1. 6.1, 6.2, 6.6 Meiosis

2. MEIOSIS Type of cell division that produces four cells
Each new cell has half the number of chromosomes as the parent cell
Meiosis reduces chromosome number and creates genetic diversity.
Meiosis makes haploid cells from diploid cells
Meiosis produces gametes  sperm and egg cells
In animals, meiosis occurs in the sex organs—ovaries and testes

3. MEIOSIS

4. 6.1 Chromosomes and Meiosis
KEY CONCEPT: Gametes have half the number of
chromosomes that body cells have.

5. each species has a characteristic number of chromosomes per cell
Common Name
Diploid numbers (2n)
Human 46
Donkey 42
Dog 78
Cat 38
Mouse 40
Frog 26
Fruit fly 8
Corn 20
Potato 48
Yeast 32
Mold 4
This number is given for body cells, not for gametes
The number of chromosomes is not related to the complexity of an organism
Your body cells each contain 46 chromosomes in 23 pairs
these cells are genetically identical to one another

6. You have body cells and gametes.
The cells in your body are divided into two major groups:
1) Body cells (somatic cells):
- make up most of your body tissues
- DNA in body cells is not passed on to offspring
2) Sex cells called gametes (egg and sperm)
- Sex cells are located in the ovaries and testes.
- Gametes have DNA that can be passed to offspring.
body cells
sex cells (sperm)
sex cells (egg)

7. autosomes and sex chromosomes
Your cells have autosomes and sex chromosomes.
Your body cells have 23 pairs of homologous chromosomes.
Autosomes: Chromosome pairs 1-22
not related to gender
Sex chromosomes: X and Y, determine gender
Females: XX
Males: XY
homologous chromosomes
sister
chromatids
sister
chromatids

8. HOMOLOGOUS CHROMOSOMES
Paired chromosomes: one inherited from the mother and the other from the father
Not the same as sister chromatids
Not identical to one another
Each has same genes controlling the same trait in same order
homologous chromosomes
From Dad
From Mom
Sister
Chromatids
Sister Chromatids

9. HOMOLOGOUS CHROMOSOMES
SAME SIZE
SAME SHAPE
CARRY GENES for the SAME TRAITS
BUT ____________________________!
(Don’t have to have the SAME CHOICES)
NOT IDENTICAL

10. Diploid and haploid cells
Diploid:(2n) contain two homologous sets of chromosomes  in humans - 46 total
Body (somatic) cells are diploid.
Ex: Skin, Muscle, Nerve, Blood Cells
Created by mitosis
Haploid: (n) have a single set of chromosomes  in humans - 23 total
Gametes - sperm and egg – are haploid
Created by meiosis

11. Fertilization: fusing of the nucleus of a haploid sperm with the nucleus of a haploid egg
Zygote: produced as a result of fertilization  fertilized egg is diploid

12. Chromosomes (diploid)46
Chromosomes (diploid)
23 chromosomes
23 chromosomes
23 chromosomes

13. Zygote divides by mitosis
Typical human has about 100 trillion cells
9-10 months of cells dividing later….

14. 6.2 Process of Meiosis
KEY CONCEPT : During meiosis, diploid cells undergo two cell divisions that result in haploid cells.

15. Process of MEIOSIS Two distinct parts:
Meiosis I: homologous chromosomes separate from each other (sister chromatids remain together)
Meiosis II: sister chromatids separate from each other

16. MITOSIS vs. MEIOSIS Key differences occur in meiosis I: Prophase I:
- tetrad formation, crossing over
Metaphase I:
- tetrads line up at midline
Anaphase I:
- homologous chromosomes separate and move to opposite poles

17. MEIOSIS I: Prophase I Has 2 steps not seen in prophase of mitosis
Mom 1
MEIOSIS I: Prophase I
Dad 1
Has 2 steps not seen in prophase of mitosis
1) Tetrad Formation: Pairing of homologous chromosomes
Dad’s #1 chromosome pairs with Mom’s #1, etc…
The four paired chromatids equals a TETRAD
2) Crossing Over: Exchange of genes

18. Meiosis I

19. MEIOSIS I PRODUCTS Two cells
Chromosomes have crossed over – unique cells
Non-identical cells
Haploid cells - only one chromosome from each homologous pair
Each chromosome is still doubled (consists of two sister chromatids)
Another cell division is needed

20. Meiosis I

21. MEIOSIS II
Events are identical to those of mitosis with one exception:
Chromosomes DO NOT duplicate prior to meiosis II

22. MEIOSIS II PRODUCTS
Four cells
Non-identical cells
Haploid cells

23. Meiosis II

24. Meiosis Review

25. http://highered. mcgraw- hill. com/olcweb/cgi/pluginpop. cgi
hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/ /120074/ bio19.swf::Stages of Meiosis

26. Meiosis: crossing over and variability

27. Meiosis: the Great Divide
WorksheetL

28. 6.6 Meiosis and Genetic Variation
KEY CONCEPT: Independent assortment and crossing over during meiosis result in genetic diversity.

29. MEIOSIS & GENETIC VARIATION
Meiosis results in increased genetic variation for three reasons:
Independent & random assortment of chromosomes
Crossing over in Prophase I
Random fertilization of gametes

30. 1. Independent & random assortment of chromosomes
Chromosomes in tetrads line up and separate randomly in meiosis
FYI: Allows for a large amount of variation to occur at fertilization; million variations for humans (223)

31. Random orientation of chromosomes
Orientation of Chromosomes During Meiosis

32. 2. Crossing over in Prophase I
Exchange of genetic material between non-sister chromatids during prophase I
Results in genetic recombination
Provides a second source of genetic variation

33. 3. Random fertilization of gametes
Refers to the fact that any egg can be fertilized by any sperm.
FYI: This means that for a particular man and woman, the number of unique combinations of genes that could occur in their offspring is 8.4 million times 8.4 million ( 70 trillion), not counting variation caused by crossing over and mutation.
FYI: STATISTICALLY, THERE HAVE NEVER BEEN 2 HUMANS CREATED THAT WERE GENETICALLY IDENTICAL!!!!

34. Genetic Variation
The genetic variation that results from mutations, meiosis, and fertilization cause the phenomenon with which we are all familiar: even in very large populations, such as the human population, every individual is genetically unique.

35. EPIGENETICS

37. KARYOTYPES Karyotype: a display of 46 chromosomes of an individual
Human body cells  chromosomes—23 pairs of homologous chromosomes
Each chromosome has a twin that resembles it in size and shape (except sex chromosomes in males)

38. HUMAN KARYOTYPES