1. Human chromosomes
Humans have 23 pairs of chromosomes (or total of 46 chromosomes)

2. Each organism must inherit a single copy of every gene from each of its “parents.”
Gametes are formed by a process that separates the two sets of genes so that each gamete ends up with just one set.
Gametes

3. Homologous Chromosomes
Draw this:
From mom From dad
The two sets of chromosomes are homologous.
Each of the chromosomes that come from the male have a corresponding chromosome from the female parent.

4. 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 humans, the diploid number is 46, which can be written as 2N=46.

5. 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 humans, the haploid number is 23, which can be written as N=23.

6. What are gametes?
The cells involved in sexual reproduction
Sexual reproduction is when two gametes from different parents join to make a new cell, called a zygote.
A zygote is a fertilized egg cell.

7. Fertilization
Egg (23) + Sperm (23)
Zygote (a fertilized egg - 46)
Embryo

8. Think-pair-share How many chromosomes are in your somatic cells?
What does it mean for a cell to be diploid?
Are gametes diploid or haploid?

9. What is meiosis?
Cell division that results in only half the chromosome number in each cell (ex: 46  23)
It results in four genetically different cells.

10. Why do cells do meiosis?
Multicellular organisms do meiosis to form gametes for sexual reproduction

11. Phases of Meiosis
Meiosis involves two cell divisions, meiosis I and meiosis II.
Draw this:
Meiosis I
Meiosis II

12. DNA is replicated forming duplicate chromosomes
Interphase I:
From mom
From dad
DNA is replicated forming duplicate chromosomes
(sister chromatid)

13. Homologous Chromosomes
Sister chromatids
Sister chromatids

14. Prophase I: Homologous chromosomes pair up to form a tetrad.
Crossing-over may occur = exchange of genetic information between homologous chromosomes

15. Watch the animation:

16. New combinations formed
Crossing-over increases genetic variation.
New combinations formed

17. Metaphase I: Spindle fibers attach to the chromosomes; homologous chromosomes line up side by side

18. Anaphase I: The fibers pull the homologous chromosomes toward opposite ends of the cell.

19. Telophase I & Cytokinesis: Nuclear membranes form
Telophase I & Cytokinesis: Nuclear membranes form. The cell separates into two cells. 1st Cell Division! 1 2

20. Meiosis II: Unlike meiosis I, neither cell goes through chromosome replication.

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

22. Metaphase II: The chromosomes line up at the center of each cell (single file)

23. Anaphase II: The sister chromatids separate and move toward opposite ends of the cell

24. Telophase II & Cytokinesis: Cells divide to produce four haploid daughter cells. 2nd Cell Division!1 2 3 4

25. Go back to your drawing of meiosis cell divisions and add the chromosomes:

26. Watch video:
Bozeman Science Meiosis Video

27. How many cells are produced by mitosis?
Two identical cells
How many cells are produced by meiosis?
Four genetically different cells
What happened during Prophase 1 to make them genetically different?
Crossing over