1. Cell Division Mitosis and Meiosis

2. Cell division
All complex organisms originated from a single fertilised egg (zygote).
Every cell in your body started here, through cell division the numbers are increased.
Cells then specialise and change into their various roles.

3. Mitosis
Mitosis is the process by which new body cells (somatic cells) are produced for:
Growth
Replacing damaged or old cells.
This is a complex process requiring 4 stages:
Prophase
Metaphase
Anaphase
Telophase

4. Brain Pop Video

5. Parent cell
Chromosomes are copied and double in number
Chromosomes now split
2 daughter cells identical to original

6. Mitosis
All daughter cells contain the same genetic information from the original parent cell from which it was copied.
These cells are diploid, meaning they possess 2 sets of chromosomes (2 copies of each gene)
Every somatic cell in your body contains the same genes, but only some act to make the cells specialised – e.g. into nerve or muscle tissue.

7. Mitosis – bone cell slides2 1
Cells split
Chromosomes copied
Parent cell 3 4 5
2 daughter cells
Copies separating

8. Plant Cells

9. Rat – epithelial cells

10. Cell division – ‘reduction division’ Production of sex cells – gametes
Meiosis
Cell division – ‘reduction division’
Production of sex cells – gametes

11. Why do we need meiosis?
Meiosis is necessary to halve the number of chromosomes going into the sex cells.
Gametes are therefore haploid, meaning they contain only one set of chromosomes (23 and not 46 like somatic cells).

12. Meiosis Video

13. Meiosis Parent cell – chromosome pair Chromosomes copied
1st division - pairs split
2nd division – produces 4 gamete cells with ½ the original no. of chromosomes

14. Meiosis – mouse testes
Parent cell
1st division
2nd division
4 gametes

15. Meiosis – division error
Chromosome pair

16. Meiosis error - fertilisation
Should the gamete with the chromosome pair be fertilised then the offspring will not be ‘normal’.
In humans this often occurs with the 21st pair – producing a child with Downs Syndrome

17. 21 trisomy – Downs Syndrome
Can you see the extra 21st chromosome?