1. Meiosis

2. Why do we need meiosis?
Meiosis is necessary to halve the number of chromosomes going into the sex cells
Why halve the chromosomes in gametes?
At fertilization the male and female sex cells will provide ½ of the chromosomes each – so the offspring has genes from both parents

3. Meiosis makes for a lot of genetic diversity
Meiosis makes for a lot of genetic diversity. This trick is accomplished through independent assortment and crossing-over.

4. Meiosis Parent cell – chromosome pair Chromosomes copied
1st division - pairs split
2nd division – produces 4 gamete cells with ½ the original no. of chromosomes

5. Meiosis – mouse testes
Parent cell
1st division
2nd division
4 gametes

6. The Stages of Meiosis:

7. Meiosis I
Interphase
Each of the chromosomes replicate

8. Prophase I
Extremely IMPORTANT!!! It is during this phase that crossing over can occur.
Crossing Over is the exchange of segments during synapsis.

9. Metaphase I
The chromosomes line up at the equator(Middle) attached by their centromeres to spindle fibers from centrioles.
Still in homologous pairs

10. Anaphase I The spindle pulls the chromosomes apart
Chromosomes appear as pairs (unlike mitosis)

11. Telophase I The cytoplasm divides, forming two new daughter cells.
cytokinesis

12. Figure 13.7 The stages of meiotic cell division: Meiosis I

13. Meiosis II : Separates sister chromatids
Proceeds similar to mitosis
THERE IS NO INTERPHASE II !
DNA does not replicate!

14. Prophase II
Each of the daughter cells forms a spindle, and the double stranded chromosomes move toward the equator

15. Metaphase II
The chromosomes are positioned on the metaphase plate in a mitosis-like fashion

16. Anaphase II The centromeres of sister chromatids finally separate
The sister chromatids of each pair move toward opposite poles
Now individual chromosomes

17. Telophase II and Cytokinesis
Nuclei form at opposite poles of the cell and cytokinesis occurs
After completion of cytokinesis there are four daughter cells
All are haploid (n)

18. Figure 13.7 The stages of meiotic cell division: Meiosis II

19. One Way Meiosis Makes Lots of Different Sex Cells (Gametes) – Independent Assortment
Independent assortment produces 2n distinct gametes, where n = the number of unique chromosomes.
In humans, n = 23 and 223 = 6,000,0000.
That’s a lot of diversity by this mechanism alone.

21. Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over
Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

22. Mitosis vs. Meiosis

23. The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in Meiosis
Mitosis
The first (and distinguishing) division of meiosis

24. Mitosis vs. Meiosis

25. Boy or Girl? The Y Chromosome “Decides”
X chromosome
Y chromosome

26. Boy or Girl? The Y Chromosome “Decides”

27. Meiosis – division error
Chromosome pair

28. Meiosis error - fertilization
Should the gamete with the chromosome pair be fertilized then the offspring will not be ‘normal’.
In humans this often occurs with the 21st pair – producing a child with Downs Syndrome

29. 21 trisomy – Downs Syndrome
Can you see the extra 21st chromosome?
Is this person male or female?