1. Chapter 11.4
Meiosis

2. Mendel’s Principles What are Mendel’s Four Main
Principles of Genetics?
Biological characteristics, traits, are determined by units known as genes.
For a single trait, different forms of the gene, alleles, may be dominant or recessive.
In sexually reproducing organisms, each organism has two copies of each gene. These genes segregate when gametes form.
The alleles for different genes segregate independently from one another.

3. Chromosome Genes Determine traits
Located in specific positions on chromosomes
Gene A
Gene B
Gene C
Gene D
Gene E

4. Chromosomes
Each organism must inherit a single copy of every gene from each of its “parents.”
When gametes are produced, the two sets of genes must be separated so that each gamete ends up with just one set.

5. Chromosome Number All organisms have different numbers of chromosomes.
A body cell in an adult fruit fly has 8 chromosomes: 4 from the fruit fly's male parent, and 4 from its female parent.
Human Chromosome Number?

6. Chromosome Number These two sets of chromosomes are homologous.
Each chromosome from the male parent has a corresponding chromosome from the female parent.
There are four homologous pairs of chromosomes!
Humans?

7. Chromosome Number
A cell that contains both sets of homologous chromosomes is said to be diploid.
The number of chromosomes in a diploid cell is sometimes represented by the symbol 2N.
For Drosophila fruit flies, the diploid number is 8, which can be written as 2N=8.
Humans Diploid Number?

8. Chromosome Number
Gametes contain only a single set of chromosomes, and therefore only a single set of genes.
These cells are haploid. Haploid cells are represented by the symbol N.
For Drosophila fruit flies, the haploid number is 4, which can be written as N=4.
Human haploid number?

9. Meiosis
Process of division in which the number of chromosomes per cell is cut in half
Creation of haploid cells
Separates homologous chromosomes
Makes gametes

10. Meiosis Meiosis involves two divisions
Meiosis I and Meiosis II.
By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells.

11. Telophase I and Cytokinesis
Meiosis I
Prophase I
Metaphase I
Anaphase I
Telophase I and Cytokinesis
Interphase I

12. Cells undergo a round of DNA replication, forming duplicate chromosomes.
Interphase I

13. There are 4 chromatids in a tetrad.
Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
There are 4 chromatids in a tetrad.
MEIOSIS I
Prophase I

14. Spindle fibers attach to the chromosomes.
MEIOSIS I
Metaphase I
Spindle fibers attach to the chromosomes.

15. MEIOSIS I
Anaphase I
The fibers pull the homologous chromosomes toward opposite ends of the cell.

16. Nuclear membranes form. The cell separates into two cells.
MEIOSIS I
Telophase I and Cytokinesis
Nuclear membranes form.
The cell separates into two cells.
The two cells produced by meiosis I have chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I.

17. Meiosis II
The two cells produced by meiosis I now enter a second meiotic division.
Unlike meiosis I, neither cell goes through chromosome replication.
Each of the cell’s chromosomes has 2 chromatids.

18. Telophase II and Cytokinesis
Meiosis II
During meiosis, the number of chromosomes per cell is cut in half through the separation of the homologous chromosomes. The result of meiosis is 4 haploid cells that are genetically different from one another and from the original cell.
Meiosis II
Telophase I and Cytokinesis I
Metaphase II
Anaphase II
Telophase II and Cytokinesis
Prophase II

19. MEIOSIS II
Prophase II
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell.
MEIOSIS II Prophase II - Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell.

20. The chromosomes line up in the center of cell.
MEIOSIS II
Metaphase II
The chromosomes line up in the center of cell.
MEIOSIS II Metaphase II - The chromosomes line up in a similar way to the metaphase state of mitosis.

21. MEIOSIS II
Anaphase II
The sister chromatids separate and move toward opposite ends of the cell.
MEIOSIS II Anaphase II - The sister chromatids separate and move toward opposite ends of the cell.

22. Meiosis II results in four haploid (N) daughter cells.
Telophase II and Cytokinesis
Meiosis II results in four haploid (N) daughter cells.
MEIOSIS II Telophase II and Cytokinesis - Meiosis II results in four haploid (N) daughter cells.

23. Gamete Formation
In male animals, meiosis results in four equal-sized gametes called sperm.
Meiosis produces four genetically different haploid cells. In males, meiosis results in four equal-sized gametes called sperm.

24. Gamete Formation
In many female animals, only one egg results from meiosis. The other three cells, called polar bodies, are usually not involved in reproduction.
Meiosis produces four genetically different haploid cells. In females, only one large egg cell results from meiosis. The other three cells, called polar bodies, usually are not involved in reproduction.

25. Comparing Mitosis and Meiosis
Mitosis results in the production of two genetically identical diploid cells. Meiosis produces four genetically different haploid cells.

26. Comparing Mitosis and Meiosis
Cells produced by mitosis have the same number of chromosomes and alleles as the original cell.
Mitosis allows an organism to grow and replace cells.
Some organisms reproduce asexually by mitosis.
Cells produced by meiosis have half the number of chromosomes as the parent cell.
These cells are genetically different from the diploid cell and from each other.
Meiosis is how sexually-reproducing organisms produce gametes.

27. Comparing Mitosis and Meiosis
Cells produced by mitosis have the same number of chromosomes and alleles as the original cell.
Mitosis allows an organism to grow and replace cells.
Some organisms reproduce asexually by mitosis.

28. Comparing Mitosis and Meiosis
Cells produced by meiosis have half the number of chromosomes as the parent cell.
These cells are genetically different from the diploid cell and from each other.
Meiosis is how sexually-reproducing organisms produce gametes.