1. The Formation of Sex Cells
Meiosis
The Formation of Sex Cells

2. You simply cannot combine an egg and sperm if they contain the
You simply cannot combine an egg and sperm if they contain the normal number of chromosomes!
The chromosome # must be reduced to half! This is Meiosis.

3. I. Overview of Meiosis
All organisms inherit two complete sets of genes, one from each parent
In animals, mom’s set of genes comes from an egg cell; dad’s set of genes comes from a sperm cell
In plants, the genes come from pollen and ovule
B. Sex cells, or gametes contain half of the normal amount of chromosomes
1. Meiosis is the process of creating haploid (N) gametes from diploid (2N) cells
occurs in eukaryotic cells only
required for sexual, but not asexual, reproduction

4. II. Chromosome Number
Eukaryotic sexual reproduction results in organisms with two sets of chromosomes
1. the two sets are said to be homologous
a. chromosomes in one set have a version of themselves in the other set
A cell with both sets of homologous chromosomes is said to be diploid (2N)
1. Ex. somatic cells (liver cell, heart cell, skin cell etc.)
A cell with one set of chromosomes is said to be haploid (N)
1. Ex. gametes (egg and sperm or ovule and pollen)

5. Each body cell of a human has 46 chromosomes, 23 from the male and 23 from the female
Each body cell of a fruit fly has 8 chromosomes, 4 from the male and 4 from the female

6. Homologous Chromosome Pairs
What are these chromosomes made of?
What species is this individual? How do you know?
What gender is this individual?
Why are there two of each chromosome?

7. III. Phases of Meiosis
Overview: Diploid cells destined to become gametes go through two rounds of cell division
A. DNA replication
1. occurs before the first round of cell division (recall the S phase of mitosis)
B. Meiosis I
1. newly synthesized chromosomes pair up with their homologes forming a tetrad
Ex. The chromosome containing the gene for eye color from mom will pair up with the chromosome containing the gene for eye color from dad
This is different from mitosis!
Tetrad

8. B. Meiosis I cont.
2. the tetrads may exchange portions of their chromatids, in a process called crossing-over
genes are exchanged between homologous chromosomes
this introduces variation!
3. homologous chromosomes separate and the cell divides (recall cytokinesis)
4. two new diploid (2N) cells are formed, each having a unique set of genes

10. note the similarities and differences to mitosis
Images of Meiosis I
note the similarities and differences to mitosis
Meiosis and Variation movie

11. 1. Performed by the two new cells produced in Meiosis I
C. Meiosis II
1. Performed by the two new cells produced in Meiosis I
each of the new haploid cell’s chromosomes contains 2 unique chromatids
2. DNA does not replicate
3. Prophase through cytokinesis occurs
4. Four unique haploid (N) cells are formed
two from each of the cells produced in meiosis I

13. 3 polar bodies donate cell materials (except DNA) and disintegrate
Males vs. Females
there is a difference!
For animals, meiosis in males produces 4 sperm from every germ cell vs. one egg for females
Spermatogenesis
Oogenesis
3 polar bodies donate cell materials (except DNA) and disintegrate

14. Compare Mitosis and Meiosis Link to Meiosis Square Dance
IV. Mitosis vs. Meiosis
1. Mitosis
Asexual reproduction
Begins with a diploid cell (2N)
Results in the formation of 2 diploid cells (2N) that are genetically identical to each other and to the original cell
Meiosis
Required for sexual reproduction
Results in the formation of 4 haploid cells (N) that are genetically different from each other and the original cell
Compare Mitosis and Meiosis
Link to Meiosis Square Dance