Science, Grade 6


Table Of Contents: Meiosis

1. Sexual Reproduction

2.1. Sex Cells - Sperm and Eggs
Meiosis is a process that occurs in male and female reproductive organs. Meiosis forms sex cells--sperm and eggs. These cells have half the number of chromosomes as the parent cell.
2.2. Egg Fertilization
During sexual reproduction, a sperm fertilizes an egg, and the genetic information from the parents is combined. The fertilized egg is called a zygote.
2.3. Sex Cell Chromosomes
A human sex cell has 23 chromosomes. At fertilization, the chromosomes from the egg and sperm combine to form a zygote with 46 chromosomes.
2.4. The Zygote
The zygote contains all the information needed to form a new organism. Every cell of the organism descends from this original cell.

2. Meiosis Overview

3.1. Meiosis Overview
The process of meiosis is divided into two stages--meiosis I and meiosis II.
3.2. Meiosis I Overview
In meiosis I, homologous pairs of chromosomes are separated and two cells are formed.
3.3. Meiosis II Overview
In meiosis II, chromatids are pulled apart, and another cell division occurs, resulting in four cells.

3. Meiosis I

4.1. DNA Replication
Before meiosis begins, the cell copies its genetic material using a process called DNA replication.
4.2. Prophase I
Prophase I: Meiosis starts when the nuclear membrane breaks down, chromatin condenses and the replicated chromosomes become visible.
4.3. Synapsis
Homologous chromosomes move together to form pairs. This is referred to as synapsis. An exchange of DNA segments called crossing-over often occurs during synapsis.
4.4. Metaphase I
Metaphase I: The homologous pairs then line up in the center of the cell.
4.5. Anaphase I
Anaphase I: Chromosomes from each pair move to opposite sides of the cell.
4.6. Telophase I and Cell Division
Telophase I and Cell Division: A nuclear membrane starts to form around each set of chromosomes, and then the cell divides.
4.7. End of Meiosis I
At the end of meiosis I, two daughter cells are formed, each with only half of the genetic material as the original cell.
4.8. Entire Process of Meiosis I Animation
Watch the entire process of meiosis I.

4. Meiosis II

5.1. Beginning of Meiosis II
At the end of Meiosis I, chromosomes in each daughter cell decondense back into chromatin and a nuclear envelope completely reforms around the genetic material. Then Meiosis II begins.
5.2. Prophase II
Prophase II: The nuclear membrane disintegrates, chromatin condenses to form chromosomes and spindle fibers form.
5.3. Metaphase II
Metaphase II: Chromosomes, composed of two chromatids, move to the middle of the cell.
5.4. Anaphase II
Anaphase II: The centromeres split and the chromatids separate. They move to opposite sides of the cell.
5.5. Telophase II and Cell Division
Telophase II and Cell Division: A nuclear membrane forms around each set of chromatids. Then the daughter cells divide to form four sex cells.
5.6. End of Meiosis II
At the end of meiosis II, each sex cell has one chromosome from each pair of the original set of chromosomes.
5.7. Sex Cells and Polar Bodies
In males, all four sex cells develop into sperm. In females, only one of the sex cells will develop into an egg. The other three cells, called polar bodies, eventually die.
5.8. Entire Process of Meiosis II Animation
Watch the entire process of meiosis II.

5. Pause and Interact

6.1. Review
Use the whiteboard tools to complete the activity.
6.2. Meiosis I Phases
Follow the onscreen instructions.
6.3. Meiosis II Phases
Follow the onscreen instructions.

6. Crossing-Over

7.1. Homologous Chromosomes
Homologous chromosomes move together during synapsis in prophase of meiosis I. Proteins hold these chromosome pairs very close together.
7.2. Crossing-Over
Sometimes homologous chromosomes exchange segments of DNA. This is called crossing-over. When crossing-over occurs, each chromatid ends up with a unique sequence of DNA.
7.3. Crossed-Over Genes
At the completion of meiosis, the new combinations of crossed-over genes are found in the sex cells.
7.4. Offspring Variation
Crossing-over adds variation to the offspring and ultimately contributes to the evolution of a species.

7. Comparing Mitosis and Meiosis

8.1. Mitosis and Meiosis Overview
Both mitosis and meiosis are processes that result in the production of new cells.
8.2. Comparing Mitosis and Meiosis
Mitosis is a process that is responsible for an organism's growth and development as well as replacement of worn out body cells. Meiosis is a process for making sex cells.
8.3. Number of Chromosomes
Mitosis forms cells that have the same number of chromosomes as the parent cell, while meiosis reduces the chromosome number by half.
8.4. Daughter Cells and Sex Cells
Mitosis results in daughter cells that are genetically identical, and meiosis results in sex cells that are genetically different.

8. Pause and Interact

9.1. Review
Use the whiteboard tools to complete the activity.

9. Vocabulary Review

10.1. Meiosis Vocabulary Matching

10. Virtual Investigation

11.1. Build the Phases of Meiosis
In this lab you will build each phase of meiosis by arranging the chromosomes and cell parts in the cell.

11. Assessment

12.1. Meiosis