1. Meiosis chapter 6

2. Mitosis Cell Division Used for growth and repair
Occurs in somatic (body) cells
Produces two new cells identical to the parent cell

3. Chromosomes

4. Chromosomes Chromosomes store genetic information
Human cells have 46 chromosomes: 23 pairs

5. Chromosomes

6. Called Sister Chromatids
Chromosomes
Duplicated chromosomes are called chromatids
chromatids held together by centromere
Called Sister Chromatids

7. Body cells are diploid = 2n
Gametes are
haploid = n
The pairs are called homologs

8. How can the cell divide and cut its chromosome number in ½?
The Problem:
How can the cell divide and cut its chromosome number in ½?
How can we go from 2n to 1n?

9. Meiosis

10. Facts About Meiosis Two divisions- Meiosis I and II
reduction- division
Original cell is diploid (2n)
Four daughter cells are haploid (1n)

11. How do you cut the chromosome number in ½?
Here is the problem:
How do you cut the chromosome number in ½?
How do you divide and get the right chromosomes into each cell?

12. A major difference between the divisions of mitosis and meiosis:
The HOMOLOGS pair up before dividing

13. Meiosis: Two Cell Divisions
Sister
chromatids
separate
Meiosis I
Meiosis II
Homologs
separate
Diploid
Diploid
Haploid

14. Prophase I Late prophase Early prophase Chromosomes condense.
Spindle forms.
Nuclear envelope fragments.
Early prophase
Homologs pair.
Crossing over occurs.

15. Crossing-Over Homologous chromosomes in a tetrad cross over each other
Pieces of chromosomes or genes are exchanged
Produces Genetic recombination in the offspring

16. Crossing-Over
Crossing-over multiplies the already huge number of different gamete types produced by independent assortment

17. Homologous pairs of chromosomes align along the equator of the cell
Metaphase I
Homologous pairs of chromosomes align along the equator of the cell

18. Anaphase I Homologs separate and move to opposite poles.
Sister chromatids remain
attached at their centromeres.

19. Telophase I Nuclear envelopes reassemble. Spindle disappears.
Cytokinesis divides cell into two.

20. Prophase II
Nuclear envelope fragments.
Spindle forms.

21. Metaphase II
Chromosomes align
along equator of cell.

22. Anaphase II Sister chromatids separate and move to opposite poles.
Equator
Pole
Sister chromatids separate and move to opposite poles.

23. Telophase II Nuclear envelope assembles. Chromosomes decondense.
Spindle disappears.
Cytokinesis divides cell into two.

24. Results of Meiosis Gametes (egg & sperm) form
Four haploid cells with one copy of each chromosome
One allele of each gene
Different combinations of alleles for different genes along the chromosome

25. Comparing Mitosis and Meiosis

26. Mitosis
Meiosis
WHAT it is used for
How many daughter cells
Are the daughters identical to the parent?
If the parent is diploid with 16 chromosomes, what will the daughter be?
Does the DNA need to replicate before division?
Does crossing over occur?

27. Comparison of Divisions
Mitosis
Meiosis
Number of divisions
Number of daughter cells
Genetically identical?
Chromosome #
Where
When
Role

28. Comparison of Divisions
Mitosis
Meiosis
Number of divisions 1 2
Number of daughter cells 4
Genetically identical?
Yes No
Chromosome #
Same as parent
Half of parent
Where
Somatic cells
Germ cells
When
Throughout life
At sexual maturity
Role
Growth and repair
Sexual reproduction