1. Chapter 17
Section
Meiosis

2. Meiosis
Meiosis – is a two-stage cell division in which the chromosome number of the parental cell is reduced by half to form a gamete.
Gamete – a sex cell, such as a sperm or egg cell.
Parent cells are said to contain a diploid number of chromosomes. (Humans = 46) represented by 2n

3. Gamete cells are said to contain a haploid number of chromosomes
Gamete cells are said to contain a haploid number of chromosomes. (Half of a normal cell) (Humans = 23) represented by n
During fertilization, a haploid (23) sperm cell unites with a haploid (23) egg cell to produce a diploid (46) zygote. represented by 2n

4. Homologous Chromosomes
Offspring carry genetic information from both parents.
Each of the 23 chromosomes you receive from your biological mother has a matching chromosome that you receive from your biological father.
These paired chromosomes are known as homologous chromosomes.

5. Homologous Chromosomes
Gene for eye colour as an example would be located in the same position on each chromosome.
**NOTE: The chromosomes depicted in this diagram have been duplicated preparing for mitosis or meiosis.
From Father
From Mother

6. Stages of Meiosis Meiosis involves two nuclear divisions. Meiosis I
Where the number of chromosomes gets reduced to half.
Meiosis II
Where the sister chromatids of the duplicated chromosome separate.
Meiosis II is almost identical to mitosis.

7. Meiosis I Meiosis I consists of 4 phases: Prophase I Metaphase I
Anaphase I
Telophase I

8. Prophase I During prophase I the following events occur:
Duplicated chromosomes condense.
Homologous chromosomes intertwine in a process called synapsis.
During synapsis there are 2 pairs of sister chromatids intertwined, since there are 4 chromatids present, this is referred to as a tetrad.

9. During synapsis, crossing over also occurs.
Crossing over is the exchange of genetic material between two homologous chromosomes.
Centrioles move to the opposite poles of the cell.
Spindle fibres form and attach to the centromeres of the chromosomes.
Nuclear membrane begins to dissolve.

10. Prophase I Nuclear membrane begins to dissolve. Chromosomes condense.
Crossing over occurs.
Homologous chromosomes synapse.
Centrioles move to opposite poles.
Spindle fibres form and attach to centromeres.

11. Metaphase I During metaphase I the following events occur:
Homologous chromosomes line up at the equatorial plate.
Nuclear membranes are completely dissolved.

12. Metaphase I Homologous chromosomes line up at the equatorial plate.
Nuclear Membrane has completely dissolved.

13. Anaphase I During anaphase I the following events occur:
The homologous chromosomes separate from each other and move to opposite poles in a process known as segregation.

14. Anaphase I
Segregation – the separation of the homologous chromosomes to opposite ends of the cell.

15. Telophase I During telophase I the following events occur:
Nuclear membrane begins to form around each mass of chromosomes.
The two nuclei are not identical because one of chromosomes comes from mom and one comes from dad.
Following telophase I, cytokinesis splits the cytoplasm.

16. Telophase I
Nuclear membranes form.
Cytokinesis splits the cytoplasm.

17. Meiosis II Meiosis II consists of 4 phases: Prophase II Metaphase II
Anaphase II
Telophase II

18. Prophase II During prophase II the following events occur:
Chromosomes are still condensed, so that does not need to occur.
Centrioles move to opposite poles.
Spindle fibres form and attach to the centromeres of duplicated chromosomes.
Nuclear membrane begins to dissolve.

19. Prophase II Nuclear membrane begins to dissolve.
Centrioles move to opposite poles.
Spindle fibres form and attach to centromeres.

20. Metaphase II During metaphase II the following events occur:
Chromosomes line up at the equatorial plate.
Nuclear membrane is completely dissolved.

21. Metaphase II Chromosomes line up at the equatorial plate.
Nuclear Membrane has completely dissolved.

22. Anaphase II During anaphase II the following events occur:
Centromeres divide and the sister chromatids separate becoming individual chromosomes.
Chromosomes move to the opposite poles of the cell.

23. Anaphase II Centromeres divide and sister chromatids separate.
Chromosomes move to opposite poles of the cell.

24. Telophase II During telophase II the following events occur:
The nuclear membrane begins to form around each mass of chromosomes.
Following telophase II, cytokinesis will split the cytoplasm.
The final result is 4 haploid gametes or sex cells.

25. Telophase II
Nuclear membranes form.
Cytokinesis splits the cytoplasm.