1. BIOLOGY Unit 4 Notes: Meiosis

2. (1) What is Meiosis? Cell division for sex-cells. Sex Cells:
Are also called Gametes.
Sperm in Males
Eggs in Females

3. (2) What happens during Meiosis?
There are 2 divisions.
Start with 1 cell and end up with 4 genetically unique cells.
Start “2n” or “Diploid”  End “1n” or “Haploid”
1n = Haploid = ½ Set of Genetic Information
In Humans that means that each sperm or egg cell should have 23 chromosomes.

4. (3) Meiosis Phases Meiosis-1: Meiosis-2:
Start with 1 cell and end with 2
Prophase, Metaphase-1, Anaphase-1, Telophase-1, Cytokinesis-1
Crossing Over occurs
Homologous Pairs of Chromosomes
Meiosis-2:
Start with 2 cells and end with 4
Metaphase-2, Anaphase-2, Telophase-2, Cytokinesis-2
Pairs of Sister Chromatids
Law of Independent Assortment
Law of Segregation

6. (4) Meiosis-1 Details Prophase: Chromosomes form + Nucleus dissolves.
Metaphase-1: Homologous pairs of chromosomes line up at the metaphase plate. Crossing Over occurs.
Anaphase-1: Homologous pairs of chromosomes separate.
Telophase-1: Chromosomes are fully separated.
Cytokinesis-1: Cytoplasm splits and the cell membranes divide.

7. (5) Meiosis-1 Vocabulary:
Homologous Pairs of Chromosomes 
Pairs of sister chromatids with similar segments of DNA (or genes.)
Looks like 2 X’s.
Crossing Over 
When nearby chunks of DNA are exchanged between homologous pairs of chromosomes.
Used to increase genetic diversity.

8. (6) Meiosis-2:
Metaphase-2: Pairs of sister chromatids line up at the metaphase plate.
Anaphase-2: Pairs of sister chromatids separate. Law of Segregation. Law of Independent Assortment.
Telophase-2: Chromosomes are fully separated. Nuclei reform.
Cytokinesis-2: Cytoplasm splits and the cell membranes divide up.

9. (Drawing Time)

10. (7) Meiosis-2 Vocabulary:
Sister Chromatids: Identical copies of chromosomes.
Law of Segregation: Chromosomes split up so that each sex cell receives a different variation of each gene.
Law of Independent Assortment: Chromosomes split up completely independent of every other chromosome and gene variation.

11. (Drawing Time)

12. (8) Meiosis Errors
When chromosomes do not separate correctly during meiosis this can cause:
Sex Cells to have too many chromosomes (more than 23).
Sex cells to have too few chromosomes (less than 23).
Individuals born from these sex cells will acquire a genetic disorder from having the wrong number of sex chromosomes.