1. Creating Genetic Diversity
Meiosis
Creating Genetic Diversity

2. Homologous Chromosomes
Chromosomes that have the genes for the same traits.
These genes must be in the same order on the chromosome.
They do not have to be identical alleles.
EXAMPLE: Can be S and S, S and s, or s and s.
ARRANGE THE CHROMOSOMES AT YOUR DESK INTO HOMOLOGOUS PAIRS

3. Homologous Chromosomes
Before Replication
After Replication
Before Replication
After Replication

4. Cell Types
A cell with two of each kind of chromosome is called a DIPLOID cell.
All Body cells
Chromosomes form homologous pairs
A cell with only one of each type of chromosome is called a HAPLOID cell.
Sex cells or gametes.
No homologous pairs
½ the chromosomes found in body cells

5. LET’S APPLY… How many chromosomes in human body cells?
How many chromosomes does MITOSIS ensure are in each new cell that forms?
How many chromosomes would be in human offspring if the egg & sperm have the same number of chromosomes as all other cells? 46 46 92

6. Therefore…
It is important that sex cells be formed by a different manner of cell division…
One that will ensure the chromosome number is reduced to half the normal number of all other cells.

7. Meiosis
Meiosis is the form of cell division that produces these haploid sex cells.
The chromosome number is reduced to half, each sex cell gets one chromosome from each homologous pair
In males this is sperm and females it is an egg.
They combine during sexual reproduction/ fertilization.
The resulting zygote will have the diploid number of chromosomes.

8. LET’S PRACTICE… A skin cell has 24 chromosomes…
how many in a blood cell of the same organism?
How many in a sperm cell of the same organism?
How many chromosomes will be in the zygote (offspring) of this organism
Now work through the practice worksheet 24 12 24

9. Stages of Meiosis Interphase Prophase I Metaphase I Anaphase I
Telophase I
Prophase II
Metaphase II
Anaphase II
Telophase II
Prepare your flipbook in order for notes
New info will be highlighted in red

10. Interphase The period between cell division
Cell is growing & metabolizing
Cell replicates its chromosomes
NO CHROMOSOMES VISIBLE

11. Prophase I DNA coils into chromosomes which are now visible
Centrioles separate & Spindles form
Nucleus disappears
Homologous chromosomes line up next to each other

12. Metaphase I Spindle is complete
Centromeres become attached to a spindle fiber.
The homologous chromosomes (tetrad) line up in the middle of the cell.

13. Anaphase I Homologous chromosomes separate
One chromsomes from each pair moves to opposite ends of the cell

14. Telophase I Chromosomes have reached opposite sides of cell
Spindles break down
Nucleus reforms
Cytoplasm divides to form two new cells.

15. Prophase II New spindles form
Chromosomes are still made up of sister chromatids (replicated copy of DNA)

16. Metaphase II Chromosomes line up at the equator
This time, they are single file as in mitosis

17. ANAPHASE II
Sister chromatids separate and move to opposite ends of each cell.

18. Telophase II Cytoplasm divides Meiosis produces four haploid cells.
Are they identical to the chromosomes that began?
What is different?
There are two chromosomes instead
Of 4
The chromosomes do not have a matched
(homologous) pair

19. IN MALES… These 4 cells would be SPERM cells
Sperm cells have tails so they can swim to find the egg
Made in the TESTES

20. IN FEMALES…
One cell would be very large…receive most of the cell material so that if fertilized it can support developing offspring
The other 3 (polar bodies) are so small they die
The egg doesn’t move
Made in the OVARIES

21. In Conclusion… Meiosis is the process by which sex cells are produced.
Meiosis reduces the number of chromosomes in sex cells to half the normal number
Meiosis keeps the number of chromosomes from doubling when offspring are produced.