1. Genetics Unit: Module 5 Meiosis Notes
©2013 Educurious Partners. All rights reserved.

2. ©2013 Educurious Partners. All rights reserved. www.educurious.org
Objectives
Identify the purpose of meiosis
Identify each stage of meiosis
Describe what happens in each stage of meiosis
Explain why meiosis is important
Describe Down Syndrome and its causes
©2013 Educurious Partners. All rights reserved.

3. ©2013 Educurious Partners. All rights reserved. www.educurious.org
Mitosis Review
Phases:
Prophase
Metaphase
Anaphase
Telophase and Cytokinesis
Result: two haploid cells
©2013 Educurious Partners. All rights reserved.

4. ©2013 Educurious Partners. All rights reserved. www.educurious.org
Why Meiosis?
Most eukaryotes are homologous or diploid: have paired chromosomes.
One comes from egg; one comes from sperm.
Each chromosome contains different genetic information (a different sequence of base pairs) that determine that organism’s physical traits.
©2013 Educurious Partners. All rights reserved.

5. ©2013 Educurious Partners. All rights reserved. www.educurious.org
Meiosis Overview
Meiosis starts with 1 diploid cell and ends up with 4 haploid cells
The cell begins with 2 copies of the DNA (diploid):
one from the cell’s mother
one from the cell’s father
The DNA is duplicated, so the cell then has 4 copies of the DNA
The cell divides TWICE, to produce 4 cells (as opposed to once to produce 2 cells like we see with mitosis)
The resulting cells have only ONE copy of the DNA (as opposed to two like we see with mitosis)
©2013 Educurious Partners. All rights reserved.

6. ©2013 Educurious Partners. All rights reserved. www.educurious.org
Meiosis Overview
Image from
©2013 Educurious Partners. All rights reserved.

7. ©2013 Educurious Partners. All rights reserved. www.educurious.org
Meiosis Overview
Meiosis is not a cycle. It starts with a diploid cell and ends up with 4 haploid cells with different genetic combinations within those cell than were originally found in the rest of the body.
©2013 Educurious Partners. All rights reserved.

8. ©2013 Educurious Partners. All rights reserved. www.educurious.org
Meiosis: Prophase I
In Prophase I, Crossing Over, genetic material is exchanged between homologous chromosomes.
The homologous chromosomes line up at exactly the same place and the arms break and reattach on to each other repeatedly. The farther genes are from each other the more likely they will be separated. Genes that are right next to each other rarely are separated so they are said to be “linked”
©2013 Educurious Partners. All rights reserved.

9. ©2013 Educurious Partners. All rights reserved. www.educurious.org
Meiosis: Metaphase I
Homologous chromosomes line up in the middle of the cell
Spindle fibers attach to the centromeres.
©2013 Educurious Partners. All rights reserved.

10. ©2013 Educurious Partners. All rights reserved. www.educurious.org
Meiosis: Anaphase I
Homologous chromosomes separate
©2013 Educurious Partners. All rights reserved.

11. ©2013 Educurious Partners. All rights reserved. www.educurious.org
Meiosis: Telophase I
2 cells are formed
Each has 2N (2 copies of the DNA)
©2013 Educurious Partners. All rights reserved.

12. ©2013 Educurious Partners. All rights reserved. www.educurious.org
Overview of Meiosis I
Diploid cell starts with 2 copies of the DNA
The DNA is duplicated
Crossing Over occurs
(Why is this important?)
Homologous chromosomes separate
2 cells result
Each cell is still diploid
©2013 Educurious Partners. All rights reserved.

13. ©2013 Educurious Partners. All rights reserved. www.educurious.org
Meiosis: Prophase II
DNA remains condensed
©2013 Educurious Partners. All rights reserved.

14. ©2013 Educurious Partners. All rights reserved. www.educurious.org
Meiosis: Metaphase II
Chromosomes line up in the center of the cell
©2013 Educurious Partners. All rights reserved.

15. ©2013 Educurious Partners. All rights reserved. www.educurious.org
Meiosis: Anaphase II
Chromatids separate
This is the point where the cells go from diploid to haploid.
©2013 Educurious Partners. All rights reserved.

