1. Meiosis: Making haploid reproductive cells

2. Goal of Meiosis
The purpose is to make haploid cells so that when 2 haploid cells combine you have a new diploid offspring that has an original combination of genes. + = Haploid cell + Haploid cell = Diploid cell (2n) (1n) (1n) that turns into a muticellular offspring

3. Meiosis Overview The Goal: Makes 4 haploid cells
Takes place in sex cells (gametes) such as egg & sperm cells
Location: Testes (male) and ovaries (female)
The Goal:
To make 4 haploid daughter cells for reproduction that are genetically different from the parent cells.
Each haploid daughter cell will contain a mixture of the parents chromosomes, so that the 4 daughter cells should be different.

4. Meiosis Involves 2 cell divisions
The cell will go through interphase (G1, S, G2)…but then will divide twice  resulting in 4 haploid cells
There are 2 parts in Meiosis:
1. Meiosis 1 and Meiosis 2

5. Meiosis I and Meiosis II
Both go through each of the phases of mitosis (PMAT)
After each one cytokinesis occurs
The goal is to reduce chromosome number to haploid so that 2 cells can combine to make a diploid organism

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

7. 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

8. Metaphase I & Anaphase I
Tetrads line up randomly in the center. Spindle fibers attach to each end of the homologous pair
Anaphase I:
Homologous pairs split randomly, called independent assortment

9. 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

10. More independent assortment

11. What is genetic recombination?
…when the combination of genes in an offspring are different from the gene combination found in the parents
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.

12. 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

13. Meiosis II (2nd cell division, similar to mitosis steps, PMAT)
Prophase II: Spindle fibers form, centrioles appear, nucleolus disappears
Metaphase II: Chromosomes move to center (along equator)
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.

14. How does Meiosis differ in males vs. females?
Since meiosis is producing sex cells (sperm & eggs) it takes place in the testes (males) & the ovaries (female)
In males:
Spermatogenesis - the formation of 4 spermatids (haploid cells) which will turn into spermatozoa (sperm cells).
In females:
Oogenesis - the formation of 1 egg cell (ova) with more cytoplasm than the other 3 cells. The remaining 3 cells are called polar bodies and will disintegrate

15. The phases of Meiosis

16. Diagrams

17. Let us compare Mitosis VS. Meiosis
Mitosis makes IDENTCIAL cells to the parent
Mitosis make DIPLOID cells
Mitosis occurs in BODY cells
Mitosis makes 2 DAUGHTER CELLS
Meiosis make DIFFERENT cells from the parent
Meiosis makes HAPLOID cells
Meiosis occurs in REPRODUCTIVE cells
Meiosis makes 4 DAUGHTER CELLS

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