1. Chapter 9 Meiosis

2. Asexual vs. sexual reproduction
asexual: one parent creates an identical copy
sexual: two parents create a new, different cell
What is MITOSIS?
ASEXUAL or SEXUAL?
asexual: one parent
sexual: two parents
What is MITOSIS?
ASEXUAL or SEXUAL?

3. Somatic (body) vs. Reproductive (sex)
somatic (body) cells: all the cells in your body EXCEPT the sex cells (ex: liver, skin, brain, etc)
reproductive (sex) cells: haploid (half the DNA) cells used for reproduction ONLY sperm (male) and egg (female)

4. Chromosome Number Remember the DNA buddies?
Homologous chromosome: chromosomes in diploid cells that work in pairs (one from dad, one from mom);
Homologs have the same genes in the same order along the chromosome, but they may not be EXACTLY the same. Ex- blue eyes vs brown eyes; blonde vs black hair

5. Chromosome Number
diploid: “two sets”; all homologous pairs are present
- human body cells have 46 chromosomes (diploid number = 46 or 23 pairs)
- fruit fly has 8 chromosomes (diploid number = 8 or 4 pair)
haploid: “one set”; no homologous pairs are present; HALF the normal amount of DNA
- human sex cells have 23 chromosomes (haploid = 23)
- fruit fly sex cells have 4 chromosomes (haploid = 4)

6. Karyotype
Karyotype: a picture of all the homologous pairs; used to make sure all DNA is present without extra or missing chromosomes; last pair = sex chromosomes (XX female, XY male)
To read: make sure no extra or missing and identify sex
Is this a male or female?

7. Human Chromosomes

8. What if?
What would happen if two diploid cells (with 2 complete sets of 23 chromosomes or 46 total) came together during reproduction?
How many chromosomes would that new cell have?
92 chromosomes!
Would it be human?

9. Mitosis
You have already learned how a cell makes a complete copy of its chromosomes in
Mitosis

10. But …
How do sex cells end up with half the chromosomes that body cells have????
Meiosis

11. Purpose of Meiosis
To produce HAPLOID (half DNA/no pairs) eggs or sperm, so fertilization can produce a fertilized egg (zygote) with new chromosome pairs (diploid).

13. “Crossing Over”
In meiosis, variety is BEST! The offspring should be DIFFERENT. To mix up the DNA, sometimes crossing over occurs between homologous pairs.
Homologous pairs break pieces off and trade to create brand new combinations. “Mom” and “Dad” chromosomes become “Mad” and “Dom”

14. Genetic Diversity
Independent assortment (where the chromosomes distribute randomly) and crossing over create new combinations of genes for DIVERSITY

15. Stages of Meiosis (See DIAGRAM)
Meiosis starts the same as mitosis with a duplication of the DNA. In order to create half cells, there are TWO divisions. One to split the pairs and one to split the duplicated chromosomes.
Interphase
Meiosis I (Division of the homologous pairs)
Prophase I
Metaphase I
Anaphase I
Telophase I
Cytokinesis
Meiosis II (Division of the chromosomes)
Prophase II
Metaphase II
Anaphase II
Telophase II

16. Prophase I Pause
Prophase I is longest phase of meiosis; crossing over occurs here
Human egg production in a baby girl actually starts before birth and pauses in prophase I until puberty, then 1 egg/month continues until the eggs are gone

17. Meiosis I: Interphase I
Prophase I
Metaphase I
Anaphase I
Cells undergo a round of DNA replication, forming duplicate Chromosomes.
Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
Spindle fibers attach to the chromosomes.
The fibers pull the homologous chromosomes toward the opposite ends of the cell.

18. Meiosis I: Prophase I Interphase I Prophase I Metaphase I Anaphase I
Cells undergo a round of DNA replication, forming duplicate Chromosomes.
Each chromosome pairs in synapsis with its corresponding homologous chromosome to form a tetrad. Crossing over may occur.
Spindle fibers attach to the chromosomes.
The fibers pull the homologous chromosomes toward the opposite ends of the cell.

19. Meiosis I: Metaphase I Interphase I Prophase I Metaphase I Anaphase I
Cells undergo a round of DNA replication, forming duplicate Chromosomes.
Each chromosome pairs in synapsis with its corresponding homologous chromosome to form a tetrad. Crossing over at chiasma.
Spindle fibers attach to the chromosomes.
The fibers pull the homologous chromosomes toward the opposite ends of the cell.

20. Meiosis I: Anaphase I Interphase I Prophase I Metaphase I Anaphase I
Cells undergo a round of DNA replication, forming duplicate Chromosomes.
Each chromosome pairs in synapsis with its corresponding homologous chromosome to form a tetrad. Crossing over at chiasma.
Spindle fibers attach to the chromosomes.
The fibers pull the homologous chromosomes toward the opposite ends of the cell.

21. At the end of Meiosis I …
2 new cells are formed; although each new cell now has 4 chromatids (as it would after mitosis), something is different
neither of the daughter cells has the two complete sets of chromosomes that it would have in a diploid cell
the two cells produced by Meiosis I have sets of chromosomes and alleles that are different from each other and different from the diploid cell that entered Meiosis I

22. In Between Meiosis I and Meiosis 2…
There is NO Interphase II
There is no DNA replication

23. Separation of Chromatids Meiosis I – Splits the homologous pairs Meiosis II – splits the chromosome

24. Meiosis II: Prophase II
Metaphase II
Anaphase II
Telophase II
The chromosomes line up in a similar way to the metaphase stage of mitosis.
The sister chromatids separate and move toward opposite ends of the cell.
Meiosis II results in four haploid (N) daughter cells.
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

25. Meiosis II: Metaphase II
Prophase II
Metaphase II
Anaphase II
Telophase II
The chromosomes line up in a similar way to the metaphase stage of mitosis.
The sister chromatids separate and move toward opposite ends of the cell.
Meiosis II results in four haploid (N) daughter cells.
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

26. Meiosis II: Anaphase II
Prophase II
Metaphase II
Anaphase II
Telophase II
The chromosomes line up in a similar way to the metaphase stage of mitosis.
The sister chromatids separate and move toward opposite ends of the cell.
Meiosis II results in four haploid (N) daughter cells.
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

27. Meiosis II: Telophase II
Prophase II
Metaphase II
Anaphase II
Telophase II
The chromosomes line up in a similar way to the metaphase stage of mitosis.
The sister chromatids separate and move toward opposite ends of the cell.
Meiosis II results in four haploid (N) daughter cells.
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

28. At the end of Meiosis II …
You have FOUR (4) daughter cells with the haploid number (N) of chromosomes!

29. If this took place in … … a male: … a female:
all four haploid cells will grow flagella and become sperm
… a female:
only 1 of the 4 haploid cells will survive and become an egg

30. A zygote with the diploid number of chromosomes!
Male + female =
A zygote with the diploid number of chromosomes!

31. Problems in Mitosis and Meiosis
What happens if something goes wrong during cell division?
In Mitosis, incorrect cell division or uncontrolled cell division leads to cancer.
Cells divide incorrectly or too fast. This can lead to the growth of tumors or abnormal cells that do not do their job correctly.
In Meiosis, incorrect cell division leads to birth defects from too many or missing chromosomes

32. Nondisjunction
Occurs when chromosomes do not separate in meiosis; 1 extra or 1 missing chromosome in gamete
Fertilization will produce a zygote with 45 or 47 chromosomes

33. Down’s Syndrome
Trisomy 21 results from nondisjunction of chromosome 21
These kids have 3 chromosomes at pair 21.