1. MEIOSIS (making sperm and egg cells…)

2. DNA Passes from Parent to Offspring
Two types of reproduction (making more of you):
Asexual reproduction → a single parent reproduces by itself
Parent and offspring (child) are genetically identical
e.g., bacteria, many plants and fungi

3. DNA Passes from Parent to Offspring
Sexual reproduction → 2 cells (different parents) unite to produce the first cell of the new offspring
parents and offspring are genetically unique

4. Review of Chromosomes Chromosome → condensed DNA, containing all genes
Normal cells = diploid (2n) → 2 copies of each chromosome (good idea! backup copy!)
In diploid cells, 1 copy came from each parent (people: 1 from mom, 1 from dad)
Offspring have the same amount of DNA as their parents…so how can parents only pass on HALF their genetic material?

5. What is meiosis?
Meiosis → special cell division for sexual reproduction
Produces haploid (1n) cells → 1 copy of each chromosome
Haploid cells = gametes (sperm/eggs)
Human gametes:
In ♂, meiosis produces (in testes) 4 sperm cells
In ♀, meiosis produces (in ovaries) 1 egg cell and 3 polar bodies (that later disintegrate)
Mitosis: cells divide once
Meiosis: cells divide twice

6. From diploid (2n) to haploid (n).
Why do ovary and testes cells go through meiosis?
Meiosis is “reduction division.” It reduces the chromosome number in the gametes by one half.
From diploid (2n) to haploid (n).
Why do meiosis?

7. Chromosome numbers… In humans, 2n = 46
A normal body cell has 46 chromosomes → 23 came from mom, 23 from dad; each set of 23 from mom is similar to the set of 23 from dad
In humans, n = 23
A normal gamete has half the normal chromosome number (n) → just one set

8. Chromosome number questions
If a horse retina cell has 36 chromosomes, how many chromosomes does a horse sperm cell have?
If n= 4 for fruit flies, how many chromosomes does a wing cell have?
If the 2n number of a pine tree is 86, how many chromosomes would be found in a pine ovum?
If n = 16 for goldfish, how many chromosomes are in a fin cell?

9. Homologous Pairs
Homologous pairs → Pairs of chromosomes with the same sequence of genes
One is from mom, one from dad
Genes are in the same order, but not identical!
e.g., blood type

10. Phases of Meiosis Remember mitosis? (I)PMAT…
In meiosis, each phase occurs twice
Meiosis I: Prophase I, Metaphase I, Anaphase I, Telophase I, Cytokinesis I
Meiosis II: Prophase II, Metaphase II, Anaphase II, and Telophase II, Cytokinesis II

11. Don’t freak out – I changed the order of the next slides…

12. Meiosis I In Prophase I, homologous chromosomes pair up in a tetrad
same gene sequences, same banding pattern, same position of centromere
Crossing-over takes place

13. A Sketch of Chromosomes Crossing Over (Prophase I)
During crossing-over, homologous chromosomes exchange pieces of DNA
Allows for a reshuffling of genetic material

14. Meiosis I results in 2 haploid daughter cells

15. Meiosis II
Meiosis
Each of the haploid daughter cells from meiosis I divide again to make 4 haploid cells.

16. Comparing Mitosis and Meiosis
The processes of mitosis and meiosis are very similar, but with some important differences:
Mitosis
Includes only 1 cycle of cell division
Occurs in body cells (somatic cells)
No crossing over of chromosomes
Results in two genetically identical diploid cells
Metaphase: 1 duplicated chromosome per spindle fiber
Meiosis
Includes 2 cycles of cell division (meiosis I and meiosis II)
Occurs only in sex cells (gametes)
Chromosomes cross over in Prophase I
Results in four genetically different haploid cells
Metaphase I: 2 duplicated chromosomes per spindle fiber

18. Now, let’s look at our flipbooks and compare!!

19. Interphase DNA replication occurs
DNA is in chromatin form (unwound, like spaghetti)

20. Prophase I DNA condenses to form chromosomes
Homologous chromosomes pair up to form tetrads
Crossing over takes place

21. Metaphase I Homologous pairs line up along center of cell
1 homologous chromosome and its duplicate are attached to each spindle fiber

22. Anaphase I
Homologous pairs separate, move to opposite ends of the cell

23. Telophase I
Nuclear membrane reappears
Spindle fibers disappear

24. Cytokinesis Cytoplasm of the two cells separate
Meiosis I results in 2 haploid cells that are genetically different

25. Prophase II
The 2 haploid cells from Meiosis I will divide again

26. Metaphase II
Chromosomes (set of sister chromatids) line up along the center of the cell
1 sister chromatid attaches to each spindle fiber

27. Anaphase II
Centromeres split, sister chromatids are pulled apart

28. Telophase II and Cytokinesis
Cytoplasm divides, resulting in 4 genetically different haploid cells