1. Reducing the Chromosome Number
Meiosis
Reducing the Chromosome Number

2. Meiosis
Chromosomes carry the DNA, our genetic code that creates all the characteristics of our body.
Human’s have 46 chromosomes, goldfish have 94 chromosomes and fruit flies have 8 chromosomes.
Now if two human’s mate, why do we not have 92 chromosomes in the offspring?
There is a process called Meiosis that happens in our reproduction cells – sperm and eggs.
Sexual reproduction is when a female reproduction cell combines with a male reproduction cell.

3. Meiosis
As we have learned, our body has cells that divide. These cells are called somatic cells. This process is called Mitosis.
Now, human’s somatic cells have 23 pairs of chromosomes for a total of 46 chromosomes.
Our sex cells are called gametes.
Male gametes are called sperm, and female gametes are called eggs.

4. Meiosis Meiosis only happens in cells that produce gametes.
Each gametes is then able to produce half of the genetic material for the offspring.
Cells with half of the chromosomes are called haploid and are symbolized with a single n.
Cells that have a complete set of chromosomes are called diploid symbolized as 2n.
Sperm (n) + egg (n) = zygote (2n)

5. Meiosis During Meiosis Diploid cells (2n)  Haploid cells (n)
If human body cells have 46 chromosomes, then gametes (sperm and egg) each have 23 chromosomes.

6. Homologous Chromosomes
A sperm will have 23 chromosomes and an egg will have 23 chromosomes
When the sperm and egg meet, the 23 chromosomes will pair up with a corresponding chromosome, this is called homologous chromosomes.

7. Homologous Chromosomes
Example: one sperm chromosome carries the DNA for eye colour, it will pair up with the egg chromosome that has the DNA for eye colour.
Thus each parent will contribute one half (n) (haploid) of the chromosomes to create a diploid (2n) somatic cell with the chromosome for eye colour.

8. Prior to Meiosis - Interphase
Recall Chromosomes are what carry our DNA.
Before Meiosis begins, DNA replication occurs in the S-phase of Interphase.
Each Duplicated Chromosome contains two (2) sister chromatids which are attached at their centromeres.

9. Meiosis I There are four (4) phases in Meiosis I:
Prophase I
Metaphase I
Anaphase I
Telophase I
NOTE: For our example we will look at two pairing of chromosomes that have replicated. Remember, the human body has 23 pairs.

10. Prophase I
Is the longest and most complex stage; 90 percent of the meiotic process is spent in prophase
Chromatids condense and shorten
Nuclear membrane disintegrates
During prophase the homologous chromosomes pair up (synapsis) and the non-sister chromatids exchange DNA.
The exchange of DNA is referred to as Crossing Over.

11. Why is Crossing Over Important?
During crossing over segments of non-sister chromatids break and reattach to the other chromatid.
Non-sister chromatids
Tetrad

12. Why crossing over is important
Since one pair can be arraigned two different ways we can create an equation 2pairs? = possible arrangements
In this case we would have 21 – 2 arrangements
But the human body has 23 pairs of chromosomes, thus
223 = 8,388,608 different arrangements.
Now that is a lot of variations. No wonder our siblings do not look like identical to us. or
The same pair of chromosomes create another type of arrangement
One pair of chromosomes create one arrangement

13. Why is Crossing Over Important?
Multiple crossovers create an infinite number of genetic possibilities for just one gamete.
Variation is important and crossing over creates variation in sexual reproduction.

14. Prophase I
Centrioles
Spindle Fibers

15. Metaphase I Shortest phase in Meiosis
Pairs of homologous chromosomes lines up at the equator of the cell.
Spindle Fibers attach to centromere
Independent assortment occurs.

16. Independent Assortment
The process of random segregation and assortment of homologous chromosomes during metaphase resulting in the production of genetically unique gametes.

17. Metaphase I
Equator
Equator OR

18. Anaphase I
Homologous chromosomes separate and are pulled to opposite ends (poles) of the cell.
This ensures that each new cell will receive only one chromosome from each homologous chromosome.
Unlike in mitosis the sister chromatids remain attached.

19. Anaphase I

20. Telophase I
One chromosome from each homologous pair is at each pole of the cell
Two nuclei form
Each nucleus contains a complete copy of the cell’s DNA
Cell divides, forming two cells

21. Cytokinesis
The cytoplasm divides via cytokinesis to yield two new daughter cells.

22. Meiosis II Meiosis II is very similar to Mitosis
There is no DNA replication, no Interphase prior to Meiosis II.
Meiosis II has four (4) stages as well, and this occurs after Meiosis I:
Prophase II
Metaphase II
Anaphase II
Telophase II

23. Prophase II
There is one chromosome of each homologous pair in each cell.
Chromosomes relax and condense again
Nuclear membrane begins to disappear
DNA exists as chromosomes

24. Metaphase II The chromosomes line up at the equator of the cell

25. Anaphase II
Chromosome breaks apart at the centromere and sister chromatids move to opposite poles of the cell.
Once the chromosomes separate, each sister chromatids is considered to be a chromosome.

26. Telophase II Telophase II
Spindle fibres begin to breakdown, and a nuclear membrane forms around each set of chromosomes
Cytoplasm begins to divide.
Telophase II

27. Cytokinesis
Each sex cell divides into 2 producing 4 haploid daughter cells
4 gametes are produced – sperm or eggs
Note that each gamete has its own nuclei as well.

28. The stages of Meiosis

29. Video on Meiosis
Another good video

30. Assignment Page 42 in Workbook
Textbook page 78 – 1-11, 14, 16, and Note, question 7, do not answer the fruit fly part.