1. DO NOW
How is meiosis similar to mitosis?

2. Sexual Reproduction - Meiosis
Humans have 46 total chromosomes = 2n
Gametes – found in the reproductive organs and only contain half the usual number of chromosomes.
Female gametes – eggs
Male gametes – sperm
Both have 23 = 1n or n

4. Fertilization
The process that takes place when an egg and sperm combine to form one new cell.
Zygote = a fertilized egg cell
23 chromosomes from an egg + 23 from a sperm = 46 total!

5. Diploid vs. Haploid Diploid cells Haploid cells
Somatic cells (regular cells) are diploid
Contains 2 sets of chromosomes
46 total chromosomes
23 pairs
“half” the # of chromosomes
Gametes (reproductive cells) are haploid

6. Meiosis
Definition: A form of cell division that halves the number of chromosomes when forming specialized reproductive cell (such as gametes or spores).
Involves 2 Divisions of the nucleus!

8. Steps of Meiosis: PMAT+C (1) PMAT+C (2)
Prophase 1
Metaphase 1
Anaphase 1
Telophase 1 and cytokinesis
Prophase 2
Metaphase 2
Anaphase 2
Telophase 2 and cytokinesis

9. Prophase 1
Chromosomes become visable. The nuclear enevlope breaks down. CROSSING OVER occurs.
Crossing over → portions of a chromatid on one homologous chromosomes are broken and exchanged with the corresponding chromatid portions of the other homologous chromosome

11. Important Thoughts … Crossing over does NOT occur during mitosis.
Does crossing over occur at the same part of the chromosome every single time?
How is crossing over related to genetic variation?

12. Metaphase 1
Pairs of homologous chromosomes move to the equator of the cell

13. Anaphase 1
Homologous chromosomes separate and move to the opposite ends of the cell.
Genetic material recombines and chromatids don’t separate at their centromeres.

14. Telophase and cytokinesis
Chromosomes gather at the poles of the cell. Both cells or poles contain 1 chromosomes from each pair of homologous chromosomes.
The cytoplasm divides, form two new cells.

15. At the end of PMAT 1 …
You have two cells. They are NOT genetically identical because of the crossing over
Next, those two cells go through PMAT again.

17. Prophase 2 or II
A new spindle forms around the chromosomes.

18. Metaphase II
Chromosomes line up at the equator and attached at their centromeres to their spindle fibers.

19. Anaphase II
Centromeres divide. Chromatids move to opposite poles of the cell.

20. Telophase II and Cytokinesis
A nuclear envelope forms around each chromosome. The spindle breaks down and cytokinesis breaks the cells apart.

21. Question … How many cells do you start with and end with in mitosis?
Are those cells genetically similar?

22. At the end of meiosis … We have 4 haploid offspring cells.
Allows for rapid generations of new genetic combinations!