1. Meiosis
6.1, 6.2, 6.6

2. 6.1 – Chromosomes & Meiosis
Key Concept:
Gametes have half the number of chromosomes that body cells have.

3. You have somatic cells and gametes.
Are body cells
Make up all cells in body except for egg and sperm cells
DNA not passed on to children
Gametes:
Are egg or sperm cells
DNA passed on
to children

4. Your cells have autosomes and sex chromosomes.
Human somatic cells have 23 pairs of chromosomes (46 total)
(1) Autosomes: pairs 1 – 22; carry genes not related to the sex of an organism
(2) Homologous chromosomes: pair of
chromosomes; one from each parent; carry the same genes but may have a different form of the gene (example: one gene for brown eyes and one gene for blue eyes)
(3) Sex chromosomes: pair 23; determines the sex of an animal; control the development of sexual characteristics (Females XX, Males XY)

7. Stop & Review Another name for body cells
Somatic cells
Another name for sperm & egg cells
Gametes
The number of pairs of chromosomes in somatic cells 23
The number of individual chromosomes in somatic cells 46
Chromosome pairs 1-22
Autosomes
Chromosome pair 23
Sex chromosomes

8. Somatic cells are diploid; gametes are haploid.
Diploid (2n)
Has two copies of each chromosome (1 from mother & 1 from father)
44 autosomes, 2 sex chromosomes
Somatic cells are diploid
Produced by mitosis
Haploid (1n)
Has one copy of each chromosome
22 autosomes, 1 sex chromosome
Gametes are haploid
Produced by meiosis

9. Chromosome number must be maintained in animals.
Many plants have more than two copies of each chromosome (can be tetraploid [4n] or hexaploid [6n])
Mitosis and meiosis are types of nuclear division that make different types of cells.

10. Mitosis makes identical diploid cells.

11. Meiosis makes different haploid cells from diploid cells.

12. Stop & Review
The number of copies of each chromosome in a diploid cell 2
The number of chromosomes in a human diploid cell 46
The number of copies of each chromosome in a haploid cell 1
The number of chromosomes in a human haploid cell 23
This process produces genetically identical diploid cells
Mitosis
This process produces genetically unique haploid cells
Meiosis

13. 6.2 – Process of meiosis Key Concept:
During meiosis, diploid cells undergo two cell divisions that result in haploid cells.

14. Cells go through two rounds of division in meiosis.
Meiosis reduces chromosome number and creates genetic diversity.

15. Homologous chromosomes (sometimes called homologues)
Pair of chromosomes
Inherit one from each parent
Carry same genes but code for different traits (different versions of the gene)
Separate during Meiosis I
Sister chromatids
Duplicates of each other
Each half of a duplicated chromosome
Attached together at the centromere
Separate in Meiosis II

16. Stop & Review These separate during Meiosis I
Homologous chromosomes
These separate during Meiosis II
Sister chromatids

17. Meiosis I Occurs after DNA has been replicated (copied)
Divides homologous chromosomes in four phases.

18. Meiosis I (1) Prophase I Chromosomes condense
Homologous chromosomes pair up
Nuclear envelope (membrane) breaks down
Spindle fibers form
Crossing over occurs

19. Meiosis I (2) Metaphase I
Homologous chromosomes are lined up along the middle of the cell (along the cell equator) by spindle fibers

20. Meiosis I
(3) Anaphase I
Homologous chromosomes move apart to opposite sides of the cell
Sister chromatids remain attached

21. Meiosis I (4) Telophase I & Cytokinesis Spindle fibers fall apart
Nuclear membranes reform
Cytoplasm splits

22. Meiosis II Divides sister chromatids in four phases.
DNA is not replicated between Meiosis I and Meiosis II.

23. Meiosis II (5) Prophase II Nuclear envelope (membrane) breaks down
Spindle fibers form

24. Meiosis II (6) Metaphase II
Spindle fibers line chromosomes up along the middle of the cell

25. Meiosis II (7) Anaphase II
Sister chromatids are pulled apart to opposite sides of the cell

26. Meiosis II (8) Telophase II & Cytokinesis
Nuclear membranes form around chromosomes
Chromosomes begin to uncoil
Spindle fibers fall apart
Cytoplasm splits

27. Mitosis Vs. Meiosis Mitosis Meiosis One cell division
Homologous chromosomes do not pair up
Results in 2 diploid cells
Daughter cells are identical to parent cell
Two cell divisions
Homologous chromosomes pair up (Metaphase I)
Results in 4 haploid cells
Daughter cells are unique

28. Stop & Review – Mitosis Or Meiosis?
This involves two rounds of cell divisions
Meiosis
This produces 4 genetically unique haploid daughter cells
This results in daughter cells that are identical to the parent cell
Mitosis
Homologous chromosomes pair up during this process
This results in 2 genetically identical daughter cells
This involves one round of cell division

29. Haploid cells develop into mature gametes.
Gametogenesis is the production of gametes.
Gametogenesis differs between males and females.
Sperm (spermatogenesis)
Become streamlined and motile (able to move)
Primary contribution to embryo is DNA only
Males produce over 250 million sperm per day
Egg (oogenesis)
Contribute DNA, cytoplasm, and organelles to the embryo
During meiosis, the egg gets most of the contents, the other 3 cells become polar bodies
Females produce a few hundred eggs over a lifetime

31. 6.6 – Meiosis & Genetic Variation
Key Concept:
Independent assortment and crossing over during meiosis result in genetic diversity.

32. Sexual reproduction creates unique combinations of genes.
Fertilization
Random
Increases unique combinations of genes
In humans, the chance of getting any one combination of chromosomes from any one set of parents is one out of x 223 (which is one out of over 70 trillion combinations)

33. Sexual reproduction creates unique combinations of genes.
Independent assortment of chromosomes
Homologous chromosomes line up randomly along the cell equator
Increases the number of unique combinations of genes
In human cells, about 223 (8 million) different combinations could result

35. Sexual reproduction creates unique combinations of genes.
Crossing over
Exchange of chromosome segments between homologous chromosomes
Increases genetic diversity
Occurs during Prophase I of Meiosis I
Results in new combinations of genes (chromosomes have a combination of genes from each parent)

37. Genetic linkage Chromosomes contain many genes.
The farther apart two genes are located on a chromosome, the more likely they are to be separated by crossing over
Genetic linkage: genes located close to each other on the same chromosome tend to be inherited together