1. Meiosis
Unit 4

2. Mendel’s principles of genetics
Each organism must inherit a single copy of every gene from both “parents”
When an organism produces its own gametes, those two sets of genes must be separated from each other so that each gamete only contains 1 set of genes.

3. Chromosome Number Example: Drosophila Has 8 chromosomes
4 from each parent
The two sets are homologous, they have a corresponding chromosome from each parent.
A diploid cell contains both sets of homologous chromosomes (2N)
The gametes of a sexually reproducing organism has only 1 set of chromosomes making them haploid (N)

4. Phases of Meiosis
Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell
Meiosis has two stages
Meiosis I
Meiosis II

5. Meiosis I Prior to meiosis I each chromosome is replicated Prophase I
Each chromosome pairs up with its corresponding homologous chromosome to form a tetrad (4 chromatids in a tetrad)
Chromosomes may exchange portions of chromatids in crossing over
results in new combinations of alleles

6. Meiosis I
Metaphase I
Spindle fibers attach to the chromosomes

7. Meiosis I
Anaphase I
The fibers pull the homologous chromosomes toward opposite ends of the cell

8. Meiosis I Telophase I and Cytokinesis
Nuclear membranes form. The cell separates into 2 cells.
This results in 2 haploid daughter cells, each with ½ the number of chromosomes as the original cell.

9. 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.

10. Meiosis II
Prophase II
Spindle fibers form

11. Meiosis II
Metaphase II
The chromosomes line up in the center of cell.

12. Meiosis II
Anaphase II
The sister chromatids separate and move toward opposite ends of the cell.

13. Meiosis II Telophase II and Cytokinesis
Nuclear membranes form and each cell separates into 2 new daughter cells.
Meiosis II results in 4 haploid daughter cells.

14. Gamete Formation Female Gametes Male Gametes
4 equal-sized sperm cells
Female Gametes
1 large egg cell 3 polar bodies

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

16. Review
During meiosis, the number of chromosomes per cell is cut in half through the separation of
Daughter cells
Homologous chromosomes
Gametes
Chromatids

17. The formation of a tetrad occurs during
Anaphase I
Metaphase II
Prophase I
Prophase II

18. If a human body cell contains 46 chromosomes, a single sperm cell should have
Between 23 and 46 chromosomes

19. In many female animals, meiosis results in the production of
Only 1 egg
1 egg and 3 polar bodies
4 eggs
1 egg and 2 polar bodies

20. Compared to egg cells formed during meiosis, daughter cells formed during mitosis are
Genetically different, while eggs are genetically identical
Genetically different, just as egg cells are
Genetically identical, just as eggs cells are
Genetically identical, while egg cells are genetically different