1. Meiosis Notes
Cell division to form the gametes, sperm (male gamete) and egg (female gamete).
Normal cells are diploid (2n): 2 copies of every gene.
Gametes are haploid (n): 1 copy of every gene

3. Number of Chromosomes

4. Overview of Meiosis 2 cell divisions.
Starts with 2 copies of each chromosome (homologous), each with 2 chromatids (copies of DNA).
In meiosis I, homologous chromosomes are separated into 2 cells.
In Meiosis II the chromatids are separated into 4 cells.

5. Meiosis I (PMAT I)
PROPHASE I - The homologous chromosomes pair together (Sometimes crossing over occurs).
METAPHASE I - The pairs of homologous chromosomes line up along the middle
ANAPHASE I - Homologous chromosomes are pulled apart.
TELOPHASE I - One cell becomes two cells with one chromosome of the pair (haploid)

6. Meiosis II (PMAT II) PROPHASE II – Prepare to divide
METAPHASE II – Chromosomes line up in the middle
ANAPHASE II – Chromatids (copies of DNA) pull apart
TELOPHASE II – The end result is four cells with one copy of each gene.

7. Stages Of Meiosis: Meiosis I
Interphase
Mother cell
Prophase I:
Condensing Chromosomes
Prophase I:
Tetrad formation/
crossing over
Metaphase I
Anaphase I
Telophase I
Meiosis II

8. Stages Of Meiosis: Meiosis II
The products of meiosis are 4 haploid cells each with a unique set of chromosomes.
Telophase I
Prophase II
Metaphase II
Anaphase II
Telophase II

9. Segregation
In humans meiosis starts with one cell containing 46 chromosomes (23 pairs) and results in four cells containing 23 chromosomes.
The copies of DNA are separated when gametes are formed.

10. Independent Assortment
Homologous chromosomes are positioned randomly so any copy can be passed to the gametes with any combination of other chromosomes
There are 2n combinations possible during meiosis with n the haploid number of chromosomes for the organism

11. How many combinations are possible in human meiosis?
Possible combinations: 2n
n=23 in humans
223=about 8,300,000 combinations

12. Crossing Over
During Prophase I, the exchange of genetic material between homologous chromosomes

13. Crossing Over Prophase I: Metaphase I Anaphase I Telophase I
Tetrad formation/
crossing over
Metaphase I
Anaphase I
Telophase I
Because of crossing over, every gamete receives a unique set of genetic information.
Telophase II

14. Fertilization
The combination of a sperm and an egg which forms a zygote.
1 sperm (1 of 8 million possible chromosome combinations) x 1 ovum (1 of 8 million different possibilities) = 64 trillion diploid combinations!

16. Fertilization Results In A Diploid Zygote
Egg
Haploid
nucleus
Sperm
Haploid
nucleus

17. Fertilization Results In A Diploid Zygote
Egg
Haploid
nucleus
Sperm
Haploid
nucleus

18. Fertilization Results In A Diploid Zygote
Egg
Haploid
nucleus
Haploid
nucleus
Sperm

19. Fertilization Results In A Diploid Zygote
Egg
Haploid
nucleus
Haploid
nucleus
Sperm

20. From Zygote to Embryo
Zygote
Diploid
Zygote 2n

21. From Zygote to Embryo
Mitosis

22. From Zygote to Embryo
Mitosis

23. From Zygote to Embryo
Mitosis

24. From Zygote to Embryo
Mitosis

25. From Zygote to Embryo

27. Twins

28. Monozygotic Twins (Identical)
1/3 of all twins are identical
Twins that form from one zygote (one egg fertilized by one sperm).
These twins have identical genes and must be the same sex.
Having identical twins is random, not genetic

29. Dizygotic Twins (fraternal)
2/3 of all twins are fraternal
Twins that form from two zygotes (two eggs fertilized by two sperm)
Can be the same sex or different sexes.
The ability to have fraternal twins is thought to be genetic.

30. The
End