1. Meiosis and Cell Regulation

2. Introduction to Meiosis
Meiosis - cell division where the numbers of chromosomes is reduced.
2 sets of cell division occur
Meiosis I and Meiosis II
4 Cells are made
Each cell has ½ the number of chromosomes as the parent cell

3. Diploid (2n) Any cell that contains 2 complete sets of chromosomes
Examples: body cells (skin, hair, heart, liver, lung, etc.)
Most cells that make up humans are diploid
In humans these cells have 46 chromosomes

5. Haploid (n) Any cell that contains 1 complete set of chromosomes
Example: gametes (sex cells) (sperm in males and eggs in females)
In humans these contain 23 chromosomes

8. Homologous Chromosomes
A pair of chromosomes that has the same genetic material.
One chromosome comes from the mother, one from the father.
Humans have 46 chromosomes, 23 homologous pairs

9. Meiosis

10. Meiosis I Prophase I Homologous chromosomes pair up to form a tetrad
Crossing over occurs and allows for genetic information to be exchanged
Nuclear envelope starts to disappear
Spindles form

11. Meiosis I Chromosomes line up in the middle of the cell
Metaphase I
Chromosomes line up in the middle of the cell
Spindle fibers attach to the chromosomes at the centromere

12. Meiosis I Homologous pairs are separated and move to opposite ends
Anaphase I
Homologous pairs are separated and move to opposite ends

13. Meiosis I Chromosomes uncoil and form 2 nuclei
Telophase I
Chromosomes uncoil and form 2 nuclei
Nuclear membrane begins to form
The spindles break down
A cleavage furrow begins to form
The cell prepares to divide

14. Meiosis II Chromosomes condense Nuclear envelope starts to disappear
Prophase II
Chromosomes condense
Nuclear envelope starts to disappear
Spindles form in each new cell

15. Meiosis II Chromosomes line up randomly at the center of the cell
Metaphase II
Chromosomes line up randomly at the center of the cell

16. Meiosis II Centromeres split
Anaphase II
Centromeres split
Sister chromatids move apart to opposite ends

17. Meiosis II 4 nuclei form around chromosomes Spindles break down
Telophase II
4 nuclei form around chromosomes
Spindles break down
The cells prepare to divide

18. After Meiosis II and Cytokinesis
4 Haploid cells are formed
So in humans, how many chromosomes in each?

19. What are we separating?

20. Benefits of Meiosis Formation of Gametes
Gamete = Sex Cell (egg and sperm)
Contain one set of chromosomes
Haploid
In humans
Typical cell = 46 chromosomes
Gamete = 23 chromosomes
Keeps chromosome number constant after fertilization

21. Benefits of Meiosis During prophase, crossing over occurs
During metaphase, chromosomes line up randomly at the equator
These 2 reasons are why we don’t look exactly like one parent or another

22. Crossing Over
Exchange of chromosomal segments between a pair of homologous chromosomes
Takes place in Prophase I of meiosis

23. Independent Assortment
Independent Segregation of genes during the formation of gametes

24. Genetic Variation in Meiosis
Variation - the genetic (and physical) differences between members of a population Organisms that reproduce sexually have greater variation, therefore a greater chance of survival

25. Importance of Variation
Diversity = Stability
As variation in a population increases, so does the likelihood that individuals will survive despite changes in the environment
If there are significant environmental changes, a trait that was once beneficial may become a disadvantage where a trait that was not helpful may become an advantage

26. Regulation In some types of cells the cell cycle occurs very quickly.
EX) Skin cells, blood cells, hair follicle cells, and cells of the digestive track.

27. Regulation
Other cells move through the cycle more slowly, and others stop.
Neurons stop dividing while you are a toddler.
Since no new neurons are made, the body doesn’t repair brain damage and spinal cord injuries

28. Regulation The cell cycle is closely controlled.
Although we do not know all of the controls, we know some.
There are a group of proteins called cyclins that regulate the timing of the cell cycle in eukaryotes

29. Two Types of Cyclins Internal Regulators Respond to internal events
EX) They respond to whether or not a full copy of DNA has been made

30. Two Types of Cyclins External Regulators Respond to the surroundings
EX) They respond to whether or not there is space around the cell to allow for new cells.

31. Cancer
In some cells the regulators on the cell cycle malfunction and result in cancer.
There are many types of cancer.
Cancer cells grow uncontrollably using up all the nutrients leaving the healthy cells to die around them

32. Cancer
We will most likely need to completely understand the regulations of the cell cycle to cure cancer.

33. Cancer One treatment for cancer is chemotherapy
Chemotherapy is the range of toxins that can be given to cancer patients that in some way harms cells that are going through the cell cycle quickly.
(Also affect cells that normally go through the cycle quickly)
Side effect—Nausea, hair loss, anemia (lack of oxygen)