1. KEY CONCEPT During meiosis, diploid cells undergo two cell divisions that result in haploid cells.

2. In your notes:
Create a Venn Diagram to Compare and Contrast Mitosis and Meiosis
Use your notes and/or your book and/or your brain, or all three!!!
You have 4 minutes to finish.

3. Meiosis occurs in two separate cycles.
Meiosis reduces chromosome number by half and creates genetic diversity.
Two cycles of Meiosis
Meiosis I
Meiosis II

4. They are not copies of each other
Meiosis I and meiosis II each have four phases, similar to those in mitosis.
Homologous chromosomes-are 2 separate chromosomes, 1 from Mom and 1 from Dad.
They are not copies of each other
Sister chromatids-are duplicated chromosomes, that are attached by a centromere, each ½ of “X” is a copy
Homologous chromosomes are divided in Meiosis I
Sister Chromatids are divided in Meiosis II.
homologous chromosomes
sister
chromatids
sister
chromatids

5. Meiosis I
Before Meiosis begins the DNA is copied during the S phase of Interphase
Meiosis I divides homologous chromosomes, producing 2 diploid cells with duplicated chromosomes
There are 4 phases of Meiosis I

6. Prophase I- Nuclear membrane starts to break down, centriols move to opposite ends of cell, and spindle fibers are formed.
Chromosomes condense and pair up as homologous chromosomes.
They pair up precisely, gene for gene, down the entire length.
The sex chromosomes also pair up.

7. 23 chromosome pairs, 1 from Mom and 1 from Dad, line up in the middle.
Metaphase I- homologous chromosomes randomly line up in the middle and spindle fibers attach to them.
23 chromosome pairs, 1 from Mom and 1 from Dad, line up in the middle.
This randomness allow for genetic variation.
Crossing over happens during Meiosis and allows for more genetic variation.
Crossing Over- Is the exchange of chromosome segments when the chromosomes are very close to each other. A piece from one chromosome breaks off and attaches to another chromosome.
In humans there are over 8.3 million combinations of chromosomes randomly lining up in the cell.

8. Anaphase I- paired homologous chromosomes separate from each other and move to opposite ends of the cell.
Sister Chromatids remain together during this step of Meiosis
Telophase I- nuclear membrane forms, spindle fibers break down, cell goes through cytokinesis
The end result is 2 cells with unique combinations of 23 pairs of chromosomes coming from both parents

9. Meiosis II
Divides sister chromatids in four phases, like Mitosis
DNA is NOT copied during Meiosis II
Prophase II- N. membrane breaks down, centriols move to opposite sides of cell, spindle fibers form
Metaphase II- Chromosomes align in the middle, spindle fibers attach to the sister chromatids
Anaphase II- Sister Chromatids are pulled apart to opposite sides of the cell
Telophase II- N. membrane starts to form, spindle fibers break down, and cell goes through cytokinesis
The end result is 4 Haploid cells with a different combination of DNA from the Diploid parent cell

10. Come in and grab a Process of Meiosis Power Notes from the table and start filling it out.
Then answer questions 1-5 on page 166.
This is your assignment today, you will have time to work on it after notes and the quiz, anything you don’t finish will be homework.

11. Haploid cells develop into mature gametes.
Gametogenesis is the production of gametes.
Gametogenesis differs between females and males.
Sperm become streamlined and motile.
Sperm primarily contribute DNA to an embryo.
Eggs contribute DNA, cytoplasm, and organelles to an embryo.
During meiosis, the egg gets most of the contents; the other cells form polar bodies.