiate so they have the right structure for what ever their job is. Some cells become so specialised that they lose the ability to divide, replicate and grow again. Instead they just die and have to be replaced by a brand new version but some will never be replaced at all. Can you think of any examples of both these types of cells in the human body?

3. Cells that are not replaced Nerve Cells – are made to last the lifetime of the organism. Once damaged they are not replaced but other nerve cells are able to make new connections to bypass the damaged area. Retinal cells – Cells in the retina which lines the back of the eye. These absorb light. The cells in the lens of the eye Cardiac muscle cells – Just increase in size

4. Cells that arise from stem cells in the adult body Sperm cells Egg cells All blood cells: WBC, RBC, and platelets Skin cells in the dermis Epithelial gut cells

5. Mitosis Overview Definition- process by which cells divide to create two identical daughter cells. Each daughter cell has the same kind and number of chromosomes as the original parent cell. Purpose- growth of the organism and replacement of old cells.

6. Mitosis is responsible for forming new cells during development. future arm and hand of embryo, five weeks old

7. Mitosis stages Interphase Prophase Metaphase Anaphase Telophase

8. Interphase Chromosomes can’t be seen. Chromosomes duplicate.

10. Prophase Chromosomes become visible. Each chromosome consists of two identical chromatids. Nuclear membrane dissolves.

12. Metaphase Chromosomes line up along equator of cell. Each chromosome becomes attached to a spindle fibre.

14. Met

15. Anaphase Centromeres split, separating each double- stranded chromosomes into two chromatids. Chromatids draw apart and move towards opposite poles of the cell.

17. Telophase and cytokinesis By this stage, the chromatids will have moved to the poles of the cell. Nuclear membrane reforms around each set of chromosomes. The cytoplasm divides by cytokinesis.

20. Cytokinesis Division of cell into two daughter cells. In animal cells, actin filaments, working with myosin, constrict the plasma membrane and divide the cell in two.

21. A frog egg.

22. Fig. 17.5, p. 350 TELOPHASE PROPHASE ANAPHASE METAPHASE nucleus cytoplasm one spindle pole one of the condensed chromosomes spindle equator microtubules organized as a spindle apparatus INTERPHASE

27. Can you identify the different stages of mitosis from the diagram below? metaphase anaphase prophase interphase telophase