1. Genes & Chromosomes
Organisms have tens of thousands of genes that determine individual traits
Genes are lined up on chromosomes
A thousand or more genes are arranged on a 1 chromosome.
Alleles – alternative forms of a gene for each variation of a trait of an organism.
Trait – any characteristic that is inherited.

2. Diploid Cells In body cells, chromosomes come in pairs (Mitosis)
A cell with two of each kind of chromosome is called a diploid cell.
It is called 2n or diploid number of chromosomes.
Contains 2 chromosomes so there are 2 alleles for any given trait.

3. Haploid Cells
In Organisms that produce gametes (for sexual reproduction) or sex cells have cells that contain one of each kind of chromosome
A cell that contains one of each kind of chromosome is called a haploid cell.
It is called n or haploid number of chromosomes.
Contains 1 chromosome so there is only 1 allele for any given trait.

4. Homologous Chromosomes
Two chromosomes of each pair in a diploid cell help determine what the individual organism looks like.
Homologous Chromosomes – The paired chromosomes.
They are not always identical due to variations of alleles.

5. What does this person pass on to the next generation?
What sex will this person be?
What does this person pass on to the next generation?

6. Why Meiosis?
When cells divide by Mitosis, the new cells have the exact same number and kind of chromosomes
If sex cells had the same number of chromosomes as a body cell, with each fertilization, the chromosome number would double!
Meiosis – A different type of cell division where gametes have half the number of chromosomes as the parents.
Therefore, when sex cells combine, they’ll create an offspring with the same number of chromosomes as the parents.

7. Meiosis: Reduction/Division
Two divisions:
Meiosis I &
Meiosis II

8. Meiosis I & Crossing Over
Meiosis occurs in 2 stages: Meiosis I & II
Meiosis I – Begins with one diploid (2n) cell
Meiosis I is very similar to Mitosis except for what happens to the chromosomes
Prophase I – each pair of homologous chromosomes comes together to form a tetrad.
Tetrad – 2 homologous chromosomes, each made of 2 sister chromatids (4 total)
These chromosomes exchange genetic material in a process called crossing over.
Can occur at any location on a chromosome and several locations at the same time.

9. Prophase I Unlike Mitosis:
Homologous chromosomes group with other homologous chromosomes to form a “tetrad”
A “chiasma” occurs at the point where the chromosomes cross over
Like Mitosis:
Nuclear membrane disappears
Spindles form
Chromatin condenses into chromatids

10. Metaphase I Homologous chromosomes line up at the metaphase plate.
Microtubules (spindles) attach to the kinetochore

11. Anaphase I Homologous chromosomes separate.
Sister chromatids remain attached.

12. Telophase I
Two daughter cells are formed with each one containing only one chromosome of the homologous pair.
The daughter cells are now diploid.

13. Meiosis I Re-cap… Metaphase I
The Centromere of each chromosome becomes attached to a spindle fiber
Spindle fibers pull the tetrads to the equator
Unlike mitosis, the chromosomes are together
Anaphase I
The homologous chromosomes separate to opposite ends of the cell
This ensures each new cell has 2 chromosomes
Telophase I
The cell divides with a new genetic combination unique to the parent cell

14. Meiosis II: Gamete formation
Prophase II:
DNA does not replicate.
Prophase II is just like mitotic prophase.

15. Metaphase II:
All of the chromosomes line up at the metaphase plate.

16. Anaphase II: Centromeres divide
Sister chromatids migrate separately to each pole.

17. Telophase II: Cell division is complete.
Four haploid daughter cells are obtained.

18. Meiosis II - Recap Meiosis II – ends with 4 haploid (n) cells
Meiosis II is identical to Mitosis
Prophase II
Spindle forms and attaches to the chromosomes
Metaphase II
Spindle attaches to the chromosomes and pulls to them to the equator
Anaphase II
Sister chromatids separate and are pulled to the poles
Telophase II
Spindle breaks down
Cytoplasm divides

19. Summary of Meiosis

20. Comparison Table of Mitosis and Meiosis
Trait
Mitosis
Meiosis
Chromosome Number
Same (Diploid)
Half (Haploid)
Genetic Similarities
Same
Diverse
Type of Cell
Body Cell
Sex Cells (eggs & sperm)
Number of Divisions
I (one)
I & II (two)
Type of Reproduction
Asexual
Sexual