16. ©2013 Educurious Partners. All rights reserved. www.educurious.org
Meiosis: Telophase II
4 cells with 1 copy of DNA are formed (each is 1N; haploid)
Nuclear membrane forms around DNA
©2013 Educurious Partners. All rights reserved.

17. ©2013 Educurious Partners. All rights reserved. www.educurious.org
Overview of Meiosis II
Like mitosis
2 cells have 2 copies of DNA
DNA replication DOES NOT occur
Chromatids separate
4 daughter cells result
Each cell is now haploid; each has 1 copy of DNA
These are gametes: egg and sperm cells
©2013 Educurious Partners. All rights reserved.

18. ©2013 Educurious Partners. All rights reserved. www.educurious.org
Why Meiosis?
Sexual Reproduction!
“Mixes up” genes: genetic recombination
Children aren’t exactly like their parents
Each parent has 2 copies of DNA in every cell (diploid)
Each child needs 2 copies of DNA in each cell (diploid)
Meiosis forms cells that have 1 copy of DNA (haploid)
Called “gametes”
1 gamete from each parent combines to make a new, complete organism (diploid)
©2013 Educurious Partners. All rights reserved.

19. ©2013 Educurious Partners. All rights reserved. www.educurious.org
Gametes Combine
Image from
©2013 Educurious Partners. All rights reserved.

20. ©2013 Educurious Partners. All rights reserved. www.educurious.org
Take-Home Message
Purpose of Meiosis:
To form haploid gametes: egg and sperm cells with 1 copy of DNA
Allows sexual reproduction
Genetic recombination so the overall gene pool is healthy
More variation in the gene pool = healthier gene pool
Meiosis I: duplicates DNA and forms 2 diploid cells
Meiosis II: divides 2 diploid cells to form 4 haploid cells
Meiosis II is like Mitosis
©2013 Educurious Partners. All rights reserved.

21. ©2013 Educurious Partners. All rights reserved. www.educurious.org
Down Syndrome
Typically, the nucleus of each cell contains 23 pairs of chromosomes, half of which are inherited from each parent. Down syndrome occurs when an individual has a full or partial extra copy of chromosome 21.
Characteristics associated with Down Syndrome:
low muscle tone
small stature
an upward slant to the eyes
a single deep crease across the center of the palm
©2013 Educurious Partners. All rights reserved.

22. ©2013 Educurious Partners. All rights reserved. www.educurious.org
Types of Down Syndrome
Three different types of down syndrome:
trisomy 21 (nondisjunction)
translocation
Mosaicism
***Trisomy 21 is the most common
©2013 Educurious Partners. All rights reserved.

23. ©2013 Educurious Partners. All rights reserved. www.educurious.org
Trisomy 21
Down syndrome is usually caused by an error in cell division called "nondisjunction.“
Nondisjunction results in an embryo with three copies of chromosome 21 instead of the usual two.
Prior to or at conception, a pair of 21st chromosomes in either the sperm or the egg fails to separate. As the embryo develops, the extra chromosome is replicated in every cell of the body.
This type of Down syndrome accounts for 95% of cases. 
©2013 Educurious Partners. All rights reserved.

24. ©2013 Educurious Partners. All rights reserved. www.educurious.org
Risk Factors
Risk increases with age of both parents.
80% of children with Down syndrome are born to women under 35 years of age. 
No scientific research supporting Down Syndrome is due to environmental factors.
Does not discriminate!
1 in 350 chance with a woman of 35 years.
Sibling with Down Syndrome is a 1 in 100 chance.
©2013 Educurious Partners. All rights reserved.

25. ©2013 Educurious Partners. All rights reserved. www.educurious.org
Venn Diagram
Compare and Contrast Meiosis and Mitosis “Mitosis in my toes, meiosis in my ovaries!”
©2013 Educurious Partners. All rights reserved.

26. ©2013 Educurious Partners. All rights reserved. www.educurious.org
Exit Ticket
What causes Down Syndrome as related to meiosis?
If a baby has Down Syndrome will its siblings have it too? Why or why not?
Why is the most common type of Down Syndrome called Trisomy 21?
©2013 Educurious Partners. All rights reserved.