1. Meiosis

2. Meiosis
Meiosis is the cell division which produces the sex cells or gametes.
Meiosis occurs in the cells of the reproductive organs:
Sperm cells – testes
Ova (eggs) – ovaries
Pollen - anthers

3. Meiosis Gametes are produced during sexual reproduction.
Sexual reproduction involves fertilization – the fusion of 2 gametes, one male and one female.
Sexual reproduction produces genetically different individuals.

4. Meiosis I
In meiosis, the chromosome replicates prior to cell division.
The chromosomes have shortened and thickened and appear as two chromatids joined by a centromere.
The nuclear membrane disappears and the spindle begins to form.
Homologous pairs of chromosomes come together.

5. Meiosis I
Homologous pairs of chromosomes line up on the equator of the cell.
Spindle fibres attach to the members of each pair.
The spindle fibres contract separating the pairs of chromosomes.
The nuclear membrane reforms temporarily and the cell divides.

6. Meiosis II The nuclear membrane disappears again.
The chromosomes (consisting of 2 chromatids) line up in the centre of the cell.
Spindle fibres attach to each chromatid.
The fibres contract separating and pulling the chromatids to each end of the cell.

7. Meiosis II
The nuclear membrane forms around each half set of chromosomes.
The cell divides.
Four daughter cells are produced, each with half the number of chromosomes as the parent cell.
These cells become gametes.
Gametes are haploid (n).

9. Sexual Reproduction and Genetic Variation
Sexual reproduction produces many different individuals (a lot of genetic variation).
This is a result of three processes:
Independent Assortment
Gametes are produced with many combinations of chromosomes from each parent.

10. Sexual Reproduction and Genetic Variation
Random Fertilisation
Random gametes combine at fertilisation
Crossing Over
As gametes are produce portions of the parent cell’s two copies of each chromosome are swapped around

11. Independent Assortment
When testes cells produce sperm and ovarian cells produce eggs the chromosome number is halved and the gametes contain different combinations of the chromosomes.
Humans can produce 223 or 8,388,608 different gametes through the independent assortment of chromosomes.

12. Independent Assortment
Male Gametes
Testis Cell
Gametes are produced with many combinations of chromosomes from each parent
Female Gametes
Ovarian Cell

13. Random Fertilization
Random gametes combine at fertilisation, any of the many different male gametes can fertilise any of the many different female gametes
Through random fertilisation, each male and female human can produce 223 x 223 = 246 or 70,368,744,177,664 genetically unique zygotes.

14. Random gametes combine at fertilisation
Random Fertilization
Male Gametes
Female Gametes
Zygotes
Random gametes combine at fertilisation

15. Crossing Over
During meiosis, two chromosomes of a homologous pair (one maternal, one paternal) can cross over and exchange segments, producing genetic variations in the resulting gametes.
This is in addition to the variation created by independent assortment and random fertilisation.

16. Crossing Over

17. Crossing Over Male Gametes Testis Cell
As gametes are produced portions of the parent cell’s two copies of each chromosome are swapped around
Female Gametes
Ovarian Cell

18. Mutation
Some times when chromosomes are being copied during mitosis mutation occurs and the copy is not exact
Most often these mutations have little effect or cause fatality
Some time the mutation results in a new variant of a characteristic (trait).

19. Comparing Mitosis and Meiosis
cell division for growth and repair (used for reproduction only in simple organisms such as bacteria)
Synapsis (crossing over) does not occur
one division
two identical daughter cells
chromosome number is maintained (two copies of each chromosome)

20. Comparing Mitosis and Meiosis
cell division for gamete production
synapsis (crossing over) occurs
two divisions
four unique daughter cells
chromosome number is reduced by half (one copy of each chromosome)