1. Meiosis and Sexual Reproduction

2. Parents can produce many types of offspring
Families have resemblances, but no two are alike

3. How to Make Offspring
When sperm and egg combine, they form a zygote (offspring). The zygote gets half of its chromosomes from Mom and half from Dad.
Sperm & Egg Zygote

4. Chromosomes Every human cell has 23 pairs of chromosomes.
1 set from Mom and 1 set from Dad

5. Homologous Chromosomes
Chromosome pairs, one from each parent, that pair up during meiosis. These pairs have the same genes in the same location, or loci.

6. Diploid (2n) vs. Haploid (1n)
Diploid - Cells with 2 copies of each chromosome.
In humans, all cells except sex cells (gametes)
Haploid - Cells with only 1 copy of each chromosome.
In humans, only sex cells (gametes)

7. Meiosis Making Sperm and Eggs
Meiosis occurs in sexual reproduction when a diploid (2n) germ cell produces 4 haploid (1n) daughter cells that can mature to become gametes (sperm or egg)

8. Goal of Meiosis Goal: Reduce genetic material by 1/2.
from mom
from dad
child
meiosis reduces
genetic content
too
much!
Just right!
Why? 1n (Mom) + 1n (Dad) = 2n (offspring)

9. 2 Phases of Meiosis Meiosis I - Homologous chromosomes separate
Meiosis II - sister chromatids separate

10. Chromatids vs. Homologous Chromosomes
Sister chromatids are 2 duplicates of a chromosome made during DNA replication.
Homologous chromosomes are a pair of the same chromosome, one from Mom, one from Dad.
Homologous
chromosomes

11. Difference Between Mitosis and Meiosis
YouTube Video
Difference Between Mitosis and Meiosis

12. Meiosis I - Prophase I Chromatids pair and condense
Crossing over occurs
Spindle forms
Nuclear envelope fragments

13. Crossing Over
Crossing over occurs during Prophase I when parts of homologous chromosomes overlap. Crossing over increases genetic variation.

14. Meiosis I - Metaphase I
Chromosome pairs align along the equator of the cell.

15. Meiosis I - Anaphase I
Chromosomes separate and move to opposite poles.
Sister chromatids remain attached at their centromeres.

16. Meiosis I - Telophase I Nuclear envelope reassembles
Spindle disappears
Cytokinesis divides cell into two

17. Meiosis II Sister chromatids carry identical genetic information.
Meiosis II produces gametes with 1 copy of each chromosome and thus one copy of each gene.

18. Meiosis II - Prophase II
Nuclear envelope fragments
Spindle forms

19. Meiosis II - Metaphase II
Chromosomes align along equator of cell

20. Meiosis II - Anaphase II
Sister chromatids separate and move to opposite poles.

21. Meiosis II - Telophase II
Nuclear envelope assembles
Chromosomes unravel
Spindles disappear
Cytokinesis divides cell into 2

22. Results of Meiosis
4 haploid cells
One copy of each chromosome

23. Meiosis in Males and FemalesX X Y Y X

24. Fertilization
Meiosis allows the creation of unique individuals through sexual reproduction.

25. Comparison of Mitosis and Meiosis
# of divisions 1 2
# of daughter cells 4
Genetically identical
yes no
Chromosome #
Same as parent
Half of parent
Where
Somatic cells
Sex cells
When
Throughout life
At sexual maturity
Role
Growth and repair
Sexual reproduction

26. YouTube Video
Meiosis

27. Mitosis Meiosis Occurs in somatic (body) cells
Occurs only in gonads (sex organs): Ovary/Testes
Produces cells for repair, maintenance, growth, asexual reproduction
Only produces gametes (sex cells: egg & sperm)
Results in identical diploid (2n) daughter cells
Reduction division results in haploid (n) cells

28. Mistakes in Meiosis
Non-disjunction = The failure of homologous chromosomes to separate properly during meiosis.

29. Mistakes in Meiosis
Trisomy: When a gamete with extra chromosomes is fertilized by a normal gamete, the zygote will have an extra chromosome.
Monosomy = When a gamete that is missing a chromosome fuses with a normal gamete during fertilization, the resulting zygote lacks a chromosome.
Polyploidy = organisms with more than the usual number of chromosome sets.

30. Stop Here

31. Chromosomes
Chromosomes are made of two identical sister chromatids, which are joined at the centromere.