1. Meiosis Formation of Gametes (Eggs & Sperm)

2. Sexual Reproduction Sexual Reproduction
two cells (egg & sperm) that are NOT identical to the original cells unite to form a zygote
Eg. Meiosis

3. Facts About Meiosis
Daughter cells contain half the number of chromosomes as the original cell
Produces gametes (eggs & sperm)
Occurs in the testes in males (Spermatogenesis)
Occurs in the ovaries in females (Oogenesis)

4. Why do we need Meiosis? Fertilization!
Two haploid (1n) gametes are brought together through fertilization to form a diploid (2n) zygote
2n = 6
1n =3

5. Meiosis Forms Haploid Gametes
Meiosis must reduce the chromosome number by half
Fertilization then restores the 2n number
from mom
from dad
child
too
much!
meiosis reduces
genetic content
The right number!

6. Replication of Chromosomes
Replicated copies are called sister chromatids
Held together at centromere
Occurs in Interphase
Replication is the process of duplicating chromosome. The new copy of a chromosome is formed by DNA synthesis during S-phase. The chromosome copies are called sister chromatids. Sister chromatids are held together at the centromere.
Homologs 6

7. Meiosis.flv
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8. Meiosis: Two Part Cell Division
Sister
chromatids
separate
Meiosis I
Meiosis II
Homologs
separate
Diploid
Diploid
Haploid

9. Meiosis I: Reduction Division
Nucleus
Spindle
fibers
Nuclear
envelope
Early Prophase I
(Chromosome number doubled)
Late Prophase I
Metaphase I
Anaphase I
Telophase I (diploid)

10. Prophase I Homologs pair Crossing over Spindle forms
Nuclear envelope fragments.

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

12. Homologous Chromosomes During Crossing-Over

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

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

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

16. Meiosis II: Reducing Chromosome Number
Prophase II
Metaphase II
Telophase II
Anaphase II
4 haploid cells

17. Prophase II
Nuclear envelope fragments.
Spindle forms.

18. Metaphase II
Chromosomes align
along equator of cell.

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

20. Telophase II Nuclear envelope assembles. Spindle disappears.
Cytokinesis divides cell into two.

21. Results of Meiosis Gametes (egg & sperm) form
Four haploid cells with one copy of each chromosome
Different combinations of genes along the chromosome

22. 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
Body cells
Sex cells
When
Throughout life
At sexual maturity
Role
Growth and repair
Sexual reproduction

23. Vocabulary Gametes – eggs and sperm Zygote – a fertilized egg
Diploid – full number of chromosomes
Haploid – half the number of chromosomes
Embryo – early stage of development
Crossing over - the exchange of genetic material between homologs
Homologs – chromosome pairs that are similar (in length, gene position and centromere location)

24. ..\..\My Videos\RealPlayer Downloads\meiosis square dancing2.flv