2. Write
Meiosis &
Sex Cell Reproduction

3. ALL LIVING THINGS reproduce
Most animals undergo SEXUAL REPRODUCTION
Combines genetic material
from 2 parents (sperm & egg)
so offspring are
genetically different
from parents

4. Review
Before we learn about sex cell reproduction, let’s revisit somatic cell reproduction…mitosis
Mitosis
occurs in somatic (body) cells
Includes all cells except sex cells(eggs and sperm)
produces cells with same genetic information
identical daughter cells
same number of chromosomes in all cells

5. Think
Why must sex cells(aka gametes) be different?
Human cells contain 46 chromosomes
If an egg with 46 chromosomes joins with a sperm with 46 chromosomes how many chromosomes would the offspring have?
If that process continued, after 10 generations human cells would have 46 X 210 chromosomes…
YIKES!!!!!
To prevent exponential growth in chromosomes, sex cells must reduce 46 chromosomes (diploid) to 23 (haploid)
egg
zygote 46 23 23 46 23
meiosis
sperm 46
fertilization 23

6. 46 chromosomes (diploid) 23 chromosomes (haploid)
Write
diploid = 2 copies aka Homologous chromosomes
(1 from mother/1 from father)
46 chromosomes (diploid)
is same as 23 pairs
23 chromosomes (haploid) 2n n

7. How does meiosis occur? Write
Two stages called Meiosis I and Meiosis II.
Each stage contains 4 distinct phases:
Prophase I, Metaphase I, Anaphase I, Telophase I
Prophase II, Metaphase II, Anaphase II, Telophase II
Telophase is followed by division of cell cytoplasm (aka cytokinesis).

8. DNA copied (chromatin)
Write
Interphase:
Same as mitosis
Protein synthesis
Cell growth
Organelles made
DNA copied (chromatin)

9. Interphase I

10. Prophase I
Write
Chromosomes formed and paired with corresponding homologous chromosome to form a tetrad.
4 chromosomes (2 sister chromatids each) in a tetrad.
DNA sections from chromatids unwind and pair with corresponding homologue
Crossing-over may occur
chromosomes overlap and exchange portions of their chromatids
produces new genetic combinations
tetrad
MOM DAD

11. Prophase I

12. Write
Metaphase I
Pairs of homologous chromosomes line up together in the middle of the cell
Alignment is random (called independent assortment)
Spindle fibers form

13. Metaphase I

14. Write
Anaphase I
Spindle fibers pull one chromosome of each homologous pair to opposite poles of the cell
Now haploid (1 chromosome of the original pair)

15. Anaphase I

16. Telophase I & Cytokinesis
Write
Telophase I & Cytokinesis
Haploid chromosomes gather at each pole
Each chromosome is made of two chromatids that are not identical due to crossing over during Prophase I
Two haploid cells form

17. Telophase 1

18. Meiosis II Meiosis II is just like Mitosis.
Prophase II: the chromosomes condense and the spindle fibers form.
Metaphase II: chromosomes (non-identical chromatids) line up singly in the middle of the cell.
Anaphase II: chromatids get pulled to opposite poles by spindle fibers.
Telophase II: cytoplasm divides into 2 cells.
After Meiosis II: total of 4 cells from the original cell.
Each contains one haploid set of chromosomes

19. The value of Meiosis Write
Keeps chromosome number same from generation to generation
1 diploid cell creates 4 haploid cells
Introduces genetic variation
crossing over (Prophase I)
independent assortment (Metaphase I)

20. Mitosis vs Meiosis Write Mitosis Meiosis
Number and type of cells made…
Cells are genetically…
Happens in these cells…
2 Diploid Cells (2n)
4 Haploid Cells
(1n)
Identical
Different
Somatic (Body)
Cells
Sex Cells