2. Chromosomes Review
Every organism has traits (eye color, hair color, height, etc) passed on to them by their parents.
The instructions for each trait is found within a gene that is found on chromosomes

4. Review!
Humans somatic cells (body cells) have how many total chromosomes?
How many pairs?
Half of your chromosomes come from your father
Half of your chromosomes come from your mother 46 23

5. 46 Total Chromosomes
23 pairs of chromosomes

6. Homologous Chromosomes
A pair of chromosomes (one from mom and one from dad) are called homologous chromosomes.
They are the same length and have the centromere in the same place.
They also carry genes for the same traits at the same place
They “read” the same genes in the same places

8. Chromosome Number
The number of chromosomes does not change from generation to generation
You have the same number of chromosomes as your parents
Gametes are sex cells (sperm and egg) that contain half the number of chromosomes
Since you get half from mom and half from dad this guarantees each generation has the same number of chromosomes

9. Chromosomes The symbol n represents the number of chromosomes.
A cell with n number of chromosomes is called haploid
In humans n = 23
Gametes are haploid

10. Chromosomes
The process in which one haploid gamete joins with another haploid gamete is called fertilization.
After fertilization the cell has 2n chromosomes
n from the female parent plus n from the male parent

11. A cell with 2n chromosomes is called a diploid
A fertilized egg is called a zygote
In humans the haploid number is?
In humans the diploid number is?
How many chromosomes in a human gamete?
How many chromosomes in a human zygote? 23 46 23 46

13. LETS MAKE A BABY!!!! X
Haploid (n) Y
Haploid (n)

14. LETS MAKE A BABY!!!! X
Haploid (n) Y
Haploid (n)

15. LETS MAKE A BABY!!!! X Y
Haploid (n)
Haploid (n)

16. LETS MAKE A BABY!!!! X Y
Haploid (n)
Haploid (n)

17. LETS MAKE A BABY!!!! Y X
Haploid (n)
Haploid (n)

18. LETS MAKE A BABY!!!! Y X
Haploid (n)
Haploid (n)

19. LETS MAKE A BABY!!!! Y X
Haploid (n)
Haploid (n)

20. LETS MAKE A BABY!!!! Y X

21. LETS MAKE A BABY!!!! Y X

22. LETS MAKE A BABY!!!! Y X

23. LETS MAKE A BABY!!!! Y X

24. LETS MAKE A BABY!!!! Y X

25. LETS MAKE A BABY!!!! Y X

26. Y X
LETS MAKE A BABY!!!!

27. Y X
LETS MAKE A BABY!!!!
Diploid (2n)

28. Y X
Awww! WE MADE A ZYGOTE!
Diploid (2n)

29. Cell Division Recall that somatic cells are formed by mitosis
In mitosis the chromosome number stays the same
However, sex cells need HALF the chromosomes so a different type of cell division is needed
Gametes are formed during meiosis

30. Meiosis Meiosis is a type of cell division that produces gametes
This produces sex cells, which are not genetically identical, through a series of cell divisions.
In Meiosis the nucleus divides twice
Important because it creates genetic variation (a variety in genes/traits)
Because the nucleus divides twice meiosis creates four daughter cells

31. Stages of Meiosis There are 2 divisions of the nucleus in meiosis
Interphase
Prophase I
Metaphase I
Anaphase I
Telophase I
Prophase II
Metaphase II
Anaphase II
Telophase II
Meiosis I
Meiosis 2

33. Review Challenge! Interphase Prophase I Metaphase I Anaphase I
Telophase I
Prophase II
Metaphase II
Anaphase II
Telophase II
Meiosis I
Meiosis 2
Which stage does DNA Replication occur?
Does Meiosis replicate DNA twice?
During Interphase
(the S phase)
No!
That is why is meiosis you end up with HALF of the original chromosome number

34. Genetic Variation Meiosis allows for new genetic combinations
The ways this can occur
Independent assortment: of the chromosomes. About 8 million combinations can be made from the original cell
Random Fertilization: Zygotes form by random joining of gametes.
B) Crossing-over: DNA is exchanged

35. Genetic Variation
Independent assortment—homologous chromosomes are randomly sorted/distributed during meiosis
Leads to several possible combinations of genes (why siblings aren’t always identical)

37. Genetic Variation
Random-fertilization—the fertilization of an egg and sperm is random
Each sperm can have a different combination of genes (from independent assortment)
It is random which combination of genes will fertilize the egg

38. Genetics Variation
Crossing-over- the exchange of genetic material that can occur between homologous chromosomes during Prophase I
Part of one chromatid is broken and exchanged with the corresponding chromatid of the other homologous chromosome
Crossing over provides new combinations of genes that are different from either parent

41. How is Meiosis Different from Mitosis?
There are 2 divisions in meiosis
Meiosis I and meiosis II
The result is 4 cells instead of 2
In meiosis II, the DNA is not replicated again. (No interphase)
The ending number of chromosomes is 23 in humans (egg has 23 and sperm has 23)This is haploid (n).

42. Mitosis Vs. Meiosis Mitosis Meiosis 2 divisions = 4 cells
Daughter cells different (crossing over; independent assortment)
Haploid cells (n)=23
Gametes (sex cells—eggs or sperm)
Sexual reproduction
Mitosis
1 division=2 cells
Daughter cells identical
Diploid cells (2n)=46
Body/Somatic cells
Asexual reproduction

43. Meiosis Animation