1. The formation of gametes.
Meiosis
The formation of gametes.

2. Meiosis
Most organisms use sexual reproduction in at least part of their life cycle.
Sexual reproduction involves the fusion of nuclei from 2 different parents.
The offspring end up with a unique combination of genes.
For this to work there needs to be a way of reducing the number of chromosomes in sex cells before reproduction.

3. Meiosis
If this did not happen, the numbers of chromosomes would double every generation.
Meiosis is the cell division process that ensures the halving of chromosome numbers.

4. Meiosis Adult ♀ Adult ♂ Egg Sperm Mitosis Meiosis Fertilisation
Zygote → embryo→ Foetus→ Baby→ Adult
Mitosis
Meiosis
Fertilisation

5. Meiosis Cell division which has 2 stages Halves the chromosome number
Used to form gametes/ sex cells
Produces 4 gametes, each with a haploid number of chromosomes.

6. Meiosis I Interphase I Same as in Mitosis.
DNA replicates, giving 2 identical chromatids joined at a centromere.
The centrioles replicate and move to each end of the nucleus.
The nucleolus disappears, as does the nuclear membrane.

7. Meiosis I
Prophase I
Homologous sets of sister chromatids undergo synapsis.
They line up side by side, attached by their centromeres, to form a tetrad (4 chromatids).
During synapsis the chromatids may exchange pieces of DNA – a process called Crossing Over.
The points on the chromatids where they break and rejoin are called Chiasmata.
The centrioles form a spindle across the nuclear material.

8. Crossing Over
This is where a piece of one chromosome is exchanged for a piece of its homologous pair.
This gives rise to new combinations of genes.
The points on the chromatids where they break and rejoin are called Chiasmata. (singular – chiasma)

10. Meiosis I
Metaphase I
The tetrads line up across the equator of the spindle.
Spindle fibres from one of the poles attach to one chromosome of each pair.
Spindle fibres from the opposite pole attach to the homologous chromosome.

11. Meiosis I Anaphase I The centromeres linking the tetrads separate.
The pairs of chromatids move as single units towards the poles.

13. Meiosis I
Telophase I
The two daughter cells form and the nuclear membrane re-forms.
Each cell now has a haploid set of chromosomes.
BUT each chromosome still has 2 chromatids.
Often the cytoplasm also divides by cytokinesis before the cell goes into the second division phase, called meiosis II

15. Meiosis II
This cell division proceeds in a similar manner to mitosis, except there is no DNA replication beforehand.
Prophase II
A spindle forms at right angles to the previous spindle.
The chromosomes move to the equator of each cell.

16. Meiosis II Metaphase II
The chromosomes line up at the equator as in mitosis.

17. Meiosis II Anaphase II The centromeres of sister chromatids separate.
The sister chromatids of each pair are now individual chromosomes.
These move towards opposite poles of the cell.

18. Meiosis II Telophase II
Nuclei begin to form at opposite poles, and cytokinesis occurs.

20. Meiosis II
At the end of meiosis, cells have half the chromosome number.
There are four cells, which are not all genetically identical, especially if there has been crossing over.

22. Gametogenesis Meiosis is different in males and females.
In males its called Spermatogenesis.
In Females its called Oogenesis.

23. Spermatogenesis In males, each cell formed becomes a sperm cell.
The sequence is uninterrupted.
Adult males produce as many as 250 – 400 million sperm cells in each ejaculation.

25. Oogenesis The formation of eggs differs in 3 ways:
During the first meiotic division cytokinesis is uneven – one cell takes all the cytoplasm going on to form the egg. The smaller cell forms polar bodies which disintegrate.
At birth the ovary has all the cells it will ever have to develop into eggs.

26. Oogenesis
Formation of the eggs includes long “resting” periods. Between birth and puberty, the egg cells enlarge and the follicles around them grow. The primary egg cells replicate their DNA.
They enter prophase I but do not change again until activated by hormones at puberty.
An egg cell activated periodically by hormones completes meiosis I, then stops.
Penetration of the sperm triggers meiosis II.