1. Meiosis

2. GENES Genes determine individual traits
Genes do not exist free in the nucleus of a cell; they are lined up on chromosomes.
A chromosome can contain a thousand or more genes along its length.

3. A cell with two of each kind of chromosome is called a diploid cell.
In the body cells of most plants and animals chromosomes occur in pairs.
One chromosome in each pair come from the male parent and the other came from the female parent.
A cell with two of each kind of chromosome is called a diploid cell.
They are said to contain 2n number of chromosomes.
Each species of organisms contains a characteristic number of chromosomes.
Humans have 23 pairs of chromosomes or 46 total.

4. Why Meiosis?
When cells divide by mitosis, the new cells have exactly the same number and kind of chromosomes as the original cell.
The body uses meiosis to provide a cell for the purpose of reproduction.
A cell produced by meiosis is said to be haploid and contain n amount of chromosomes
This cell is called a gamete.
An example of a gamete is sperm (in males) and eggs (in females)
Produced by meiosis, gametes have half the number of chromosomes as a parent’s body cell.

5. Meiosis consists of 2 separate division, known as meiosis I and meiosis II.
Meiosis I begins with one diploid (2n) cell. (just like mitosis).
By the end of meiosis II there are four haploid (n) cells. (this is different then mitosis).
When a haploid (n) sperm cell fertilizes a haploid egg (n) cell, the resulting cell (called a zygote) once again has a diploid (2n) number of cells.

6. The Phases of Meiosis Interphase Prophase I Metaphase I Anaphase I
Telophase I
Prophase II
metaphaseII
Anaphase II
Telophase II

7. Meiosis I: interphase Similar to mitosis
The cell replicates its chromosomes. After replication each chromosome consists of 2 identical sister chromatids, held together by a centromere.

8. Meiosis I : prophase I Steps similar to prophase in mitosis:
The DNA of the chromosomes coil up.
Centrioles migrate to opposite poles of the cell.
The spindle apparatus forms
The nuclear envelope begins to disintegrate

9. Meiosis: prophase I continued:
Steps different than prophase in mitosis:
As the DNA coils, homologous chromosomes line up with each other, gene by gene along their length, to form a four-part structure called a tetrad.
Homologous chromosomes have genes for the same trait, such as height.
A tetrad consist of two homologous chromosomes, each made up of two sister chromatids. The chromatids in a tetrad pair so tightly that crossing over can occur.

10. Meiosis: prophase I continued
Crossing over is when non-sister chromatids from homologous chromosomes break and exchange genetic material.

12. Crossing over:
There are an average of two to three crossovers for each pair of homologous chromosomes
Crossing over aids in genetic diversity.

13. Meiosis I: metaphase I
The centromeres of each chromosome become attached to a spindle fiber
The spindle fibers pull the tetrads into the middle or equator, of the spindle.
this is different then in mitosis:
In meiosis the homologous chromosomes are lined up side by side as tetrads. In mitosis chromosomes line up on the spindle’s equator independently of each other.

15. Meiosis I: anaphase I
Homologous chromosomes, each with its two chromatids, separate and move to opposite ends of the cells.
The centromeres holding the sister chromatids together do not split as they do during anaphase in mitosis.

17. Meiosis I: telophase I Spindle is broken down Chromosomes uncoil
Cytoplasm divides to yield two new cells
Each cell has half the genetic information as the original cell because it has only one chromosome from each homologous pair
The daughter cells are genetically different from each other and the parent.
Another cell division is needed because each chromosome is still doubled.

19. Meiosis II: Prophase II
DNA coil
A spindle forms
Centrioles migrate to opposite poles of the cell.
Nuclear envelope disintegrates

20. Meiosis II: metaphase II
The chromosomes, made up of sister chromatids, are pulled to the center of the cell and line up randomly at the equator.

21. Meiosis II: anaphase II
The centromere of each chromosome splits allowing the sister chromatids to separate and move to opposite poles.

22. Meiosis II: telophase II
Nuclei re-form
The spindles break down
The cytoplasm divides

23. The events of meiosis II are identical to those of mitosis except that the chromosomes do not replicate before they divide.

24. At the end of meiosis II, four haploid cells have been formed from one diploid cell.