1. Meiosis: Making haploid reproductive cells Ch. 11 sec. 11.4 p 275-278+ =
Haploid cell + Haploid cell = Diploid cell (2n) (1n) (1n) that turns into a muticellular offspring

2. Meiosis Overview Location: Testes (male) and ovaries (female)
Number of Divisions: 2
Start Meiosis: 1 diploid cell
End Meiosis: 4 haploid cells (gametes; sperm/egg)
The Goal:
To make 4 haploid daughter cells
To introduce genetic variation

3. 2 Parts: Meiosis I & Meiosis II
The cell will go through interphase (G1, S, G2) once…but will divide (PMAT) twice, resulting in 4 haploid cells
Chromosomes replicate
before Meiosis I, but NOT
before Meiosis II,
*This reduces the
chromosome
number to haploid

4. Meiosis 1: Prophase 1 DNA coils into chromosomes Spindle fibers appear
Nucleus/nucleolus disappear
*Homologous chromosomes pair up into tetrads of 4 chromatids, called synapsis
*Crossing over may occur causing genetic recombination

5. What is crossing over?
When a homologous pair bumps into each other during prophase I and they exchange portions one chromatid  leads to genetic recombination

6. Metaphase I & Anaphase I
Tetrads line up randomly in the center. Spindle fibers attach to each chromosome
Anaphase I:
Homologous pairs (NOT chromatids) split from one another
Independent Assortment – The chromosomes line up randomly during Metaphase I. This allows for different combinations of chromosomes in the cells produced.
Independent Assortment Demo – Have kids line up randomly in the room. Record order of students over several trials. Did they all wind up in the same spot each time?

7. Independent Assortment
The random lining up of homologous pairs during metaphase I. This allows for different gametes to be possible and also leads toward genetic recombination

8. More independent assortment

9. What is genetic recombination?
…when the gene combination in an offspring is different from the gene combination found in the parents
Genetic recombination DOES NOT take place during mitosis!
Occurs in 2 ways in meiosis
1. Crossing over where homologous pairs exchange portions of one chromatid
2. Independent assortment where the homologous pairs line up randomly or without influences like which parent they came from or whether they are dominant or recessive.

10. Telophase I and Cytokinesis I
Chromosomes reach poles
Cytokinesis occurs forming 2 haploid cells with 2 copies of their chromosomes
Nuclear membrane may or may not reform before meiosis II

11. Meiosis II (2nd cell division, similar to mitosis steps, PMAT)
Prophase II: Spindle fibers form and nucleus breaks down (if present)
Metaphase II: Chromosomes move to center
Anaphase II: Chromatids separate toward the poles
Telophase II: Nuclear membrane reforms. Spindle fibers disassemble.
Cytokinesis occurs forming 4 haploid cells with 1 copy of each chromosome.

12. How does Meiosis differ in males vs. females?
In males: (testes)
Spermatogenesis - all
4 haploid cells will
mature into
sperm cells
In females: (ovaries)
Oogenesis – Cytokinesis divides the cells unevenly, resulting in one large cell (egg) and 3 smaller cells (polar bodies)

13. The phases of Meiosis

14. Let us compare Mitosis VS. Meiosis
Mitosis makes IDENTCIAL cells to the parent
Mitosis make DIPLOID cells
Mitosis creates BODY cells
Mitosis makes 2 DAUGHTER CELLS
Meiosis make DIFFERENT cells from the parent
Meiosis makes HAPLOID cells
Meiosis creates REPRODUCTIVE cells
Meiosis makes 4 DAUGHTER CELLS
How are these processes similar?

16. Animations Johnkyrk.com