1. Meiosis
11.4

2. Homologous Chromosomes
Homologous chromosomes: pairs of chromosomes with genes for the same traits, but they can have different information about those traits

3. Humans
Humans have 46 chromosomes or 23 pairs of homologous chromosomes.
Somatic (body) cells contain 46 chromosomes.
46 is the diploid number for humans.
- Diploid means that a cell has both homologs for each chromosome
Somatic cells reproduce by mitosis

4. ½ the chromosomes in a cell come from Mom, ½ the chromosomes in a cell come from Dad

5. The Homolog’s

6. Reproductive Cells
Gametes(eggs and sperm) are called haploid because they contain one set of chromosomes.
Eggs also called Ovum and sperm contain 23 chromosomes.
@Eggs and sperm are produced by meiosis- a process that reduces chromosomes by

7. What is Meiosis?
A type of cell division where ONE diploid cell (46 chromosomes) become FOUR haploid cells(23 chromosomes each).
The cells produced by meiosis are gametes (sex cells) 46 23 23 23 23

8. Who does it? Plants Fungi Animals (including you)
Males start meiosis at puberty.
Females start meiosis in utero.

9. Where does it occur? In your gonads Females- the ovary
Males-the testicles

10. Why does it occur? Genetic Variation
Mixes genes from Mom and Dad to create a unique baby
@Reduces the chromosome number by
Ensures that each cell produced has one copy from each homologous pair of chromosomes
23 in sperm + 23 in egg unite (fertilization) leads to a 46 zygote(baby)

11. How does it occur? In two stages called Meiosis I and Meiosis II.
Each stage contains 4 distinct phases
prophase
metaphase
anaphase
teleophase
telophase is followed by the division of cytoplasm known as cytokinesis.

12. Prophase 1 Homologous chromosomes join to form a tetrad
-chromosomes in the tetrad cross over and exchange genes

13. Crossing Over @Crossing over produces new genetic combinations@
- It is another way to increase
genetic diversity

14. Metaphase 1
During Metaphase 1, the tetrads (groups of 4 homologous chromosomes) line up down the center of the cell
- they assort independently
-Chromosomes from each
parent line up independently of each other

15. Anaphase I
During anaphase I, spindle fibers pull each homologous chromosome pair toward opposite ends of the cell. .

16. Telophase I and Cytokinesis
During telophase I, a nuclear membrane forms around each cluster of chromosomes.
Cytokinesis follows telophase I, forming two new cells.

17. Meiosis II
@The two cells produced by meiosis I now enter a second meiotic
Unlike the first division, neither cell goes through a round of chromosome replication before entering meiosis II.

18. Prophase II
As the cells enter prophase II, their chromosomes—each consisting of two chromatids—become visible.
The chromosomes do not pair to form tetrads, because the homologous pairs were already separated during meiosis I.

19. Metaphase II
During metaphase of meiosis II, chromosomes line up in the middle of each cell.

20. Anaphase II
As the cell enters anaphase, the paired chromatids separate.

21. Telophase II, and Cytokinesis
These four daughter cells now contain the haploid number (N)—just two chromosomes each.

22. Independent Assortment
@Independent assortment increases genetic
- There are 23 pairs of homologous chromosomes
in humans
- independent assortment ensures that there are
223 possible combinations
223= 8,388,608 combinations
- When two gametes join to form the zygote in fertilization the possible number of combinations is
223 X 223 = 64 trillion

23. Independent Assortment

25. What happens in prophase I that produces new genetic combinations?
Crossing over

26. How many chromosomes do human sperm/eggs have?

27. How is mitosis different from meiosis?
Mitosis creates 2 daughter cells with same amount of chromosomes
Meiosis creates 4 daughter cells with half amount of chromosomes
Meiosis goes through 2 cycles
Mitosis occurs in somatic cells, Meiosis in sex cells