1. Meiosis

2. Meiosis: An Introduction
An organism must inherit a single copy of every gene from its parents.
Meiosis is a process in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.

3. Chromosome Number
Different organisms have different numbers of chromosomes.
Humans have 46 chromosomes, 22 pairs of autosomes and one pair of sex chromosomes.
A pair (or set) of chromosmes (one from each parent) are called “homologous”

4. The “Diploid” Condition
A cell that contains both sets of homologous chromosomes is said to be diploid.
The number of chromosomes in a diploid cell is sometimes represented by the symbol 2N.
For humans, the diploid number is 46, which can be written as 2N=46.
Diploid

5. The “Haploid” Condition
Meiosis results in the production of haploid gametes (sex cells) which have only 23 chromosomes.
Meiosis involves two divisions, meiosis I and meiosis II.
By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells (N).

7. Meiosis I
Interphase I
Interphase I is essentially identical to the process previously outlined in the somatic cell cycle.
Cells undergo a round of DNA replication, forming duplicate chromosomes.

8. Meiosis I: Prophase I
During Prophase I each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
There are 4 chromatids in a tetrad.
Prophase I
Tetrad

9. Meiosis I: Synapsis
Tetrads formed during Prophase I exchange portions of their chromatids in a process called crossing over.
Crossing-over produces new allele combinations and is a key source of variation in multicellular organisms.
Tetrad
Chromatid
Crossing-over occurs during meiosis. (1) Homologous chromosomes form a tetrad. (2) Chromatids cross over one another. (3) The crossed sections of the chromatids are exchanged.
Allele 9

10. Meiosis I: Metaphase I
How is this different from Mitosis?
During Metaphase I, the 23 homologous pairs (tetrads) line up along the equator of the cell.
The assortment of these chromosomes is independent, a primary source of variation within a species.
Metaphase I

11. combinations possible
Alignment at
metaphase I
Possible Chromosome Combinations
combinations possible or or
Independent
Assortment
at Metaphase I or 11

12. Meiosis I: Anaphase I
During Anaphase I the homologous pairs separate (via shortening spindle fibers) and move toward the poles of the cell.
Note the difference from mitosis; while the homologous pairs have left one another, the chromatids are still in tact at the centromere.
Anaphase I

13. Telophase I and Cytokinesis
Meiosis: Telophase I
Nuclear membranes form and the spindle breaks down.
Cleavage results in two cells.
The two haploid (N) cells produced by meiosis I have chromosomes from each homologous pair.
The cells are different from each other (remember independent assortment) and from the diploid cell that entered meiosis I.
Telophase I and Cytokinesis

14. Meiosis II
The 2 haploid cells produced by meiosis I now enter a second meiotic division which results in the production of 4 genetically different haploid cells.
No replication takes place. Remember, the chromosomes already replicated during Interphase I.

15. Meiosis II Telophase II and Cytokinesis II
Telophase I and Cytokinesis I
Prophase II
Metaphase II
Anaphase II

16. Meiosis II
During Prophase II, each of the cell’s 23 chromosomes has 2 chromatids.
Spindle fibers reform and move chromosomes toward cell equator. 16

17. Meiosis II During Metaphase II Chromosomes move to midline of dividing
cell, facing opposite
poles. 17

18. Meiosis II
During Anaphase II, chromatids separate and move toward opposite poles of the cell. 18

19. Meiosis II
During Telophase II, the nuclear membrane reforms around chromosomes in each of the four new cells, and a second round of cytokinesis produces four new haploid cells

20. Sperm or
Ovum 20

21. Gamete Development…
Spermatogenesis: four new cells from meiosis called spermatids in males develop into mature sperm cells

22. Gamete Development (cont.)
Oogenesis: During cytokinesis following meiosis in females, cytoplasm is divided unequally among the four new cells.
One cell receives most of the original cell’s cytoplasm and develops into a mature egg called an ovum. The other three cells called polar bodies die off.

23. Gamete Development (cont.)

24. Sexual Reproduction
When the nuclei of the two gametes combine during fertilization, the 2n number is restored (2n = 46).
The fusion of two gametes produces the first cell of the new animal called a zygote.

25. “OMG! A zygote?...
How romantic!”
“I want one of my haploid gametes to fertilize one of your haploid gametes…”

26. That’s All Folks!