1. SEXUAL REPRODUCTION

2. I. Sexual Reproduction: Meiosis
Why does it take place?
Each organism must inherit a single copy of every gene from each of its parent
Gamete or sex cells are formed by a process that separates the two sets of genes so that each gamete ends up with just half the original number of genes

3. YOUR TURN !
Draw the resulting cell and explain why this would not be correct.

4. from mom
from dad
child
too
much!
meiosis reduces
genetic content
Just right!

5. DNA molecules are packaged into chromosomes
Number of chromosomes
DNA molecules are packaged into chromosomes
All organisms have different numbers of chromosomes
Human somatic or body cells have 46 chromosomes
Human gametes or sperm / eggs have 23 chromosomes
They have a pair of chromosomes that are homologous

6. YOUR TURN !
In your own define homologous.

7. Chromosomes Continued
Each chromosomes that comes from the male parent has a corresponding chromosome that comes from the female parent
A cell that contains 2 sets of homologous chromosomes is said to be diploid
Sometimes represented by the symbol 2n
Sex cells contain only a single set of chromosomes, and therefore a single set of genes
These cells are called haploid and represented by the symbol n (1n)

8. YOUR TURN !
If the original organism had 4 chromosomes, draw what a diploid cell and haploid cell would look like in this organism

9. Meiosis: cell division in two parts
Sister
chromatids
separate
Diploid
Meiosis I
(reduction
division)
Meiosis II
Haploid
Chromatids
separate
(equational
division) 2n n
Result: one copy of each chromosome in a gamete.

10. Meiosis I Stages:
Prophase I - each chromosome pairs with its corresponding homologous chromosome to form a tetrad

11. Prophase I: Crossing over
Portions of the homologous chromosomes break off and attach to adjacent chromatids on the homologous chromosomes in a process called crossing over, which produces a new combination of genes

12. Metaphase I – tetrads line up in the middle of the cell

13. Anaphase I – the centromeres pull the chromosomes toward the opposite ends of the cell

14. Telophase I and cytokinesis – cell separates into two cells that have half the number of chromosomes, but they contain two copies of that chromosome

15. YOUR TURN !
Draw a cell in each of the phases of Meiosis I
Prophase I:

16. Meiosis II Stages:
Prophase II – meiosis I resulted in two haploid (n) daughter cells, each with half the number of chromosomes as the original cell

17. Metaphase II – the chromosomes line up in the middle of the cell

18. Anaphase II – the sister chromatids separate and move toward opposite ends of the cell

19. Telophase II and cytokinesis – each cell is separated into two cells with half the number of chromosomes as the parent; at the end of meiosis II four haploid cells are formed

20. YOUR TURN ! Draw a cell in each of the phases of Meiosis II
Prophase II:

21. Meiosis II : the equational division
Prophase II
(haploid)
Metaphase II
Anaphase II
Telophase II
Four
Non-identical
haploid
daughter cells

22. Fertilization:
Fertilization occurs when two haploid cells combine ( sperm and egg)
The union of the sperm and egg forms a zygote
The zygote has the same number of chromosomes as the original parent (2n)

23. YOUR TURN !
In your own words define zygote

24. “Putting It All Together” - Fertilization

25. Review
1. Copy the following tables in your notes and complete then in order to compare mitosis and meiosis

26. Mitosis Meiosis Number of divisions Number of daughter cells
Genetically identical?
Chromosome #
What type of cell
When would it occur
Role in an organism

27. MITOSIS
MEIOSIS
Occurs in
Produces
Results in

28. Mitosis Meiosis Number of divisions 1 2 Number of daughter cells 4
Genetically identical?
Yes No
Chromosome #
Same as parent
Half of parent
What type of cell
Somatic cells
Sex cells
When would it occur
Throughout life
At sexual maturity
Role in an organism
Growth and repair
Sexual reproduction

29. MITOSIS MEIOSIS Occurs in somatic (body) cells
Occurs in gametes (sex cells)
Produces cells for repair,
maintenance, growth, asexual reproduction
Only produces gametes (sex cells: egg/sperm)
Results in identical diploid (2n) daughter cells
Results in reduction division results in haploid (n) cells