1. Meiosis &
Sexual Reproduction

2. Aaaargh! I’m seeing double!
Cell Division
Mitosis – Asexual Reproduction
produce cells with same information
identical daughter cells
exact copies
clones
same number of chromosomes
same genetic information
Aaaargh! I’m seeing double!

3. Asexual Reproduction Single-celled eukaryotes
yeast
Paramecium
Amoeba
Simple multicellular eukaryotes
Hydra
budding
What are the disadvantages of asexual reproduction?
What are the advantages?

4. + How About The Rest of Us? 46 46 92 What if we did, then….
What if a complex multicellular organism (like us) wants to reproduce?
We use sexual reproduction
joining of egg + sperm
Do we make egg & sperm by mitosis? NO
What if we did, then…. 46 + 46 92
egg
sperm
zygote
Doesn’t work!

5. How Do We Make Sperm & Eggs?
We make them by MEIOSIS
For humans, this involves reducing 46 chromosomes  23 chromosomes 46 23 23
zygote 46
meiosis
egg 23 46 23
fertilization
sperm

6. Meiosis & Mitosis Mitosis to make copies of cells
growth
repair
Development
Meiosis to make gametes
sperm & egg

7. Starting Meiosis The whole goal of meiosis is to make sex cells.
A diploid (all chromosomes) cell divides to make four haploid (half chromosome) daughter cells.

8. How do you get these chromosomes?
Starting Meiosis
The DNA in the diploid cell is stored in homologous chromosome pairs
23 Homologous Pairs – 46 Individual Chromosomes
Homologous means “the same”
How do you get these chromosomes?

9. Chromosomes
These homologous chromosomes carry genetic information for the same traits
One chromosome in the pair comes from mom, the other from dad.
DAD
MOM
gene for eye color
gene for eye color
blue
green
gene for hair color
gene for hair color
brown
black

11. egg
sperm

12. Meiosis 1
In Meiosis I, the nucleus divides and produces two new cells. Each new cell has one duplicated chromosome from each similar pair.

13. Meiosis 2
In Meiosis II, the nuclei divide and the chromatids separate, producing four cells with half the number of chromosomes of the original nucleus.

14. new gametes made by offspring
The Value of Meiosis
Allows for genetic change over time
meiosis introduces genetic variation
gametes of offspring do not have same genes as gametes from parents
New combinations MAY be more beneficial than others. This causes EVOLUTION.
Consider the greater variation with 23 pairs of chromosomes = mixing and matching
from Dad
variation
from Mom
offspring
new gametes made by offspring

15. How does this explain: family resemblance & differences!
Why are the kids so similar to the parents but not exact?
Michael & Kirk Douglas
Baldwin brothers
Martin & Charlie Sheen, Emilio Estevez

16. Any Questions?? What are the advantages of asexual reproduction?
What are the DISadvantages of asexual reproduction?
Any Questions??
What are the advantages of sexual reproduction?
What are the DISadvantages of sexual reproduction?