Cell Division Mitosis and Meiosis

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.

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

Brain Pop Video http://www.brainpop.com/science/cellularlifeandgenetics/mitosis/

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

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.

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

Plant Cells

Rat – epithelial cells

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

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).

Meiosis Video

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

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

Meiosis – division error Chromosome pair

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

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