2. Mitosis Review
#1 – A somatic (body) cell has a diploid number of 24 chromosomes. How many chromosomes will each of the daughter cells have after mitosis is complete?

3. Mitosis Review
#2 – list the 6 phases of the cell cycle in order, including the 4 phases of mitosis.

4. Mitosis Review
#3 – briefly describe the outcome of mitosis, comparing the end result with the starting cell. (i.e., number of chromosomes, cells, etc)

5. Meiosis

6. Each band on a chromosome represents hundreds of genetic traits.
Remember… chromosomes carry genetic information.
Each band on a chromosome represents hundreds of genetic traits.

7. Where do they come from?
One chromosome in the pair came from the male parent and the other came from the female parent.
Is this cell diploid or haploid?
In a karyotype - chromosomes are ordered by size
What is the banding?

8. In the body cells of animals and most plants, chromosomes occur in pairs.
XX is female
XY is male
These cells that have two of each kind of chromosome are called diploid cells. (2n)

9. homologous chromosomes- a pair of matching chromosomes
Synapsis is the pairing of homologous chromosomes
Each pair of homologous chromosomes has genes for the same traits.

10. On homologous chromosomes, genes are arranged in the same order
Terminal
Because there are different possible alleles for the same gene, the two chromosomes are not always identical.
Axial
Inflated D d
Constricted T t
Short
Tall

11. Why would this be a problem in fertilization?
MITOSIS VS MEIOSIS
When cells divide in mitosis, the new cells have exactly the same number of chromosomes.
Why would this be a problem in fertilization?

12. The problem is solved by a process of cell division called MEIOSIS
MITOSIS VS MEIOSIS
The problem is solved by a process of cell division called MEIOSIS
This allows offspring to have half the number of chromosomes as their parents.

14. These haploid cells are called sex cells — gametes.
Male gametes are called sperm. (1n)
Female gametes are called eggs. (1n)
When a sperm fertilizes an egg, the resulting zygote once again has the normal number of chromosomes (2n)

15. Each species contains a unique number of chromosomes.
Chromosome Numbers of Common Organisms
Organism
Body Cell (2n)
Gamete (n)
Fruit fly 8 4
Garden pea 14 7
Corn 20 10
Tomato 24 12
Leopard Frog 26 13
Apple 34 17
Human 46 23
Chimpanzee 48 24
Dog 78 39
Adder’s tongue fern
1260
630

16. Meiosis consists of two parts (divisions): MEIOSIS I and MEIOSIS II.

17. This process starts with 1 diploid cell (2n) and ends with 4 haploid cells (1n).

18. However, what happens to the chromosomes in meiosis is very different.
Events that occur in Meiosis I are similar to mitosis (a spindle forms and the cytoplasm divides)
However, what happens to the chromosomes in meiosis is very different.

19. Interphase During interphase, the cell replicates its chromosomes.
After replication, each chromosome consists of two identical sister chromatids, held together by a centromere.
Interphase

20. Prophase I a. The chromosomes coil up and a spindle forms.
b. As the chromosomes coil, homologous chromosomes line up with each other gene by gene along their length, to form a four-part structure called a tetrad.
Prophase I

21. Prophase I c. The chromatids in a tetrad pair tightly.
d. They can pair so tightly that crossing over occurs.
e. Crossing over is the exchange of genetic material between two non-sister chromatids
Prophase I

22. Prophase I
Crossing over can occur at any location on a chromosome, and it can occur at several locations at the same time.
F. Genetic recombination results because a new mixture of genetic material is created.
Prophase I

23. Meiosis Provides for Genetic Variation
this is when we get a new mixture of genetic material that is different from either parent cell
g. Cells that are formed
by mitosis are identical
to each other and to the
parent cell.

24. Genetic recombination is a major source of variation among organisms.
MEIOSIS I
MEIOSIS II
Possible gametes
Possible gametes
Chromosome A
Chromosome B
Chromosome a
Chromosome b

26. Prophase I
It is estimated that during prophase I of meiosis in humans, there is an average of two to three crossovers for each pair of homologous chromosomes.
Sister chromatids
Nonsister chromatids
Crossing over in tetrad
Tetrad
Homologous chromosomes
Gametes

27. Metaphase I
During metaphase I, the centromere of each chromosome becomes attached to a spindle fiber.
The spindle fibers pull the tetrads into the middle of the spindle.
Metaphase I

28. Anaphase I
Anaphase I begins as homologous chromosomes, each with two chromatids, separate and move to opposite ends of the cell.
This critical step ensures that each new cell will receive only one chromosome from each homologous pair.
Anaphase I

29. Telophase I
(Events occur in the reverse order from the events of prophase I.)
The spindle is broken down, the chromosomes uncoil*, and the cytoplasm divides to yield two new cells.
*note: some cells decondense the DNA after telophase I, while others do not.
Telophase I

30. The phases of meiosis II
The second division in meiosis is simply a mitotic division of meiosis I.
Meiosis II consists of prophase II, metaphase II, anaphase II, telophase II.
Meiosis II

31. Prophase II
During prophase II, a spindle forms in each of the two new cells and the spindle fibers attach to the chromosomes.
Prophase II

32. Metaphase II
The chromosomes, still made up of sister chromatids, are pulled to the center of the cell and line up randomly at the equator during metaphase II.
Metaphase II

33. Anaphase II
Anaphase II begins as the centromere of each chromosome splits, allowing the sister chromatids to separate and move to opposite poles.
Anaphase II

34. Telophase II Finally nuclei reform, the spindles break down.

35. Cytokinesis II
Splits the cytoplasm, resulting in 4 new haploid cells.

36. End results:
At the end of meiosis II, four haploid cells have been formed from one diploid cell.
These haploid cells will become gametes, transmitting the genes they contain to offspring.

37. Meiosis is the process by which both sperm cells and egg cells (gametes) are made.
Sperm + egg = new individual
Production of sperm cells = spermatogenesis
Production of egg cells = oogenesis

38. spermatogenesis
oogenesis

40. The Phases of Meiosis
6:00

41. Spermatogenesis 4. 5. 3. 1. 2.
Meiosis I
Meiosis II

42. Oogenesis 5. 3. 6. 1. 2. 4. Meiosis I (uneven) Meiosis II
1n polar bodies die 1. 2. 4.
Meiosis I (uneven)
Meiosis II

43. KEYSTONE QUESTIONS
Compare the processes and outcomes of mitotic and meiotic nuclear divisions.