1. 3.3 Meiosis

2. Discovery of meiosis
Discovered by microscope observation around 1880’s.
One species studied was Parascaris equorum aka horse threadworm.

3. Review
A diploid human cell has 46 chromosomes. These are arranged as 23 pairs of chromosomes – homologous chromosomes
The pair are not identical. Homologous means similar not identical. One from father, one from mother.

4. n – number of unique chromosomes in an organism
Eukaryotes have n pairs of chromosomes = 2n per cell
Haploid cells = n
Diploid cells = 2n
This is shorthand for writing the number of chromosomes in a haploid or diploid cell.

6. Meiosis A diploid nucleus divides producing four haploid nuclei
Divided into meiosis I and meiosis II
Meiosis I results in two haploid cells
Meiosis II results in four haploid cells
During prophase I, the chromosomes have doubled (2n chromosomes).
No replication after telophase I, nucleus has double chromatids but one chromosome of each type.
Prophase II there are double chromatids.
What is the purpose of meiosis? Crossing over = variation within a population!

7. Crossing over Occurs during meiosis I
Exchange of genetic material between bivalents
Chromatids are a new combination of genes
See pg. 86 in orange textbook.

8. Task
You must be able to recognise and draw diagrams to represent the stages of meiosis. These are very similar to mitosis, but with a few important differences.
Crossing over during prophase I
Random orientation during metaphase I
Separation of chromatids in meiosis II
Notes/diagrams on pg. 164/65 will help you to do this.