1. Meiosis Notes
Unit 6

2. Recall: Genes & Chromosomes
DNA is divided into segments
The segments are called genes which contain the instructions to make specific proteins
Different genes are turned “on” in different types of cells

3. Alleles – alternative forms of a gene for each variation of a trait of an organism.
Trait – any characteristic that is inherited.

4. 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.

5. Haploid Cells
Organisms that produce gametes or sex cells have cells that contain one of each kind of chromosome
These cells are called a haploid cell (n).
Contains 1 chromosome so there is only 1 allele for any given trait.

6. Homologous Chromosomes
Homologous Chromosomes – The paired chromosomes.
They are not always identical due to variations of alleles.

7. Why Meiosis?
Meiosis – A different type of cell division where gametes have half the number of chromosomes as the parent.
Therefore, when sex cells combine, they’ll create an offspring (baby) with the same number of chromosomes as the parents.

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

9. Meiosis I & Crossing Over

10. Meiosis I – Begins with one diploid (2n) cell
Meiosis I is very similar to Mitosis except for what happens to the chromosomes

11. Prophase I Like Mitosis: Nuclear membrane disappears Spindles form
Chromatin condenses into chromatids
Unlike Mitosis:
Homologous chromosomes form “tetrads”
Chromosomes cross over

12. What is “crossing-over”?
Occurs when the tetrads form
Crossing-over is an exchange of information between chromosomes

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

14. Anaphase I Homologous chromosomes separate.
Sister chromatids remain attached.
Sister Chromatids

15. Telophase I and Cytokinesis:
Two daughter cells are formed with each one containing only one chromosome of the homologous pair.
The daughter cells are now haploid (have half of the original information).

16. Meiosis II: Gamete formation

17. Prophase II: DNA does not replicate (no Interphase before).
Prophase II is just like mitotic prophase.

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

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

20. Telophase II and Cytokinesis:
Cell division is complete.
Four haploid daughter cells are obtained.

21. Summary of Meiosis

22. Comparison Table of Mitosis and Meiosis
Trait
Mitosis
Meiosis
Chromosome Number compared to parent cell
Same (Diploid)
Half (Haploid)
Genetic Similarities
Same
Different
Type of Cell
Body Cell
Sex Cells (eggs & sperm)
Number of Divisions
I (one)
I & II (two)
Type of Reproduction
Asexual
Sexual (requires fertilization)