1. Chapter 9 Meiosis

2. Asexual vs. sexual reproduction
asexual: one parent
sexual: two parents
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 (muscle, liver, heart, tissue, etc.) EXCEPT for sex cells
reproductive (sex) cells: sperm (male) and egg (female) cells

4. Chromosome Number
Pairs of matching chromosomes are homologous chromosomes
homologous: a chromosome that has a corresponding chromosome from the opposite sex parent (one from the dad, one from the mom)
Homologs have the same genes, in the same order along the chromosome

5. Karyotype Homologous chromosomes can be arranged into a karyotype
Humans have 23 pairs of chromosomes
Is this a male or female?

6. Chromosome Number
diploid: “two sets”; both sets of homologous chromosomes
- human body cells have 46 chromosomes (diploid = 46)
- fruit fly has 8 chromosomes (diploid = 8)
haploid: “one set”; a single set of chromosomes (or half the needed DNA)
- human sex cells have 23 chromosomes (haploid = 23)
- fruit fly sex cells have 4 chromosomes (haploid = 4)

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. Life Cycle
Germ (or sex) cells fuse during fertilization to make a zygote

12. Purpose of Meiosis
To produce eggs and sperm with only 23 chromosomes, so fertilization can produce a fertilized egg (zygote) with 46 chromosomes.

13. Meiosis
Specialized type of cell division that produces haploid cells from diploid parent cell

14. Stages of Meiosis 2 cell divisions after replication:
Meiosis I
Meiosis II
Each division has 4 stages:
Prophase
Metaphase
Anaphase
Telophase

16. 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.

17. 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.

18. “Crossing Over”
In meiosis, chromosomes get together in homologous (similar) pairs & crossing over occurs.
Pieces from one chromosome break off and rejoin the other!

19. Genetic Diversity
Independent assortment and crossing over create new combinations of genes in gametes

20. Prophase I Pause Prophase I is longest phase of meiosis
Production of proteins and structures in egg
In humans, meiosis starts before birth, pauses in prophase I until puberty, then 1 egg/month continues for up to 40 years

21. 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.

22. 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.

23. 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

24. There is NO Interphase II so there is no DNA replication
Now on to Meiosis II …
There is NO Interphase II
so there is no DNA replication

25. Separation of Chromatids

26. 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.

27. 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.

28. 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.

29. 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.

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

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

32. A zygote with 46 chromosomes!
Male + female =
A zygote with 46 chromosomes!

33. 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 non-disjunction.

34. Nondisjunction
Occurs when chromosomes do not separate in either anaphase I or anaphase II
Gamete is missing 1 chromosome or has 1 extra chromosome
Fertilization will produce a zygote with 45 or 47 chromosomes

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